ecma-262 5.1.txt

ECMAScript Language Specification - ECMA-262 Edition 5.1

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Standard ECMA-262
=================

5.1 Edition / June 2011
=======================

ECMAScript® Language Specification
==================================

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Copyright notice
================

Copyright © 2011 Ecma International

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Contents
========

1.  Copyright notice
2.  Introduction
3.  1 Scope
4.  2 Conformance
5.  3 Normative references
6.  4 Overview
    1.  4.1 Web Scripting
    2.  4.2 Language Overview
        1.  4.2.1 Objects
        2.  4.2.2 The Strict Variant of ECMAScript

    3.  4.3 Terms and definitions
        1.  4.3.1 type
        2.  4.3.2 primitive value
        3.  4.3.3 object
        4.  4.3.4 constructor
        5.  4.3.5 prototype
        6.  4.3.6 native object
        7.  4.3.7 built-in object
        8.  4.3.8 host object
        9.  4.3.9 undefined value
        10. 4.3.10 Undefined type
        11. 4.3.11 null value
        12. 4.3.12 Null type
        13. 4.3.13 Boolean value
        14. 4.3.14 Boolean type
        15. 4.3.15 Boolean object
        16. 4.3.16 String value
        17. 4.3.17 String type
        18. 4.3.18 String object
        19. 4.3.19 Number value
        20. 4.3.20 Number type
        21. 4.3.21 Number object
        22. 4.3.22 Infinity
        23. 4.3.23 NaN
        24. 4.3.24 function
        25. 4.3.25 built-in function
        26. 4.3.26 property
        27. 4.3.27 method
        28. 4.3.28 built-in method
        29. 4.3.29 attribute
        30. 4.3.30 own property
        31. 4.3.31 inherited property

7.  5 Notational Conventions
    1.  5.1 Syntactic and Lexical Grammars
        1.  5.1.1 Context-Free Grammars
        2.  5.1.2 The Lexical and RegExp Grammars
        3.  5.1.3 The Numeric String Grammar
        4.  5.1.4 The Syntactic Grammar
        5.  5.1.5 The JSON Grammar
        6.  5.1.6 Grammar Notation

    2.  5.2 Algorithm Conventions

8.  6 Source Text
9.  7 Lexical Conventions
    1.  7.1 Unicode Format-Control Characters
    2.  7.2 White Space
    3.  7.3 Line Terminators
    4.  7.4 Comments
    5.  7.5 Tokens
    6.  7.6 Identifier Names and Identifiers
        1.  7.6.1 Reserved Words

    7.  7.7 Punctuators
    8.  7.8 Literals
        1.  7.8.1 Null Literals
        2.  7.8.2 Boolean Literals
        3.  7.8.3 Numeric Literals
        4.  7.8.4 String Literals
        5.  7.8.5 Regular Expression Literals

    9.  7.9 Automatic Semicolon Insertion
        1.  7.9.1 Rules of Automatic Semicolon Insertion
        2.  7.9.2 Examples of Automatic Semicolon Insertion

10. 8 Types
    1.  8.1 The Undefined Type
    2.  8.2 The Null Type
    3.  8.3 The Boolean Type
    4.  8.4 The String Type
    5.  8.5 The Number Type
    6.  8.6 The Object Type
        1.  8.6.1 Property Attributes
        2.  8.6.2 Object Internal Properties and Methods

    7.  8.7 The Reference Specification Type
        1.  8.7.1 GetValue (V)
        2.  8.7.2 PutValue (V, W)

    8.  8.8 The List Specification Type
    9.  8.9 The Completion Specification Type
    10. 8.10 The Property Descriptor and Property Identifier Specification Types
        1.  8.10.1 IsAccessorDescriptor ( Desc )
        2.  8.10.2 IsDataDescriptor ( Desc )
        3.  8.10.3 IsGenericDescriptor ( Desc )
        4.  8.10.4 FromPropertyDescriptor ( Desc )
        5.  8.10.5 ToPropertyDescriptor ( Obj )

    11. 8.11 The Lexical Environment and Environment Record Specification Types
    12. 8.12 Algorithms for Object Internal Methods
        1.  8.12.1 [[GetOwnProperty]] (P)
        2.  8.12.2 [[GetProperty]] (P)
        3.  8.12.3 [[Get]] (P)
        4.  8.12.4 [[CanPut]] (P)
        5.  8.12.5 [[Put]] ( P, V, Throw )
        6.  8.12.6 [[HasProperty]] (P)
        7.  8.12.7 [[Delete]] (P, Throw)
        8.  8.12.8 [[DefaultValue]] (hint)
        9.  8.12.9 [[DefineOwnProperty]] (P, Desc, Throw)

11. 9 Type Conversion and Testing
    1.  9.1 ToPrimitive
    2.  9.2 ToBoolean
    3.  9.3 ToNumber
        1.  9.3.1 ToNumber Applied to the String Type

    4.  9.4 ToInteger
    5.  9.5 ToInt32: (Signed 32 Bit Integer)
    6.  9.6 ToUint32: (Unsigned 32 Bit Integer)
    7.  9.7 ToUint16: (Unsigned 16 Bit Integer)
    8.  9.8 ToString
        1.  9.8.1 ToString Applied to the Number Type

    9.  9.9 ToObject
    10. 9.10 CheckObjectCoercible
    11. 9.11 IsCallable
    12. 9.12 The SameValue Algorithm

12. 10 Executable Code and Execution Contexts
    1.  10.1 Types of Executable Code
        1.  10.1.1 Strict Mode Code

    2.  10.2 Lexical Environments
        1.  10.2.1 Environment Records
        2.  10.2.2 Lexical Environment Operations
        3.  10.2.3 The Global Environment

    3.  10.3 Execution Contexts
        1.  10.3.1 Identifier Resolution

    4.  10.4 Establishing an Execution Context
        1.  10.4.1 Entering Global Code
        2.  10.4.2 Entering Eval Code
        3.  10.4.3 Entering Function Code

    5.  10.5 Declaration Binding Instantiation
    6.  10.6 Arguments Object

13. 11 Expressions
    1.  11.1 Primary Expressions
        1.  11.1.1 The this Keyword
        2.  11.1.2 Identifier Reference
        3.  11.1.3 Literal Reference
        4.  11.1.4 Array Initialiser
        5.  11.1.5 Object Initialiser
        6.  11.1.6 The Grouping Operator

    2.  11.2 Left-Hand-Side Expressions
        1.  11.2.1 Property Accessors
        2.  11.2.2 The new Operator
        3.  11.2.3 Function Calls
        4.  11.2.4 Argument Lists
        5.  11.2.5 Function Expressions

    3.  11.3 Postfix Expressions
        1.  11.3.1 Postfix Increment Operator
        2.  11.3.2 Postfix Decrement Operator

    4.  11.4 Unary Operators
        1.  11.4.1 The delete Operator
        2.  11.4.2 The void Operator
        3.  11.4.3 The typeof Operator
        4.  11.4.4 Prefix Increment Operator
        5.  11.4.5 Prefix Decrement Operator
        6.  11.4.6 Unary + Operator
        7.  11.4.7 Unary - Operator
        8.  11.4.8 Bitwise NOT Operator ( ~ )
        9.  11.4.9 Logical NOT Operator ( ! )

    5.  11.5 Multiplicative Operators
        1.  11.5.1 Applying the * Operator
        2.  11.5.2 Applying the / Operator
        3.  11.5.3 Applying the % Operator

    6.  11.6 Additive Operators
        1.  11.6.1 The Addition operator ( + )
        2.  11.6.2 The Subtraction Operator ( - )
        3.  11.6.3 Applying the Additive Operators to Numbers

    7.  11.7 Bitwise Shift Operators
        1.  11.7.1 The Left Shift Operator ( << )
        2.  11.7.2 The Signed Right Shift Operator ( >> )
        3.  11.7.3 The Unsigned Right Shift Operator ( >>> )

    8.  11.8 Relational Operators
        1.  11.8.1 The Less-than Operator ( < )
        2.  11.8.2 The Greater-than Operator ( > )
        3.  11.8.3 The Less-than-or-equal Operator ( <= )
        4.  11.8.4 The Greater-than-or-equal Operator ( >= )
        5.  11.8.5 The Abstract Relational Comparison Algorithm
        6.  11.8.6 The instanceof operator
        7.  11.8.7 The in operator

    9.  11.9 Equality Operators
        1.  11.9.1 The Equals Operator ( == )
        2.  11.9.2 The Does-not-equals Operator ( != )
        3.  11.9.3 The Abstract Equality Comparison Algorithm
        4.  11.9.4 The Strict Equals Operator ( === )
        5.  11.9.5 The Strict Does-not-equal Operator ( !== )
        6.  11.9.6 The Strict Equality Comparison Algorithm

    10. 11.10 Binary Bitwise Operators
    11. 11.11 Binary Logical Operators
    12. 11.12 Conditional Operator ( ? : )
    13. 11.13 Assignment Operators
        1.  11.13.1 Simple Assignment ( = )
        2.  11.13.2 Compound Assignment ( op= )

    14. 11.14 Comma Operator ( , )

14. 12 Statements
    1.  12.1 Block
    2.  12.2 Variable Statement
        1.  12.2.1 Strict Mode Restrictions

    3.  12.3 Empty Statement
    4.  12.4 Expression Statement
    5.  12.5 The if Statement
    6.  12.6 Iteration Statements
        1.  12.6.1 The do-while Statement
        2.  12.6.2 The while Statement
        3.  12.6.3 The for Statement
        4.  12.6.4 The for-in Statement

    7.  12.7 The continue Statement
    8.  12.8 The break Statement
    9.  12.9 The return Statement
    10. 12.10 The with Statement
        1.  12.10.1 Strict Mode Restrictions

    11. 12.11 The switch Statement
    12. 12.12 Labelled Statements
    13. 12.13 The throw Statement
    14. 12.14 The try Statement
        1.  12.14.1 Strict Mode Restrictions

    15. 12.15 The debugger statement

15. 13 Function Definition
    1.  13.1 Strict Mode Restrictions
    2.  13.2 Creating Function Objects
        1.  13.2.1 [[Call]]
        2.  13.2.2 [[Construct]]
        3.  13.2.3 The [[ThrowTypeError]] Function Object

16. 14 Program
    1.  14.1 Directive Prologues and the Use Strict Directive

17. 15 Standard Built-in ECMAScript Objects
    1.  15.1 The Global Object
        1.  15.1.1 Value Properties of the Global Object
        2.  15.1.2 Function Properties of the Global Object
        3.  15.1.3 URI Handling Function Properties
        4.  15.1.4 Constructor Properties of the Global Object
        5.  15.1.5 Other Properties of the Global Object

    2.  15.2 Object Objects
        1.  15.2.1 The Object Constructor Called as a Function
        2.  15.2.2 The Object Constructor
        3.  15.2.3 Properties of the Object Constructor
        4.  15.2.4 Properties of the Object Prototype Object
        5.  15.2.5 Properties of Object Instances

    3.  15.3 Function Objects
        1.  15.3.1 The Function Constructor Called as a Function
        2.  15.3.2 The Function Constructor
        3.  15.3.3 Properties of the Function Constructor
        4.  15.3.4 Properties of the Function Prototype Object
        5.  15.3.5 Properties of Function Instances

    4.  15.4 Array Objects
        1.  15.4.1 The Array Constructor Called as a Function
        2.  15.4.2 The Array Constructor
        3.  15.4.3 Properties of the Array Constructor
        4.  15.4.4 Properties of the Array Prototype Object
        5.  15.4.5 Properties of Array Instances

    5.  15.5 String Objects
        1.  15.5.1 The String Constructor Called as a Function
        2.  15.5.2 The String Constructor
        3.  15.5.3 Properties of the String Constructor
        4.  15.5.4 Properties of the String Prototype Object
        5.  15.5.5 Properties of String Instances

    6.  15.6 Boolean Objects
        1.  15.6.1 The Boolean Constructor Called as a Function
        2.  15.6.2 The Boolean Constructor
        3.  15.6.3 Properties of the Boolean Constructor
        4.  15.6.4 Properties of the Boolean Prototype Object
        5.  15.6.5 Properties of Boolean Instances

    7.  15.7 Number Objects
        1.  15.7.1 The Number Constructor Called as a Function
        2.  15.7.2 The Number Constructor
        3.  15.7.3 Properties of the Number Constructor
        4.  15.7.4 Properties of the Number Prototype Object
        5.  15.7.5 Properties of Number Instances

    8.  15.8 The Math Object
        1.  15.8.1 Value Properties of the Math Object
        2.  15.8.2 Function Properties of the Math Object

    9.  15.9 Date Objects
        1.  15.9.1 Overview of Date Objects and Definitions of Abstract Operators
        2.  15.9.2 The Date Constructor Called as a Function
        3.  15.9.3 The Date Constructor
        4.  15.9.4 Properties of the Date Constructor
        5.  15.9.5 Properties of the Date Prototype Object
        6.  15.9.6 Properties of Date Instances

    10. 15.10 RegExp (Regular Expression) Objects
        1.  15.10.1 Patterns
        2.  15.10.2 Pattern Semantics
        3.  15.10.3 The RegExp Constructor Called as a Function
        4.  15.10.4 The RegExp Constructor
        5.  15.10.5 Properties of the RegExp Constructor
        6.  15.10.6 Properties of the RegExp Prototype Object
        7.  15.10.7 Properties of RegExp Instances

    11. 15.11 Error Objects
        1.  15.11.1 The Error Constructor Called as a Function
        2.  15.11.2 The Error Constructor
        3.  15.11.3 Properties of the Error Constructor
        4.  15.11.4 Properties of the Error Prototype Object
        5.  15.11.5 Properties of Error Instances
        6.  15.11.6 Native Error Types Used in This Standard
        7.  15.11.7 NativeError Object Structure

    12. 15.12 The JSON Object
        1.  15.12.1 The JSON Grammar
        2.  15.12.2 parse ( text [ , reviver ] )
        3.  15.12.3 stringify ( value [ , replacer [ , space ] ] )

18. 16 Errors
19. Annex A (informative) Grammar Summary
    1.  A.1 Lexical Grammar
    2.  A.2 Number Conversions
    3.  A.3 Expressions
    4.  A.4 Statements
    5.  A.5 Functions and Programs
    6.  A.6 Universal Resource Identifier Character Classes
    7.  A.7 Regular Expressions
    8.  A.8 JSON
        1.  A.8.1 JSON Lexical Grammar
        2.  A.8.2 JSON Syntactic Grammar

20. Annex B (informative) Compatibility
    1.  B.1 Additional Syntax
        1.  B.1.1 Numeric Literals
        2.  B.1.2 String Literals

    2.  B.2 Additional Properties
        1.  B.2.1 escape (string)
        2.  B.2.2 unescape (string)
        3.  B.2.3 String.prototype.substr (start, length)
        4.  B.2.4 Date.prototype.getYear ( )
        5.  B.2.5 Date.prototype.setYear (year)
        6.  B.2.6 Date.prototype.toGMTString ( )

21. Annex C (informative) The Strict Mode of ECMAScript
22. Annex D (informative) Corrections and Clarifications in the 5th Edition with Possible 3rd
    Edition Compatibility Impact
23. Annex E (informative) Additions and Changes in the 5th Edition that Introduce Incompatibilities
    with the 3rd Edition
24. Annex F (informative) Technically Significant Corrections and Clarifications in the 5.1 Edition
25. Bibliography

Introduction
============

This Ecma Standard is based on several originating technologies, the most well known being
JavaScript (Netscape) and JScript (Microsoft). The language was invented by Brendan Eich at Netscape
and first appeared in that company’s Navigator 2.0 browser. It has appeared in all subsequent
browsers from Netscape and in all browsers from Microsoft starting with Internet Explorer 3.0.

The development of this Standard started in November 1996. The first edition of this Ecma Standard
was adopted by the Ecma General Assembly of June 1997.

That Ecma Standard was submitted to ISO/IEC JTC 1 for adoption under the fast-track procedure, and
approved as international standard ISO/IEC 16262, in April 1998. The Ecma General Assembly of June
1998 approved the second edition of ECMA-262 to keep it fully aligned with ISO/IEC 16262. Changes
between the first and the second edition are editorial in nature.

The third edition of the Standard introduced powerful regular expressions, better string handling,
new control statements, try/catch exception handling, tighter definition of errors, formatting for
numeric output and minor changes in anticipation of forthcoming internationalisation facilities and
future language growth. The third edition of the ECMAScript standard was adopted by the Ecma General
Assembly of December 1999 and published as ISO/IEC 16262:2002 in June 2002.

Since publication of the third edition, ECMAScript has achieved massive adoption in conjunction with
the World Wide Web where it has become the programming language that is supported by essentially all
web browsers. Significant work was done to develop a fourth edition of ECMAScript. Although that
work was not completed and not published as the fourth edition of ECMAScript, it informs continuing
evolution of the language. The fifth edition of ECMAScript (published as ECMA-262 5th edition)
codifies de facto interpretations of the language specification that have become common among
browser implementations and adds support for new features that have emerged since the publication of
the third edition. Such features include accessor properties, reflective creation and inspection of
objects, program control of property attributes, additional array manipulation functions, support
for the JSON object encoding format, and a strict mode that provides enhanced error checking and
program security.

This present edition 5.1 of the ECMAScript Standard is fully aligned with third edition of the
international standard ISO/IEC 16262:2011.

ECMAScript is a vibrant language and the evolution of the language is not complete. Significant
technical enhancement will continue with future editions of this specification.

This Ecma Standard has been adopted by the General Assembly of June 2011.

ECMAScript Language Specification

1 Scope
=======

This Standard defines the ECMAScript scripting language.

2 Conformance
=============

A conforming implementation of ECMAScript must provide and support all the types, values, objects,
properties, functions, and program syntax and semantics described in this specification.

A conforming implementation of this Standard shall interpret characters in conformance with the
Unicode Standard, Version 3.0 or later and ISO/IEC 10646-1 with either UCS-2 or UTF-16 as the
adopted encoding form, implementation level 3. If the adopted ISO/IEC 10646-1 subset is not
otherwise specified, it is presumed to be the BMP subset, collection 300. If the adopted encoding
form is not otherwise specified, it presumed to be the UTF-16 encoding form.

A conforming implementation of ECMAScript is permitted to provide additional types, values, objects,
properties, and functions beyond those described in this specification. In particular, a conforming
implementation of ECMAScript is permitted to provide properties not described in this specification,
and values for those properties, for objects that are described in this specification.

A conforming implementation of ECMAScript is permitted to support program and regular expression
syntax not described in this specification. In particular, a conforming implementation of ECMAScript
is permitted to support program syntax that makes use of the “future reserved words” listed in
7.6.1.2 of this specification.

3 Normative references
======================

The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the
referenced document (including any amendments) applies.

ISO/IEC 9899:1996, Programming Languages – C, including amendment 1 and technical corrigenda 1 and 2

ISO/IEC 10646-1:1993, Information Technology – Universal Multiple-Octet Coded Character Set (UCS)
plus its amendments and corrigenda

4 Overview
==========

This section contains a non-normative overview of the ECMAScript language.

ECMAScript is an object-oriented programming language for performing computations and manipulating
computational objects within a host environment. ECMAScript as defined here is not intended to be
computationally self-sufficient; indeed, there are no provisions in this specification for input of
external data or output of computed results. Instead, it is expected that the computational
environment of an ECMAScript program will provide not only the objects and other facilities
described in this specification but also certain environment-specific host objects, whose
description and behaviour are beyond the scope of this specification except to indicate that they
may provide certain properties that can be accessed and certain functions that can be called from an
ECMAScript program.

A scripting language is a programming language that is used to manipulate, customise, and automate
the facilities of an existing system. In such systems, useful functionality is already available
through a user interface, and the scripting language is a mechanism for exposing that functionality
to program control. In this way, the existing system is said to provide a host environment of
objects and facilities, which completes the capabilities of the scripting language. A scripting
language is intended for use by both professional and non-professional programmers.

ECMAScript was originally designed to be a Web scripting language, providing a mechanism to enliven
Web pages in browsers and to perform server computation as part of a Web-based client-server
architecture. ECMAScript can provide core scripting capabilities for a variety of host environments,
and therefore the core scripting language is specified in this document apart from any particular
host environment.

Some of the facilities of ECMAScript are similar to those used in other programming languages; in
particular Java™, Self, and Scheme as described in:

Gosling, James, Bill Joy and Guy Steele. The Java™ Language Specification. Addison Wesley Publishing
Co., 1996.

Ungar, David, and Smith, Randall B. Self: The Power of Simplicity. OOPSLA ’87 Conference
Proceedings, pp. 227–241, Orlando, FL, October 1987.

IEEE Standard for the Scheme Programming Language. IEEE Std 1178-1990.

4.1 Web Scripting
=================

A web browser provides an ECMAScript host environment for client-side computation including, for
instance, objects that represent windows, menus, pop-ups, dialog boxes, text areas, anchors, frames,
history, cookies, and input/output. Further, the host environment provides a means to attach
scripting code to events such as change of focus, page and image loading, unloading, error and
abort, selection, form submission, and mouse actions. Scripting code appears within the HTML and the
displayed page is a combination of user interface elements and fixed and computed text and images.
The scripting code is reactive to user interaction and there is no need for a main program.

A web server provides a different host environment for server-side computation including objects
representing requests, clients, and files; and mechanisms to lock and share data. By using
browser-side and server-side scripting together, it is possible to distribute computation between
the client and server while providing a customised user interface for a Web-based application.

Each Web browser and server that supports ECMAScript supplies its own host environment, completing
the ECMAScript execution environment.

4.2 Language Overview
=====================

The following is an informal overview of ECMAScript—not all parts of the language are described.
This overview is not part of the standard proper.

ECMAScript is object-based: basic language and host facilities are provided by objects, and an
ECMAScript program is a cluster of communicating objects. An ECMAScript object is a collection of
properties each with zero or more attributes that determine how each property can be used—for
example, when the Writable attribute for a property is set to false, any attempt by executed
ECMAScript code to change the value of the property fails. Properties are containers that hold other
objects, primitive values, or functions. A primitive value is a member of one of the following
built-in types: Undefined, Null, Boolean, Number, and String; an object is a member of the remaining
built-in type Object; and a function is a callable object. A function that is associated with an
object via a property is a method.

ECMAScript defines a collection of built-in objects that round out the definition of ECMAScript
entities. These built-in objects include the global object, the Object object, the Function object,
the Array object, the String object, the Boolean object, the Number object, the Math object, the
Date object, the RegExp object, the JSON object, and the Error objects Error, EvalError, RangeError,
ReferenceError, SyntaxError, TypeError and URIError.

ECMAScript also defines a set of built-in operators. ECMAScript operators include various unary
operations, multiplicative operators, additive operators, bitwise shift operators, relational
operators, equality operators, binary bitwise operators, binary logical operators, assignment
operators, and the comma operator.

ECMAScript syntax intentionally resembles Java syntax. ECMAScript syntax is relaxed to enable it to
serve as an easy-to-use scripting language. For example, a variable is not required to have its type
declared nor are types associated with properties, and defined functions are not required to have
their declarations appear textually before calls to them.

4.2.1 Objects
=============

ECMAScript does not use classes such as those in C++, Smalltalk, or Java. Instead objects may be
created in various ways including via a literal notation or via constructors which create objects
and then execute code that initialises all or part of them by assigning initial values to their
properties. Each constructor is a function that has a property named “prototype” that is used to
implement prototype-based inheritance and shared properties. Objects are created by using
constructors in new expressions; for example, new       Date(2009,11) creates a new Date object.
Invoking a constructor without using new has consequences that depend on the constructor. For
example, Date() produces a string representation of the current date and time rather than an object.

Every object created by a constructor has an implicit reference (called the object’s prototype) to
the value of its constructor’s “prototype” property. Furthermore, a prototype may have a non-null
implicit reference to its prototype, and so on; this is called the prototype chain. When a reference
is made to a property in an object, that reference is to the property of that name in the first
object in the prototype chain that contains a property of that name. In other words, first the
object mentioned directly is examined for such a property; if that object contains the named
property, that is the property to which the reference refers; if that object does not contain the
named property, the prototype for that object is examined next; and so on.

[An image of lots of boxes and arrows.]

Figure 1 — Object/Prototype Relationships

In a class-based object-oriented language, in general, state is carried by instances, methods are
carried by classes, and inheritance is only of structure and behaviour. In ECMAScript, the state and
methods are carried by objects, and structure, behaviour, and state are all inherited.

All objects that do not directly contain a particular property that their prototype contains share
that property and its value. Figure 1 illustrates this:

CF is a constructor (and also an object). Five objects have been created by using new expressions:
cf1, cf2, cf3, cf4, and cf5. Each of these objects contains properties named q1 and q2. The dashed
lines represent the implicit prototype relationship; so, for example, cf3’s prototype is CFp. The
constructor, CF, has two properties itself, named P1 and P2, which are not visible to CFp, cf1, cf2,
cf3, cf4, or cf5. The property named CFP1 in CFp is shared by cf1, cf2, cf3, cf4, and cf5 (but not
by CF), as are any properties found in CFp’s implicit prototype chain that are not named q1, q2, or
CFP1. Notice that there is no implicit prototype link between CF and CFp.

Unlike class-based object languages, properties can be added to objects dynamically by assigning
values to them. That is, constructors are not required to name or assign values to all or any of the
constructed object’s properties. In the above diagram, one could add a new shared property for cf1,
cf2, cf3, cf4, and cf5by assigning a new value to the property in CFp.

4.2.2 The Strict Variant of ECMAScript
======================================

The ECMAScript Language recognises the possibility that some users of the language may wish to
restrict their usage of some features available in the language. They might do so in the interests
of security, to avoid what they consider to be error-prone features, to get enhanced error checking,
or for other reasons of their choosing. In support of this possibility, ECMAScript defines a strict
variant of the language. The strict variant of the language excludes some specific syntactic and
semantic features of the regular ECMAScript language and modifies the detailed semantics of some
features. The strict variant also specifies additional error conditions that must be reported by
throwing error exceptions in situations that are not specified as errors by the non-strict form of
the language.

The strict variant of ECMAScript is commonly referred to as the strict mode of the language. Strict
mode selection and use of the strict mode syntax and semantics of ECMAScript is explicitly made at
the level of individual ECMAScript code units. Because strict mode is selected at the level of a
syntactic code unit, strict mode only imposes restrictions that have local effect within such a code
unit. Strict mode does not restrict or modify any aspect of the ECMAScript semantics that must
operate consistently across multiple code units. A complete ECMAScript program may be composed for
both strict mode and non-strict mode ECMAScript code units. In this case, strict mode only applies
when actually executing code that is defined within a strict mode code unit.

In order to conform to this specification, an ECMAScript implementation must implement both the full
unrestricted ECMAScript language and the strict mode variant of the ECMAScript language as defined
by this specification. In addition, an implementation must support the combination of unrestricted
and strict mode code units into a single composite program.

4.3 Terms and definitions
=========================

For the purposes of this document, the following terms and definitions apply.

4.3.1 type
==========

set of data values as defined in Clause 8 of this specification

4.3.2 primitive value
=====================

member of one of the types Undefined, Null, Boolean, Number, or String as defined in Clause 8

NOTE A primitive value is a datum that is represented directly at the lowest level of the language
implementation.

4.3.3 object
============

member of the type Object

NOTE An object is a collection of properties and has a single prototype object. The prototype may be
the null value.

4.3.4 constructor
=================

function object that creates and initialises objects

NOTE The value of a constructor’s “prototype” property is a prototype object that is used to
implement inheritance and shared properties.

4.3.5 prototype
===============

object that provides shared properties for other objects

NOTE When a constructor creates an object, that object implicitly references the constructor’s
“prototype” property for the purpose of resolving property references. The constructor’s “prototype”
property can be referenced by the program expression constructor.prototype, and properties added to
an object’s prototype are shared, through inheritance, by all objects sharing the prototype.
Alternatively, a new object may be created with an explicitly specified prototype by using the
Object.create built-in function.

4.3.6 native object
===================

object in an ECMAScript implementation whose semantics are fully defined by this specification
rather than by the host environment

NOTE Standard native objects are defined in this specification. Some native objects are built-in;
others may be constructed during the course of execution of an ECMAScript program.

4.3.7 built-in object
=====================

object supplied by an ECMAScript implementation, independent of the host environment, that is
present at the start of the execution of an ECMAScript program

NOTE Standard built-in objects are defined in this specification, and an ECMAScript implementation
may specify and define others. Every built-in object is a native object. A built-in constructor is a
built-in object that is also a constructor.

4.3.8 host object
=================

object supplied by the host environment to complete the execution environment of ECMAScript

NOTE Any object that is not native is a host object.

4.3.9 undefined value
=====================

primitive value used when a variable has not been assigned a value

4.3.10 Undefined type
=====================

type whose sole value is the undefined value

4.3.11 null value
=================

primitive value that represents the intentional absence of any object value

4.3.12 Null type
================

type whose sole value is the null value

4.3.13 Boolean value
====================

member of the Boolean type

NOTE There are only two Boolean values, true and false.

4.3.14 Boolean type
===================

type consisting of the primitive values true and false

4.3.15 Boolean object
=====================

member of the Object type that is an instance of the standard built-in Boolean constructor

NOTE A Boolean object is created by using the Boolean constructor in a new expression, supplying a
Boolean value as an argument. The resulting object has an internal property whose value is the
Boolean value. A Boolean object can be coerced to a Boolean value.

4.3.16 String value
===================

primitive value that is a finite ordered sequence of zero or more 16-bit unsigned integer

NOTE A String value is a member of the String type. Each integer value in the sequence usually
represents a single 16-bit unit of UTF-16 text. However, ECMAScript does not place any restrictions
or requirements on the values except that they must be 16-bit unsigned integers.

4.3.17 String type
==================

set of all possible String values

4.3.18 String object
====================

member of the Object type that is an instance of the standard built-in String constructor

NOTE A String object is created by using the String constructor in a new expression, supplying a
String value as an argument. The resulting object has an internal property whose value is the String
value. A String object can be coerced to a String value by calling the String constructor as a
function (15.5.1).

4.3.19 Number value
===================

primitive value corresponding to a double-precision 64-bit binary format IEEE 754 value

NOTE A Number value is a member of the Number type and is a direct representation of a number.

4.3.20 Number type
==================

set of all possible Number values including the special “Not-a-Number” (NaN) values, positive
infinity, and negative infinity

4.3.21 Number object
====================

member of the Object type that is an instance of the standard built-in Number constructor

NOTE A Number object is created by using the Number constructor in a new expression, supplying a
Number value as an argument. The resulting object has an internal property whose value is the Number
value. A Number object can be coerced to a Number value by calling the Number constructor as a
function (15.7.1).

4.3.22 Infinity
===============

number value that is the positive infinite Number value

4.3.23 NaN
==========

number value that is a IEEE 754 “Not-a-Number” value

4.3.24 function
===============

member of the Object type that is an instance of the standard built-in Function constructor and that
may be invoked as a subroutine

NOTE In addition to its named properties, a function contains executable code and state that
determine how it behaves when invoked. A function’s code may or may not be written in ECMAScript.

4.3.25 built-in function
========================

built-in object that is a function

NOTE Examples of built-in functions include parseInt and Math.exp. An implementation may provide
implementation-dependent built-in functions that are not described in this specification.

4.3.26 property
===============

association between a name and a value that is a part of an object

NOTE Depending upon the form of the property the value may be represented either directly as a data
value (a primitive value, an object, or a function object) or indirectly by a pair of accessor
functions.

4.3.27 method
=============

function that is the value of a property

NOTE When a function is called as a method of an object, the object is passed to the function as its
this value.

4.3.28 built-in method
======================

method that is a built-in function

NOTE Standard built-in methods are defined in this specification, and an ECMAScript implementation
may specify and provide other additional built-in methods.

4.3.29 attribute
================

internal value that defines some characteristic of a property

4.3.30 own property
===================

property that is directly contained by its object

4.3.31 inherited property
=========================

property of an object that is not an own property but is a property (either own or inherited) of the
object’s prototype

5 Notational Conventions
========================

5.1 Syntactic and Lexical Grammars
==================================

5.1.1 Context-Free Grammars
===========================

A context-free grammar consists of a number of productions. Each production has an abstract symbol
called a nonterminal as its left-hand side, and a sequence of zero or more nonterminal and terminal
symbols as its right-hand side. For each grammar, the terminal symbols are drawn from a specified
alphabet.

Starting from a sentence consisting of a single distinguished nonterminal, called the goal symbol, a
given context-free grammar specifies a language, namely, the (perhaps infinite) set of possible
sequences of terminal symbols that can result from repeatedly replacing any nonterminal in the
sequence with a right-hand side of a production for which the nonterminal is the left-hand side.

5.1.2 The Lexical and RegExp Grammars
=====================================

A lexical grammar for ECMAScript is given in clause 7. This grammar has as its terminal symbols
characters (Unicode code units) that conform to the rules for SourceCharacter defined in Clause 6.
It defines a set of productions, starting from the goal symbol InputElementDiv or
InputElementRegExp, that describe how sequences of such characters are translated into a sequence of
input elements.

Input elements other than white space and comments form the terminal symbols for the syntactic
grammar for ECMAScript and are called ECMAScript tokens. These tokens are the reserved words,
identifiers, literals, and punctuators of the ECMAScript language. Moreover, line terminators,
although not considered to be tokens, also become part of the stream of input elements and guide the
process of automatic semicolon insertion (7.9). Simple white space and single-line comments are
discarded and do not appear in the stream of input elements for the syntactic grammar. A
MultiLineComment (that is, a comment of the form “/*…*/” regardless of whether it spans more than
one line) is likewise simply discarded if it contains no line terminator; but if a MultiLineComment
contains one or more line terminators, then it is replaced by a single line terminator, which
becomes part of the stream of input elements for the syntactic grammar.

A RegExp grammar for ECMAScript is given in 15.10. This grammar also has as its terminal symbols the
characters as defined by SourceCharacter. It defines a set of productions, starting from the goal
symbol Pattern, that describe how sequences of characters are translated into regular expression
patterns.

Productions of the lexical and RegExp grammars are distinguished by having two colons “::” as
separating punctuation. The lexical and RegExp grammars share some productions.

5.1.3 The Numeric String Grammar
================================

Another grammar is used for translating Strings into numeric values. This grammar is similar to the
part of the lexical grammar having to do with numeric literals and has as its terminal symbols
SourceCharacter. This grammar appears in 9.3.1.

Productions of the numeric string grammar are distinguished by having three colons “:::” as
punctuation.

5.1.4 The Syntactic Grammar
===========================

The syntactic grammar for ECMAScript is given in clauses 11, 12, 13 and 14. This grammar has
ECMAScript tokens defined by the lexical grammar as its terminal symbols (5.1.2). It defines a set
of productions, starting from the goal symbol Program, that describe how sequences of tokens can
form syntactically correct ECMAScript programs.

When a stream of characters is to be parsed as an ECMAScript program, it is first converted to a
stream of input elements by repeated application of the lexical grammar; this stream of input
elements is then parsed by a single application of the syntactic grammar. The program is
syntactically in error if the tokens in the stream of input elements cannot be parsed as a single
instance of the goal nonterminal Program, with no tokens left over.

Productions of the syntactic grammar are distinguished by having just one colon “:” as punctuation.

The syntactic grammar as presented in clauses 11, 12, 13 and 14 is actually not a complete account
of which token sequences are accepted as correct ECMAScript programs. Certain additional token
sequences are also accepted, namely, those that would be described by the grammar if only semicolons
were added to the sequence in certain places (such as before line terminator characters).
Furthermore, certain token sequences that are described by the grammar are not considered acceptable
if a terminator character appears in certain “awkward” places.

5.1.5 The JSON Grammar
======================

The JSON grammar is used to translate a String describing a set of ECMAScript objects into actual
objects. The JSON grammar is given in 15.12.1.

The JSON grammar consists of the JSON lexical grammar and the JSON syntactic grammar. The JSON
lexical grammar is used to translate character sequences into tokens and is similar to parts of the
ECMAScript lexical grammar. The JSON syntactic grammar describes how sequences of tokens from the
JSON lexical grammar can form syntactically correct JSON object descriptions.

Productions of the JSON lexical grammar are distinguished by having two colons “::” as separating
punctuation. The JSON lexical grammar uses some productions from the ECMAScript lexical grammar. The
JSON syntactic grammar is similar to parts of the ECMAScript syntactic grammar. Productions of the
JSON syntactic grammar are distinguished by using one colon “:” as separating punctuation.

5.1.6 Grammar Notation
======================

Terminal symbols of the lexical, RegExp, and numeric string grammars, and some of the terminal
symbols of the other grammars, are shown in fixed width font, both in the productions of the
grammars and throughout this specification whenever the text directly refers to such a terminal
symbol. These are to appear in a program exactly as written. All terminal symbol characters
specified in this way are to be understood as the appropriate Unicode character from the ASCII
range, as opposed to any similar-looking characters from other Unicode ranges.

Nonterminal symbols are shown in italic type. The definition of a nonterminal is introduced by the
name of the nonterminal being defined followed by one or more colons. (The number of colons
indicates to which grammar the production belongs.) One or more alternative right-hand sides for the
nonterminal then follow on succeeding lines. For example, the syntactic definition:

WhileStatement :

while ( Expression ) Statement

states that the nonterminal WhileStatement represents the token while, followed by a left
parenthesis token, followed by an Expression, followed by a right parenthesis token, followed by a
Statement. The occurrences of Expression and Statement are themselves nonterminals. As another
example, the syntactic definition:

ArgumentList :

AssignmentExpression

ArgumentList , AssignmentExpression

states that an ArgumentList may represent either a single AssignmentExpression or an ArgumentList,
followed by a comma, followed by an AssignmentExpression. This definition of ArgumentList is
recursive, that is, it is defined in terms of itself. The result is that an ArgumentList may contain
any positive number of arguments, separated by commas, where each argument expression is an
AssignmentExpression. Such recursive definitions of nonterminals are common.

The subscripted suffix “opt”, which may appear after a terminal or nonterminal, indicates an
optional symbol. The alternative containing the optional symbol actually specifies two right-hand
sides, one that omits the optional element and one that includes it. This means that:

VariableDeclaration :

Identifier Initialiseropt

is a convenient abbreviation for:

VariableDeclaration :

Identifier

Identifier Initialiser

and that:

IterationStatement :

for ( ExpressionNoInopt ; Expressionopt ; Expressionopt ) Statement

is a convenient abbreviation for:

IterationStatement :

for ( ; Expressionopt ; Expressionopt ) Statement

for ( ExpressionNoIn ; Expressionopt ; Expressionopt ) Statement

which in turn is an abbreviation for:

IterationStatement :

for ( ; ; Expressionopt ) Statement

for ( ; Expression ; Expressionopt ) Statement

for ( ExpressionNoIn ; ; Expressionopt ) Statement

for ( ExpressionNoIn ; Expression ; Expressionopt ) Statement

which in turn is an abbreviation for:

IterationStatement :

for ( ; ; ) Statement

for ( ; ; Expression ) Statement

for ( ; Expression ; ) Statement

for ( ; Expression ; Expression ) Statement

for ( ExpressionNoIn ; ; ) Statement

for ( ExpressionNoIn ; ; Expression ) Statement

for ( ExpressionNoIn ; Expression ; ) Statement

for ( ExpressionNoIn ; Expression ; Expression ) Statement

so the nonterminal IterationStatement actually has eight alternative right-hand sides.

When the words “one of” follow the colon(s) in a grammar definition, they signify that each of the
terminal symbols on the following line or lines is an alternative definition. For example, the
lexical grammar for ECMAScript contains the production:

NonZeroDigit :: one of

1 2 3 4 5 6 7 8 9

which is merely a convenient abbreviation for:

NonZeroDigit ::

1

2

3

4

5

6

7

8

9

If the phrase “[empty]” appears as the right-hand side of a production, it indicates that the
production’s right-hand side contains no terminals or nonterminals.

If the phrase “[lookahead ∉ set]” appears in the right-hand side of a production, it indicates that
the production may not be used if the immediately following input token is a member of the given
set. The set can be written as a list of terminals enclosed in curly braces. For convenience, the
set can also be written as a nonterminal, in which case it represents the set of all terminals to
which that nonterminal could expand. For example, given the definitions

DecimalDigit :: one of

0 1 2 3 4 5 6 7 8 9

DecimalDigits ::

DecimalDigit

DecimalDigits DecimalDigit

the definition

LookaheadExample ::

n [lookahead ∉ {1, 3, 5, 7, 9}] DecimalDigits

DecimalDigit [lookahead ∉ DecimalDigit]

matches either the letter n followed by one or more decimal digits the first of which is even, or a
decimal digit not followed by another decimal digit.

If the phrase “[no LineTerminator here]” appears in the right-hand side of a production of the
syntactic grammar, it indicates that the production is a restricted production: it may not be used
if a LineTerminator occurs in the input stream at the indicated position. For example, the
production:

ThrowStatement :

throw [no LineTerminator here] Expression ;

indicates that the production may not be used if a LineTerminator occurs in the program between the
throw token and the Expression.

Unless the presence of a LineTerminator is forbidden by a restricted production, any number of
occurrences of LineTerminator may appear between any two consecutive tokens in the stream of input
elements without affecting the syntactic acceptability of the program.

When an alternative in a production of the lexical grammar or the numeric string grammar appears to
be a multi-character token, it represents the sequence of characters that would make up such a
token.

The right-hand side of a production may specify that certain expansions are not permitted by using
the phrase “but not” and then indicating the expansions to be excluded. For example, the production:

Identifier ::

IdentifierName but not ReservedWord

means that the nonterminal Identifier may be replaced by any sequence of characters that could
replace IdentifierName provided that the same sequence of characters could not replace ReservedWord.

Finally, a few nonterminal symbols are described by a descriptive phrase in sans-serif type in cases
where it would be impractical to list all the alternatives:

SourceCharacter ::

any Unicode code unit

5.2 Algorithm Conventions
=========================

The specification often uses a numbered list to specify steps in an algorithm. These algorithms are
used to precisely specify the required semantics of ECMAScript language constructs. The algorithms
are not intended to imply the use of any specific implementation technique. In practice, there may
be more efficient algorithms available to implement a given feature.

In order to facilitate their use in multiple parts of this specification, some algorithms, called
abstract operations, are named and written in parameterised functional form so that they may be
referenced by name from within other algorithms.

When an algorithm is to produce a value as a result, the directive “return x” is used to indicate
that the result of the algorithm is the value of x and that the algorithm should terminate. The
notation Result(n) is used as shorthand for “the result of step n”.

For clarity of expression, algorithm steps may be subdivided into sequential substeps. Substeps are
indented and may themselves be further divided into indented substeps. Outline numbering conventions
are used to identify substeps with the first level of substeps labelled with lower case alphabetic
characters and the second level of substeps labelled with lower case roman numerals. If more than
three levels are required these rules repeat with the fourth level using numeric labels. For
example:

1.  Top-level step
    1.  Substep.
    2.  Substep
        1.  Subsubstep.
        2.  Subsubstep.
            1.  Subsubsubstep
                1.  Subsubsubsubstep

A step or substep may be written as an “if” predicate that conditions its substeps. In this case,
the substeps are only applied if the predicate is true. If a step or substep begins with the word
“else”, it is a predicate that is the negation of the preceding “if” predicate step at the same
level.

A step may specify the iterative application of its substeps.

A step may assert an invariant condition of its algorithm. Such assertions are used to make explicit
algorithmic invariants that would otherwise be implicit. Such assertions add no additional semantic
requirements and hence need not be checked by an implementation. They are used simply to clarify
algorithms.

Mathematical operations such as addition, subtraction, negation, multiplication, division, and the
mathematical functions defined later in this clause should always be understood as computing exact
mathematical results on mathematical real numbers, which do not include infinities and do not
include a negative zero that is distinguished from positive zero. Algorithms in this standard that
model floating-point arithmetic include explicit steps, where necessary, to handle infinities and
signed zero and to perform rounding. If a mathematical operation or function is applied to a
floating-point number, it should be understood as being applied to the exact mathematical value
represented by that floating-point number; such a floating-point number must be finite, and if it is
+0 or −0 then the corresponding mathematical value is simply 0.

The mathematical function abs(x) yields the absolute value of x, which is −x if x is negative (less
than zero) and otherwise is x itself.

The mathematical function sign(x) yields 1 if x is positive and −1 if x is negative. The sign
function is not used in this standard for cases when x is zero.

The notation “x modulo y” (y must be finite and nonzero) computes a value k of the same sign as y
(or zero) such that abs(k) < abs(y) and x−k = q × y for some integer q.

The mathematical function floor(x) yields the largest integer (closest to positive infinity) that is
not larger than x.

NOTE floor(x) = x−(x modulo 1).

If an algorithm is defined to “throw an exception”, execution of the algorithm is terminated and no
result is returned. The calling algorithms are also terminated, until an algorithm step is reached
that explicitly deals with the exception, using terminology such as “If an exception was thrown…”.
Once such an algorithm step has been encountered the exception is no longer considered to have
occurred.

6 Source Text
=============

ECMAScript source text is represented as a sequence of characters in the Unicode character encoding,
version 3.0 or later. The text is expected to have been normalised to Unicode Normalization Form C
(canonical composition), as described in Unicode Technical Report #15. Conforming ECMAScript
implementations are not required to perform any normalisation of text, or behave as though they were
performing normalisation of text, themselves. ECMAScript source text is assumed to be a sequence of
16-bit code units for the purposes of this specification. Such a source text may include sequences
of 16-bit code units that are not valid UTF-16 character encodings. If an actual source text is
encoded in a form other than 16-bit code units it must be processed as if it was first converted to
UTF-16.

Syntax
------

SourceCharacter ::

any Unicode code unit

Throughout the rest of this document, the phrase “code unit” and the word “character” will be used
to refer to a 16-bit unsigned value used to represent a single 16-bit unit of text. The phrase
“Unicode character” will be used to refer to the abstract linguistic or typographical unit
represented by a single Unicode scalar value (which may be longer than 16 bits and thus may be
represented by more than one code unit). The phrase “code point” refers to such a Unicode scalar
value. “Unicode character” only refers to entities represented by single Unicode scalar values: the
components of a combining character sequence are still individual “Unicode characters,” even though
a user might think of the whole sequence as a single character.

In string literals, regular expression literals, and identifiers, any character (code unit) may also
be expressed as a Unicode escape sequence consisting of six characters, namely \u plus four
hexadecimal digits. Within a comment, such an escape sequence is effectively ignored as part of the
comment. Within a string literal or regular expression literal, the Unicode escape sequence
contributes one character to the value of the literal. Within an identifier, the escape sequence
contributes one character to the identifier.

NOTE Although this document sometimes refers to a “transformation” between a “character” within a
“string” and the 16-bit unsigned integer that is the code unit of that character, there is actually
no transformation because a “character” within a “string” is actually represented using that 16-bit
unsigned value.

ECMAScript differs from the Java programming language in the behaviour of Unicode escape sequences.
In a Java program, if the Unicode escape sequence \u000A, for example, occurs within a single-line
comment, it is interpreted as a line terminator (Unicode character 000A is line feed) and therefore
the next character is not part of the comment. Similarly, if the Unicode escape sequence \u000A
occurs within a string literal in a Java program, it is likewise interpreted as a line terminator,
which is not allowed within a string literal—one must write \n instead of \u000A to cause a line
feed to be part of the string value of a string literal. In an ECMAScript program, a Unicode escape
sequence occurring within a comment is never interpreted and therefore cannot contribute to
termination of the comment. Similarly, a Unicode escape sequence occurring within a string literal
in an ECMAScript program always contributes a character to the String value of the literal and is
never interpreted as a line terminator or as a quote mark that might terminate the string literal.

7 Lexical Conventions
=====================

The source text of an ECMAScript program is first converted into a sequence of input elements, which
are tokens, line terminators, comments, or white space. The source text is scanned from left to
right, repeatedly taking the longest possible sequence of characters as the next input element.

There are two goal symbols for the lexical grammar. The InputElementDiv symbol is used in those
syntactic grammar contexts where a leading division (/) or division-assignment (/=) operator is
permitted. The InputElementRegExp symbol is used in other syntactic grammar contexts.

NOTE There are no syntactic grammar contexts where both a leading division or division-assignment,
and a leading RegularExpressionLiteral are permitted. This is not affected by semicolon insertion
(see 7.9); in examples such as the following:

    a = b/hi/g.exec(c).map(d);

where the first non-whitespace, non-comment character after a LineTerminator is slash (/) and the
syntactic context allows division or division-assignment, no semicolon is inserted at the
LineTerminator. That is, the above example is interpreted in the same way as:

    a = b / hi / g.exec(c).map(d);

Syntax
------

InputElementDiv ::

WhiteSpace

LineTerminator

Comment

Token

DivPunctuator

InputElementRegExp ::

WhiteSpace

LineTerminator

Comment

Token

RegularExpressionLiteral

7.1 Unicode Format-Control Characters
=====================================

The Unicode format-control characters (i.e., the characters in category “Cf” in the Unicode
Character Database such as left-to-right mark or right-to-left mark) are control codes used to
control the formatting of a range of text in the absence of higher-level protocols for this (such as
mark-up languages).

It is useful to allow format-control characters in source text to facilitate editing and display.
All format control characters may be used within comments, and within string literals and regular
expression literals.

<ZWNJ> and <ZWJ> are format-control characters that are used to make necessary distinctions when
forming words or phrases in certain languages. In ECMAScript source text, <ZWNJ> and <ZWJ> may also
be used in an identifier after the first character.

<BOM> is a format-control character used primarily at the start of a text to mark it as Unicode and
to allow detection of the text’s encoding and byte order. <BOM> characters intended for this purpose
can sometimes also appear after the start of a text, for example as a result of concatenating files.
<BOM> characters are treated as white space characters (see 7.2).

The special treatment of certain format-control characters outside of comments, string literals, and
regular expression literals is summarised in Table 1.

Table 1 — Format-Control Character Usage

  Code Unit Value   Name                    Formal Name   Usage
  ----------------- ----------------------- ------------- ----------------
  \u200C            Zero width non-joiner   <ZWNJ>        IdentifierPart
  \u200D            Zero width joiner       <ZWJ>         IdentifierPart
  \uFEFF            Byte Order Mark         <BOM>         Whitespace

7.2 White Space
===============

White space characters are used to improve source text readability and to separate tokens
(indivisible lexical units) from each other, but are otherwise insignificant. White space characters
may occur between any two tokens and at the start or end of input. White space characters may also
occur within a StringLiteral or a RegularExpressionLiteral (where they are considered significant
characters forming part of the literal value) or within a Comment, but cannot appear within any
other kind of token.

The ECMAScript white space characters are listed in Table 2.

Table 2 — Whitespace Characters

+-----------------------------------+-----------------------------------+-----------------------------------+
| Code Unit Value                   | Name                              | Formal Name                       |
+===================================+===================================+===================================+
| \u0009                            | Tab                               | <TAB>                             |
+-----------------------------------+-----------------------------------+-----------------------------------+
| \u000B                            | Vertical Tab                      | <VT>                              |
+-----------------------------------+-----------------------------------+-----------------------------------+
| \u000C                            | Form Feed                         | <FF>                              |
+-----------------------------------+-----------------------------------+-----------------------------------+
| \u0020                            | Space                             | <SP>                              |
+-----------------------------------+-----------------------------------+-----------------------------------+
| \u00A0                            | No-break space                    | <NBSP>                            |
+-----------------------------------+-----------------------------------+-----------------------------------+
|     \uFEFF                        | Byte Order Mark                   | <BOM>                             |
|                                   |                                   |                                   |
| Other category “Zs”               | Any other Unicode “space          | <USP>                             |
|                                   | separator”                        |                                   |
+-----------------------------------+-----------------------------------+-----------------------------------+

ECMAScript implementations must recognise all of the white space characters defined in Unicode 3.0.
Later editions of the Unicode Standard may define other white space characters. ECMAScript
implementations may recognise white space characters from later editions of the Unicode Standard.

Syntax
------

WhiteSpace ::

<TAB>

<VT>

<FF>

<SP>

<NBSP>

<BOM>

<USP>

7.3 Line Terminators
====================

Like white space characters, line terminator characters are used to improve source text readability
and to separate tokens (indivisible lexical units) from each other. However, unlike white space
characters, line terminators have some influence over the behaviour of the syntactic grammar. In
general, line terminators may occur between any two tokens, but there are a few places where they
are forbidden by the syntactic grammar. Line terminators also affect the process of automatic
semicolon insertion (7.9). A line terminator cannot occur within any token except a StringLiteral.
Line terminators may only occur within a StringLiteral token as part of a LineContinuation.

A line terminator can occur within a MultiLineComment (7.4) but cannot occur within a
SingleLineComment.

Line terminators are included in the set of white space characters that are matched by the \s class
in regular expressions.

The ECMAScript line terminator characters are listed in Table 3.

Table 3 — Line Terminator Characters

  Code Unit Value   Name                  Formal Name
  ----------------- --------------------- -------------
  \u000A            Line Feed             <LF>
  \u000D            Carriage Return       <CR>
  \u2028            Line separator        <LS>
  \u2029            Paragraph separator   <PS>

Only the characters in Table 3 are treated as line terminators. Other new line or line breaking
characters are treated as white space but not as line terminators. The character sequence <CR><LF>
is commonly used as a line terminator. It should be considered a single character for the purpose of
reporting line numbers.

Syntax
------

LineTerminator ::

<LF>

<CR>

<LS>

<PS>

LineTerminatorSequence ::

<LF>

<CR> [lookahead ∉ <LF> ]

<LS>

<PS>

<CR> <LF>

7.4 Comments
============

Comments can be either single or multi-line. Multi-line comments cannot nest.

Because a single-line comment can contain any character except a LineTerminator character, and
because of the general rule that a token is always as long as possible, a single-line comment always
consists of all characters from the // marker to the end of the line. However, the LineTerminator at
the end of the line is not considered to be part of the single-line comment; it is recognised
separately by the lexical grammar and becomes part of the stream of input elements for the syntactic
grammar. This point is very important, because it implies that the presence or absence of
single-line comments does not affect the process of automatic semicolon insertion (see 7.9).

Comments behave like white space and are discarded except that, if a MultiLineComment contains a
line terminator character, then the entire comment is considered to be a LineTerminator for purposes
of parsing by the syntactic grammar.

Syntax
------

Comment ::

MultiLineComment

SingleLineComment

MultiLineComment ::

/* MultiLineCommentCharsopt */

MultiLineCommentChars ::

MultiLineNotAsteriskChar MultiLineCommentCharsopt

* PostAsteriskCommentCharsopt

PostAsteriskCommentChars ::

MultiLineNotForwardSlashOrAsteriskChar MultiLineCommentCharsopt

* PostAsteriskCommentCharsopt

MultiLineNotAsteriskChar ::

SourceCharacter but not *

MultiLineNotForwardSlashOrAsteriskChar ::

SourceCharacter but not one of / or *

SingleLineComment ::

// SingleLineCommentCharsopt

SingleLineCommentChars ::

SingleLineCommentChar SingleLineCommentCharsopt

SingleLineCommentChar ::

SourceCharacter but not LineTerminator

7.5 Tokens
==========

Syntax
------

Token ::

IdentifierName

Punctuator

NumericLiteral

StringLiteral

NOTE The DivPunctuator and RegularExpressionLiteral productions define tokens, but are not included
in the Token production.

7.6 Identifier Names and Identifiers
====================================

Identifier Names are tokens that are interpreted according to the grammar given in the “Identifiers”
section of chapter 5 of the Unicode standard, with some small modifications. An Identifier is an
IdentifierName that is not a ReservedWord (see 7.6.1). The Unicode identifier grammar is based on
both normative and informative character categories specified by the Unicode Standard. The
characters in the specified categories in version 3.0 of the Unicode standard must be treated as in
those categories by all conforming ECMAScript implementations.

This standard specifies specific character additions: The dollar sign ($) and the underscore (_) are
permitted anywhere in an IdentifierName.

Unicode escape sequences are also permitted in an IdentifierName, where they contribute a single
character to the IdentifierName, as computed by the CV of the UnicodeEscapeSequence (see 7.8.4). The
\ preceding the UnicodeEscapeSequence does not contribute a character to the IdentifierName. A
UnicodeEscapeSequence cannot be used to put a character into an IdentifierName that would otherwise
be illegal. In other words, if a \ UnicodeEscapeSequence sequence were replaced by its
UnicodeEscapeSequence’s CV, the result must still be a valid IdentifierName that has the exact same
sequence of characters as the original IdentifierName. All interpretations of identifiers within
this specification are based upon their actual characters regardless of whether or not an escape
sequence was used to contribute any particular characters.

Two IdentifierName that are canonically equivalent according to the Unicode standard are not equal
unless they are represented by the exact same sequence of code units (in other words, conforming
ECMAScript implementations are only required to do bitwise comparison on IdentifierName values). The
intent is that the incoming source text has been converted to normalised form C before it reaches
the compiler.

ECMAScript implementations may recognise identifier characters defined in later editions of the
Unicode Standard. If portability is a concern, programmers should only employ identifier characters
defined in Unicode 3.0.

Syntax
------

Identifier ::

IdentifierName but not ReservedWord

IdentifierName ::

IdentifierStart

IdentifierName IdentifierPart

IdentifierStart ::

UnicodeLetter

$

_

\ UnicodeEscapeSequence

IdentifierPart ::

IdentifierStart

UnicodeCombiningMark

UnicodeDigit

UnicodeConnectorPunctuation

<ZWNJ>

<ZWJ>

UnicodeLetter ::

any character in the Unicode categories “Uppercase letter (Lu)”, “Lowercase letter (Ll)”, “Titlecase
letter (Lt)”, “Modifier letter (Lm)”, “Other letter (Lo)”, or “Letter number (Nl)”.

UnicodeCombiningMark ::

any character in the Unicode categories “Non-spacing mark (Mn)” or “Combining spacing mark (Mc)”

UnicodeDigit ::

any character in the Unicode category “Decimal number (Nd)”

UnicodeConnectorPunctuation ::

any character in the Unicode category “Connector punctuation (Pc)”

The definitions of the nonterminal UnicodeEscapeSequence is given in 7.8.4

7.6.1 Reserved Words
====================

A reserved word is an IdentifierName that cannot be used as an Identifier.

Syntax
------

ReservedWord ::

Keyword

FutureReservedWord

NullLiteral

BooleanLiteral

7.6.1.1 Keywords
================

The following tokens are ECMAScript keywords and may not be used as Identifiers in ECMAScript
programs.

Syntax
------

Keyword :: one of

  ---------- ---------- ------------ --------
  break      do         instanceof   typeof
  case       else       new          var
  catch      finally    return       void
  continue   for        switch       while
  debugger   function   this         with
  default    if         throw        
  delete     in         try          
  ---------- ---------- ------------ --------

7.6.1.2 Future Reserved Words
=============================

The following words are used as keywords in proposed extensions and are therefore reserved to allow
for the possibility of future adoption of those extensions.

Syntax
------

FutureReservedWord :: one of

  ------- -------- --------- -------
  class   enum     extends   super
  const   export   import    
  ------- -------- --------- -------

The following tokens are also considered to be FutureReservedWords when they occur within strict
mode code (see 10.1.1). The occurrence of any of these tokens within strict mode code in any context
where the occurrence of a FutureReservedWord would produce an error must also produce an equivalent
error:

  ------------ --------- ----------- -------- -------
  implements   let       private     public   yield
  interface    package   protected   static   
  ------------ --------- ----------- -------- -------

7.7 Punctuators
===============

Syntax
------

Punctuator :: one of

  ----- ------ ----- ----- ----- -----
  {     }      (     )     [     ]
  .     ;      ,     <     >     <=
  >=    ==     !=    ===   !==   
  +     -      *     %     ++    –
  <<    >>     >>>   &     |     ^
  !     ~      &&    ||    ?     :
  =     +=     -=    *=    %=    <<=
  >>=   >>>=   &=    |=    ^=    
  ----- ------ ----- ----- ----- -----

DivPunctuator :: one of

  --- ---- -- -- -- --
  /   /=            
  --- ---- -- -- -- --

7.8 Literals
============

Syntax
------

Literal ::

NullLiteral

BooleanLiteral

NumericLiteral

StringLiteral

RegularExpressionLiteral

7.8.1 Null Literals
===================

Syntax
------

NullLiteral ::

null

Semantics
---------

The value of the null literal null is the sole value of the Null type, namely null.

7.8.2 Boolean Literals
======================

Syntax
------

BooleanLiteral ::

true

false

Semantics
---------

The value of the Boolean literal true is a value of the Boolean type, namely true.

The value of the Boolean literal false is a value of the Boolean type, namely false.

7.8.3 Numeric Literals
======================

Syntax
------

NumericLiteral ::

DecimalLiteral

HexIntegerLiteral

DecimalLiteral ::

DecimalIntegerLiteral . DecimalDigitsopt ExponentPartopt

. DecimalDigits ExponentPartopt

DecimalIntegerLiteral ExponentPartopt

DecimalIntegerLiteral ::

0

NonZeroDigit DecimalDigitsopt

DecimalDigits ::

DecimalDigit

DecimalDigits DecimalDigit

DecimalDigit :: one of

0 1 2 3 4 5 6 7 8 9

NonZeroDigit :: one of

1 2 3 4 5 6 7 8 9

ExponentPart ::

ExponentIndicator SignedInteger

ExponentIndicator :: one of

e E

SignedInteger ::

DecimalDigits

+ DecimalDigits

- DecimalDigits

HexIntegerLiteral ::

0x HexDigit

0X HexDigit

HexIntegerLiteral HexDigit

HexDigit :: one of

0 1 2 3 4 5 6 7 8 9 a b c d e f A B C D E F

The source character immediately following a NumericLiteral must not be an IdentifierStart or
DecimalDigit.

NOTE For example:

    3in

is an error and not the two input elements 3 and in.

Semantics
---------

A numeric literal stands for a value of the Number type. This value is determined in two steps:
first, a mathematical value (MV) is derived from the literal; second, this mathematical value is
rounded as described below.

-   The MV of NumericLiteral :: DecimalLiteral is the MV of DecimalLiteral.

-   The MV of NumericLiteral :: HexIntegerLiteral is the MV of HexIntegerLiteral.

-   The MV of DecimalLiteral :: DecimalIntegerLiteral . is the MV of DecimalIntegerLiteral.

-   The MV of DecimalLiteral :: DecimalIntegerLiteral . DecimalDigits is the MV of
    DecimalIntegerLiteral plus (the MV of DecimalDigits times 10–n), where n is the number of
    characters in DecimalDigits.

-   The MV of DecimalLiteral :: DecimalIntegerLiteral . ExponentPart is the MV of
    DecimalIntegerLiteral times 10e, where e is the MV of ExponentPart.

-   The MV of DecimalLiteral :: DecimalIntegerLiteral . DecimalDigits ExponentPart is (the MV of
    DecimalIntegerLiteral plus (the MV of DecimalDigits times 10–n)) times 10e, where n is the
    number of characters in DecimalDigits and e is the MV of ExponentPart.

-   The MV of DecimalLiteral :: . DecimalDigits is the MV of DecimalDigits times 10–n, where n is
    the number of characters in DecimalDigits.

-   The MV of DecimalLiteral :: . DecimalDigits ExponentPart is the MV of DecimalDigits times 10e–n,
    where n is the number of characters in DecimalDigits and e is the MV of ExponentPart.

-   The MV of DecimalLiteral :: DecimalIntegerLiteral is the MV of DecimalIntegerLiteral.

-   The MV of DecimalLiteral :: DecimalIntegerLiteral ExponentPart is the MV of
    DecimalIntegerLiteral times 10e, where e is the MV of ExponentPart.

-   The MV of DecimalIntegerLiteral :: 0 is 0.

-   The MV of DecimalIntegerLiteral :: NonZeroDigit is the MV of NonZeroDigit.

-   The MV of DecimalIntegerLiteral :: NonZeroDigit DecimalDigits is (the MV of NonZeroDigit times
    10n) plus the MV of DecimalDigits, where n is the number of characters in DecimalDigits.

-   The MV of DecimalDigits :: DecimalDigit is the MV of DecimalDigit.

-   The MV of DecimalDigits :: DecimalDigits DecimalDigit is (the MV of DecimalDigits times 10) plus
    the MV of DecimalDigit.

-   The MV of ExponentPart :: ExponentIndicator SignedInteger is the MV of SignedInteger.

-   The MV of SignedInteger :: DecimalDigits is the MV of DecimalDigits.

-   The MV of SignedInteger :: + DecimalDigits is the MV of DecimalDigits.

-   The MV of SignedInteger :: - DecimalDigits is the negative of the MV of DecimalDigits.

-   The MV of DecimalDigit :: 0 or of HexDigit :: 0 is 0.

-   The MV of DecimalDigit :: 1 or of NonZeroDigit :: 1 or of HexDigit :: 1 is 1.

-   The MV of DecimalDigit :: 2 or of NonZeroDigit :: 2 or of HexDigit :: 2 is 2.

-   The MV of DecimalDigit :: 3 or of NonZeroDigit :: 3 or of HexDigit :: 3 is 3.

-   The MV of DecimalDigit :: 4 or of NonZeroDigit :: 4 or of HexDigit :: 4 is 4.

-   The MV of DecimalDigit :: 5 or of NonZeroDigit :: 5 or of HexDigit :: 5 is 5.

-   The MV of DecimalDigit :: 6 or of NonZeroDigit :: 6 or of HexDigit :: 6 is 6.

-   The MV of DecimalDigit :: 7 or of NonZeroDigit :: 7 or of HexDigit :: 7 is 7.

-   The MV of DecimalDigit :: 8 or of NonZeroDigit :: 8 or of HexDigit :: 8 is 8.

-   The MV of DecimalDigit :: 9 or of NonZeroDigit :: 9 or of HexDigit :: 9 is 9.

-   The MV of HexDigit :: a or of HexDigit :: A is 10.

-   The MV of HexDigit :: b or of HexDigit :: B is 11.

-   The MV of HexDigit :: c or of HexDigit :: C is 12.

-   The MV of HexDigit :: d or of HexDigit :: D is 13.

-   The MV of HexDigit :: e or of HexDigit :: E is 14.

-   The MV of HexDigit :: f or of HexDigit :: F is 15.

-   The MV of HexIntegerLiteral :: 0x HexDigit is the MV of HexDigit.

-   The MV of HexIntegerLiteral :: 0X HexDigit is the MV of HexDigit.

-   The MV of HexIntegerLiteral :: HexIntegerLiteral HexDigit is (the MV of HexIntegerLiteral times
    16) plus the MV of HexDigit.

Once the exact MV for a numeric literal has been determined, it is then rounded to a value of the
Number type. If the MV is 0, then the rounded value is +0; otherwise, the rounded value must be the
Number value for the MV (as specified in 8.5), unless the literal is a DecimalLiteral and the
literal has more than 20 significant digits, in which case the Number value may be either the Number
value for the MV of a literal produced by replacing each significant digit after the 20th with a 0
digit or the Number value for the MV of a literal produced by replacing each significant digit after
the 20th with a 0 digit and then incrementing the literal at the 20th significant digit position. A
digit is significant if it is not part of an ExponentPart and

-   it is not 0; or
-   there is a nonzero digit to its left and there is a nonzero digit, not in the ExponentPart, to
    its right.

A conforming implementation, when processing strict mode code (see 10.1.1), must not extend the
syntax of NumericLiteral to include OctalIntegerLiteral as described in B.1.1.

7.8.4 String Literals
=====================

A string literal is zero or more characters enclosed in single or double quotes. Each character may
be represented by an escape sequence. All characters may appear literally in a string literal except
for the closing quote character, backslash, carriage return, line separator, paragraph separator,
and line feed. Any character may appear in the form of an escape sequence.

Syntax
------

StringLiteral ::

" DoubleStringCharactersopt "

' SingleStringCharactersopt '

DoubleStringCharacters ::

DoubleStringCharacter DoubleStringCharactersopt

SingleStringCharacters ::

SingleStringCharacter SingleStringCharactersopt

DoubleStringCharacter ::

SourceCharacter but not one of " or \ or LineTerminator

\ EscapeSequence

LineContinuation

SingleStringCharacter ::

SourceCharacter but not one of ' or \ or LineTerminator

\ EscapeSequence

LineContinuation

LineContinuation ::

\ LineTerminatorSequence

EscapeSequence ::

CharacterEscapeSequence

0 [lookahead ∉ DecimalDigit]

HexEscapeSequence

UnicodeEscapeSequence

CharacterEscapeSequence ::

SingleEscapeCharacter

NonEscapeCharacter

SingleEscapeCharacter :: one of

' " \ b f n r t v

NonEscapeCharacter ::

SourceCharacter but not one of EscapeCharacter or LineTerminator

EscapeCharacter ::

SingleEscapeCharacter

DecimalDigit

x

u

HexEscapeSequence ::

x HexDigit HexDigit

UnicodeEscapeSequence ::

u HexDigit HexDigit HexDigit HexDigit

The definition of the nonterminal HexDigit is given in 7.8.3. SourceCharacter is defined in clause
6.

Semantics
---------

A string literal stands for a value of the String type. The String value (SV) of the literal is
described in terms of character values (CV) contributed by the various parts of the string literal.
As part of this process, some characters within the string literal are interpreted as having a
mathematical value (MV), as described below or in 7.8.3.

-   The SV of StringLiteral :: "" is the empty character sequence.

-   The SV of StringLiteral :: '' is the empty character sequence.

-   The SV of StringLiteral :: " DoubleStringCharacters " is the SV of DoubleStringCharacters.

-   The SV of StringLiteral :: ' SingleStringCharacters ' is the SV of SingleStringCharacters.

-   The SV of DoubleStringCharacters :: DoubleStringCharacter is a sequence of one character, the CV
    of DoubleStringCharacter.

-   The SV of DoubleStringCharacters :: DoubleStringCharacter DoubleStringCharacters is a sequence
    of the CV of DoubleStringCharacter followed by all the characters in the SV of
    DoubleStringCharacters in order.

-   The SV of SingleStringCharacters :: SingleStringCharacter is a sequence of one character, the CV
    of SingleStringCharacter.

-   The SV of SingleStringCharacters :: SingleStringCharacter SingleStringCharacters is a sequence
    of the CV of SingleStringCharacter followed by all the characters in the SV of
    SingleStringCharacters in order.

-   The SV of LineContinuation :: \ LineTerminatorSequence is the empty character sequence.

-   The CV of DoubleStringCharacter :: SourceCharacter but not one of " or \ or LineTerminator is
    the SourceCharacter character itself.

-   The CV of DoubleStringCharacter :: \ EscapeSequence is the CV of the EscapeSequence.

-   The CV of DoubleStringCharacter :: LineContinuation is the empty character sequence.

-   The CV of SingleStringCharacter :: SourceCharacter but not one of ' or \ or LineTerminator is
    the SourceCharacter character itself.

-   The CV of SingleStringCharacter :: \ EscapeSequence is the CV of the EscapeSequence.

-   The CV of SingleStringCharacter :: LineContinuation is the empty character sequence.

-   The CV of EscapeSequence :: CharacterEscapeSequence is the CV of the CharacterEscapeSequence.

-   The CV of EscapeSequence :: 0 [lookahead ∉ DecimalDigit] is a <NUL> character (Unicode value
    0000).

-   The CV of EscapeSequence :: HexEscapeSequence is the CV of the HexEscapeSequence.

-   The CV of EscapeSequence :: UnicodeEscapeSequence is the CV of the UnicodeEscapeSequence.

-   The CV of CharacterEscapeSequence :: SingleEscapeCharacter is the character whose code unit
    value is determined by the SingleEscapeCharacter according to Table 4:

Table 4 — String Single Character Escape Sequences

  Escape Sequence   Code Unit Value   Name                   Symbol
  ----------------- ----------------- ---------------------- --------
  \b                \u0008            backspace              <BS>
  \t                \u0009            horizontal tab         <HT>
  \n                \u000A            line feed (new line)   <LF>
  \v                \u000B            vertical tab           <VT>
  \f                \u000C            form feed              <FF>
  \r                \u000D            carriage return        <CR>
  \"                \u0022            double quote           "
  \'                \u0027            single quote           '
  \\                \u005C            backslash              \

-   The CV of CharacterEscapeSequence :: NonEscapeCharacter is the CV of the NonEscapeCharacter.

-   The CV of NonEscapeCharacter :: SourceCharacter but not one of EscapeCharacter or LineTerminator
    is the SourceCharacter character itself.

-   The CV of HexEscapeSequence :: x HexDigit HexDigit is the character whose code unit value is (16
    times the MV of the first HexDigit) plus the MV of the second HexDigit.

-   The CV of UnicodeEscapeSequence :: u HexDigit HexDigit HexDigit HexDigit is the character whose
    code unit value is (4096 times the MV of the first HexDigit) plus (256 times the MV of the
    second HexDigit) plus (16 times the MV of the third HexDigit) plus the MV of the fourth
    HexDigit.

A conforming implementation, when processing strict mode code (see 10.1.1), may not extend the
syntax of EscapeSequence to include OctalEscapeSequence as described in B.1.2.

NOTE A line terminator character cannot appear in a string literal, except as part of a
LineContinuation to produce the empty character sequence. The correct way to cause a line terminator
character to be part of the String value of a string literal is to use an escape sequence such as \n
or \u000A.

7.8.5 Regular Expression Literals
=================================

A regular expression literal is an input element that is converted to a RegExp object (see 15.10)
each time the literal is evaluated. Two regular expression literals in a program evaluate to regular
expression objects that never compare as === to each other even if the two literals’ contents are
identical. A RegExp object may also be created at runtime by new RegExp (see 15.10.4) or calling the
RegExp constructor as a function (15.10.3).

The productions below describe the syntax for a regular expression literal and are used by the input
element scanner to find the end of the regular expression literal. The Strings of characters
comprising the RegularExpressionBody and the RegularExpressionFlags are passed uninterpreted to the
regular expression constructor, which interprets them according to its own, more stringent grammar.
An implementation may extend the regular expression constructor’s grammar, but it must not extend
the RegularExpressionBody and RegularExpressionFlags productions or the productions used by these
productions.

Syntax
------

RegularExpressionLiteral ::

/ RegularExpressionBody / RegularExpressionFlags

RegularExpressionBody ::

RegularExpressionFirstChar RegularExpressionChars

RegularExpressionChars ::

[empty]

RegularExpressionChars RegularExpressionChar

RegularExpressionFirstChar ::

RegularExpressionNonTerminator but not one of * or \ or / or [

RegularExpressionBackslashSequence

RegularExpressionClass

RegularExpressionChar ::

RegularExpressionNonTerminator but not one of \ or / or [

RegularExpressionBackslashSequence

RegularExpressionClass

RegularExpressionBackslashSequence ::

\ RegularExpressionNonTerminator

RegularExpressionNonTerminator ::

SourceCharacter but not LineTerminator

RegularExpressionClass ::

[ RegularExpressionClassChars ]

RegularExpressionClassChars ::

[empty]

RegularExpressionClassChars RegularExpressionClassChar

RegularExpressionClassChar ::

RegularExpressionNonTerminator but not one of ] or \

RegularExpressionBackslashSequence

RegularExpressionFlags ::

[empty]

RegularExpressionFlags IdentifierPart

NOTE Regular expression literals may not be empty; instead of representing an empty regular
expression literal, the characters // start a single-line comment. To specify an empty regular
expression, use: /(?:)/.

Semantics
---------

A regular expression literal evaluates to a value of the Object type that is an instance of the
standard built-in constructor RegExp. This value is determined in two steps: first, the characters
comprising the regular expression’s RegularExpressionBody and RegularExpressionFlags production
expansions are collected uninterpreted into two Strings Pattern and Flags, respectively. Then each
time the literal is evaluated, a new object is created as if by the expression new RegExp(Pattern,
Flags) where RegExp is the standard built-in constructor with that name. The newly constructed
object becomes the value of the RegularExpressionLiteral. If the call to new RegExp would generate
an error as specified in 15.10.4.1, the error must be treated as an early error (Clause 16).

7.9 Automatic Semicolon Insertion
=================================

Certain ECMAScript statements (empty statement, variable statement, expression statement, do-while
statement, continue statement, break statement, return statement, and throw statement) must be
terminated with semicolons. Such semicolons may always appear explicitly in the source text. For
convenience, however, such semicolons may be omitted from the source text in certain situations.
These situations are described by saying that semicolons are automatically inserted into the source
code token stream in those situations.

7.9.1 Rules of Automatic Semicolon Insertion
============================================

There are three basic rules of semicolon insertion:

1.  When, as the program is parsed from left to right, a token (called the offending token) is
    encountered that is not allowed by any production of the grammar, then a semicolon is
    automatically inserted before the offending token if one or more of the following conditions is
    true:
    -   The offending token is separated from the previous token by at least one LineTerminator.
    -   The offending token is }.

2.  When, as the program is parsed from left to right, the end of the input stream of tokens is
    encountered and the parser is unable to parse the input token stream as a single complete
    ECMAScript Program, then a semicolon is automatically inserted at the end of the input stream.
3.  When, as the program is parsed from left to right, a token is encountered that is allowed by
    some production of the grammar, but the production is a restricted production and the token
    would be the first token for a terminal or nonterminal immediately following the annotation “[no
    LineTerminator here]” within the restricted production (and therefore such a token is called a
    restricted token), and the restricted token is separated from the previous token by at least one
    LineTerminator, then a semicolon is automatically inserted before the restricted token.

However, there is an additional overriding condition on the preceding rules: a semicolon is never
inserted automatically if the semicolon would then be parsed as an empty statement or if that
semicolon would become one of the two semicolons in the header of a for statement (see 12.6.3).

NOTE The following are the only restricted productions in the grammar:

PostfixExpression :

LeftHandSideExpression [no LineTerminator here] ++

LeftHandSideExpression [no LineTerminator here] --

ContinueStatement :

continue [no LineTerminator here] Identifier ;

BreakStatement :

break [no LineTerminator here] Identifier ;

ReturnStatement :

return [no LineTerminator here] Expression ;

ThrowStatement :

throw [no LineTerminator here] Expression ;

The practical effect of these restricted productions is as follows:

When a ++ or -- token is encountered where the parser would treat it as a postfix operator, and at
least one LineTerminator occurred between the preceding token and the ++ or -- token, then a
semicolon is automatically inserted before the ++ or -- token.

When a continue, break, return, or throw token is encountered and a LineTerminator is encountered
before the next token, a semicolon is automatically inserted after the continue, break, return, or
throw token.

The resulting practical advice to ECMAScript programmers is:

A postfix ++ or -- operator should appear on the same line as its operand.

An Expression in a return or throw statement should start on the same line as the return or throw
token.

An Identifier in a break or continue statement should be on the same line as the break or continue
token.

7.9.2 Examples of Automatic Semicolon Insertion
===============================================

The source

    { 1 2 } 3

is not a valid sentence in the ECMAScript grammar, even with the automatic semicolon insertion
rules. In contrast, the source

    { 12 } 3

is also not a valid ECMAScript sentence, but is transformed by automatic semicolon insertion into
the following:

    { 1;2 ;} 3;

which is a valid ECMAScript sentence.

The source

    for (a; b)

is not a valid ECMAScript sentence and is not altered by automatic semicolon insertion because the
semicolon is needed for the header of a for statement. Automatic semicolon insertion never inserts
one of the two semicolons in the header of a for statement.

The source

    returna + b

is transformed by automatic semicolon insertion into the following:

    return;a + b;

NOTE The expression a + b is not treated as a value to be returned by the return statement, because
a LineTerminator separates it from the token return.

The source

    a = b++c

is transformed by automatic semicolon insertion into the following:

    a = b;++c;

NOTE The token ++ is not treated as a postfix operator applying to the variable b, because a
LineTerminator occurs between b and ++.

The source

    if (a > b)else c = d

is not a valid ECMAScript sentence and is not altered by automatic semicolon insertion before the
else token, even though no production of the grammar applies at that point, because an automatically
inserted semicolon would then be parsed as an empty statement.

The source

    a = b + c(d + e).print()

is not transformed by automatic semicolon insertion, because the parenthesised expression that
begins the second line can be interpreted as an argument list for a function call:

    a = b + c(d + e).print()

In the circumstance that an assignment statement must begin with a left parenthesis, it is a good
idea for the programmer to provide an explicit semicolon at the end of the preceding statement
rather than to rely on automatic semicolon insertion.

8 Types
=======

Algorithms within this specification manipulate values each of which has an associated type. The
possible value types are exactly those defined in this clause. Types are further subclassified into
ECMAScript language types and specification types.

An ECMAScript language type corresponds to values that are directly manipulated by an ECMAScript
programmer using the ECMAScript language. The ECMAScript language types are Undefined, Null,
Boolean, String, Number, and Object.

A specification type corresponds to meta-values that are used within algorithms to describe the
semantics of ECMAScript language constructs and ECMAScript language types. The specification types
are Reference, List, Completion, Property Descriptor, Property Identifier, Lexical Environment, and
Environment Record. Specification type values are specification artefacts that do not necessarily
correspond to any specific entity within an ECMAScript implementation. Specification type values may
be used to describe intermediate results of ECMAScript expression evaluation but such values cannot
be stored as properties of objects or values of ECMAScript language variables.

Within this specification, the notation “Type(x)” is used as shorthand for “the type of x” where
“type” refers to the ECMAScript language and specification types defined in this clause.

8.1 The Undefined Type
======================

The Undefined type has exactly one value, called undefined. Any variable that has not been assigned
a value has the value undefined.

8.2 The Null Type
=================

The Null type has exactly one value, called null.

8.3 The Boolean Type
====================

The Boolean type represents a logical entity having two values, called true and false.

8.4 The String Type
===================

The String type is the set of all finite ordered sequences of zero or more 16-bit unsigned integer
values (“elements”). The String type is generally used to represent textual data in a running
ECMAScript program, in which case each element in the String is treated as a code unit value (see
Clause 6). Each element is regarded as occupying a position within the sequence. These positions are
indexed with nonnegative integers. The first element (if any) is at position 0, the next element (if
any) at position 1, and so on. The length of a String is the number of elements (i.e., 16-bit
values) within it. The empty String has length zero and therefore contains no elements.

When a String contains actual textual data, each element is considered to be a single UTF-16 code
unit. Whether or not this is the actual storage format of a String, the characters within a String
are numbered by their initial code unit element position as though they were represented using
UTF-16. All operations on Strings (except as otherwise stated) treat them as sequences of
undifferentiated 16-bit unsigned integers; they do not ensure the resulting String is in normalised
form, nor do they ensure language-sensitive results.

NOTE The rationale behind this design was to keep the implementation of Strings as simple and
high-performing as possible. The intent is that textual data coming into the execution environment
from outside (e.g., user input, text read from a file or received over the network, etc.) be
converted to Unicode Normalised Form C before the running program sees it. Usually this would occur
at the same time incoming text is converted from its original character encoding to Unicode (and
would impose no additional overhead). Since it is recommended that ECMAScript source code be in
Normalised Form C, string literals are guaranteed to be normalised (if source text is guaranteed to
be normalised), as long as they do not contain any Unicode escape sequences.

8.5 The Number Type
===================

The Number type has exactly 18437736874454810627 (that is, 2^64−2^53+3) values, representing the
double-precision 64-bit format IEEE 754 values as specified in the IEEE Standard for Binary
Floating-Point Arithmetic, except that the 9007199254740990 (that is, 2^53−2) distinct
“Not-a-Number” values of the IEEE Standard are represented in ECMAScript as a single special NaN
value. (Note that the NaN value is produced by the program expression NaN.) In some implementations,
external code might be able to detect a difference between various Not-a-Number values, but such
behaviour is implementation-dependent; to ECMAScript code, all NaN values are indistinguishable from
each other.

There are two other special values, called positive Infinity and negative Infinity. For brevity,
these values are also referred to for expository purposes by the symbols +∞ and −∞, respectively.
(Note that these two infinite Number values are produced by the program expressions +Infinity (or
simply Infinity) and -Infinity.)

The other 18437736874454810624 (that is, 2^64−2^53) values are called the finite numbers. Half of
these are positive numbers and half are negative numbers; for every finite positive Number value
there is a corresponding negative value having the same magnitude.

Note that there is both a positive zero and a negative zero. For brevity, these values are also
referred to for expository purposes by the symbols +0 and −0, respectively. (Note that these two
different zero Number values are produced by the program expressions +0 (or simply 0) and -0.)

The 18437736874454810622 (that is, 2^64−2^53−2) finite nonzero values are of two kinds:

18428729675200069632 (that is, 2^64−2^54) of them are normalised, having the form

s × m × 2e

where s is +1 or −1, m is a positive integer less than 2^53 but not less than 2^52, and e is an
integer ranging from −1074 to 971, inclusive.

The remaining 9007199254740990 (that is, 2^53−2) values are denormalised, having the form

s × m × 2e

where s is +1 or −1, m is a positive integer less than 2^52, and e is −1074.

Note that all the positive and negative integers whose magnitude is no greater than 2^53 are
representable in the Number type (indeed, the integer 0 has two representations, +0 and -0).

A finite number has an odd significand if it is nonzero and the integer m used to express it (in one
of the two forms shown above) is odd. Otherwise, it has an even significand.

In this specification, the phrase “the Number value for x” where x represents an exact nonzero real
mathematical quantity (which might even be an irrational number such as π) means a Number value
chosen in the following manner. Consider the set of all finite values of the Number type, with −0
removed and with two additional values added to it that are not representable in the Number type,
namely 2^1024 (which is +1 × 2^53 × 2^971) and −2^1024 (which is −1 × 2^53 × 2^971). Choose the
member of this set that is closest in value to x. If two values of the set are equally close, then
the one with an even significand is chosen; for this purpose, the two extra values 2^1024 and
−2^1024 are considered to have even significands. Finally, if 2^1024 was chosen, replace it with +∞;
if −2^1024 was chosen, replace it with −∞; if +0 was chosen, replace it with −0 if and only if x is
less than zero; any other chosen value is used unchanged. The result is the Number value for x.
(This procedure corresponds exactly to the behaviour of the IEEE 754 “round to nearest” mode.)

Some ECMAScript operators deal only with integers in the range −2^31 through 2^31−1, inclusive, or
in the range 0 through 2^32−1, inclusive. These operators accept any value of the Number type but
first convert each such value to one of 2^32 integer values. See the descriptions of the ToInt32 and
ToUint32 operators in 9.5 and 9.6, respectively.

8.6 The Object Type
===================

An Object is a collection of properties. Each property is either a named data property, a named
accessor property, or an internal property:

-   A named data property associates a name with an ECMAScript language value and a set of Boolean
    attributes.

-   A named accessor property associates a name with one or two accessor functions, and a set of
    Boolean attributes. The accessor functions are used to store or retrieve an ECMAScript language
    value that is associated with the property.

-   An internal property has no name and is not directly accessible via ECMAScript language
    operators. Internal properties exist purely for specification purposes.

There are two kinds of access for named (non-internal) properties: get and put, corresponding to
retrieval and assignment, respectively.

8.6.1 Property Attributes
=========================

Attributes are used in this specification to define and explain the state of named properties. A
named data property associates a name with the attributes listed in Table 5

Table 5 — Attributes of a Named Data Property

  Attribute Name     Value Domain                   Description
  ------------------ ------------------------------ -------------------------------------------------------------------------------------------------------------------------------------------------------
  [[Value]]          Any ECMAScript language type   The value retrieved by reading the property.
  [[Writable]]       Boolean                        If false, attempts by ECMAScript code to change the property’s [[Value]] attribute using [[Put]] will not succeed.
  [[Enumerable]]     Boolean                        If true, the property will be enumerated by a for-in enumeration (see 12.6.4). Otherwise, the property is said to be non-enumerable.
  [[Configurable]]   Boolean                        If false, attempts to delete the property, change the property to be an accessor property, or change its attributes (other than [[Value]]) will fail.

A named accessor property associates a name with the attributes listed in Table 6.

Table 6 — Attributes of a Named Accessor Property

  Attribute Name     Value Domain          Description
  ------------------ --------------------- --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
  [[Get]]            Object or Undefined   If the value is an Object it must be a function Object. The function’s [[Call]] internal method (8.6.2) is called with an empty arguments list to return the property value each time a get access of the property is performed.
  [[Set]]            Object or Undefined   If the value is an Object it must be a function Object. The function’s [[Call]] internal method (8.6.2) is called with an arguments list containing the assigned value as its sole argument each time a set access of the property is performed. The effect of a property’s [[Set]] internal method may, but is not required to, have an effect on the value returned by subsequent calls to the property’s [[Get]] internal method.
  [[Enumerable]]     Boolean               If true, the property is to be enumerated by a for-in enumeration (see 12.6.4). Otherwise, the property is said to be non-enumerable.
  [[Configurable]]   Boolean               If false, attempts to delete the property, change the property to be a data property, or change its attributes will fail.

If the value of an attribute is not explicitly specified by this specification for a named property,
the default value defined in Table 7 is used.

Table 7 — Default Attribute Values

  Attribute Name     Default Value
  ------------------ ---------------
  [[Value]]          undefined
  [[Get]]            undefined
  [[Set]]            undefined
  [[Writable]]       false
  [[Enumerable]]     false
  [[Configurable]]   false

8.6.2 Object Internal Properties and Methods
============================================

This specification uses various internal properties to define the semantics of object values. These
internal properties are not part of the ECMAScript language. They are defined by this specification
purely for expository purposes. An implementation of ECMAScript must behave as if it produced and
operated upon internal properties in the manner described here. The names of internal properties are
enclosed in double square brackets [[ ]]. When an algorithm uses an internal property of an object
and the object does not implement the indicated internal property, a TypeError exception is thrown.

The Table 8 summarises the internal properties used by this specification that are applicable to all
ECMAScript objects. The Table 9 summarises the internal properties used by this specification that
are only applicable to some ECMAScript objects. The descriptions in these tables indicate their
behaviour for native ECMAScript objects, unless stated otherwise in this document for particular
kinds of native ECMAScript objects. Host objects may support these internal properties with any
implementation-dependent behaviour as long as it is consistent with the specific host object
restrictions stated in this document.

The “Value Type Domain” columns of the following tables define the types of values associated with
internal properties. The type names refer to the types defined in Clause 8 augmented by the
following additional names. “any” means the value may be any ECMAScript language type. “primitive”
means Undefined, Null, Boolean, String, or Number. “SpecOp” means the internal property is an
internal method, an implementation provided procedure defined by an abstract operation
specification. “SpecOp” is followed by a list of descriptive parameter names. If a parameter name is
the same as a type name then the name describes the type of the parameter. If a “SpecOp” returns a
value, its parameter list is followed by the symbol “→” and the type of the returned value.

Table 8 — Internal Properties Common to All Objects

+-----------------------------------+-----------------------------------+-----------------------------------+
| Internal Property                 | Value Type Domain                 | Description                       |
+===================================+===================================+===================================+
| [[Prototype]]                     | Object or Null                    | The prototype of this object.     |
+-----------------------------------+-----------------------------------+-----------------------------------+
| [[Class]]                         | String                            | A String value indicating a       |
|                                   |                                   | specification defined             |
|                                   |                                   | classification of objects.        |
+-----------------------------------+-----------------------------------+-----------------------------------+
| [[Extensible]]                    | Boolean                           | If true, own properties may be    |
|                                   |                                   | added to the object.              |
+-----------------------------------+-----------------------------------+-----------------------------------+
| [[Get]]                           | SpecOp(propertyName) → any        | Returns the value of the named    |
|                                   |                                   | property.                         |
+-----------------------------------+-----------------------------------+-----------------------------------+
| [[GetOwnProperty]]                | SpecOp (propertyName) →           | Returns the Property Descriptor   |
|                                   |                                   | of the named own property of this |
|                                   | Undefined or Property Descriptor  | object, or undefined if absent.   |
+-----------------------------------+-----------------------------------+-----------------------------------+
| [[GetProperty]]                   | SpecOp (propertyName) →           | Returns the fully populated       |
|                                   |                                   | Property Descriptor of the named  |
|                                   | Undefined or Property Descriptor  | property of this object, or       |
|                                   |                                   | undefined if absent.              |
+-----------------------------------+-----------------------------------+-----------------------------------+
| [[Put]]                           | SpecOp (propertyName, any,        | Sets the specified named property |
|                                   | Boolean)                          | to the value of the second        |
|                                   |                                   | parameter. The flag controls      |
|                                   |                                   | failure handling.                 |
+-----------------------------------+-----------------------------------+-----------------------------------+
| [[CanPut]]                        | SpecOp (propertyName) → Boolean   | Returns a Boolean value           |
|                                   |                                   | indicating whether a [[Put]]      |
|                                   |                                   | operation with PropertyName can   |
|                                   |                                   | be performed.                     |
+-----------------------------------+-----------------------------------+-----------------------------------+
| [[HasProperty]]                   | SpecOp (propertyName) → Boolean   | Returns a Boolean value           |
|                                   |                                   | indicating whether the object     |
|                                   |                                   | already has a property with the   |
|                                   |                                   | given name.                       |
+-----------------------------------+-----------------------------------+-----------------------------------+
| [[Delete]]                        | SpecOp (propertyName, Boolean) →  | Removes the specified named own   |
|                                   | Boolean                           | property from the object. The     |
|                                   |                                   | flag controls failure handling.   |
+-----------------------------------+-----------------------------------+-----------------------------------+
| [[DefaultValue]]                  | SpecOp (Hint) → primitive         | Hint is a String. Returns a       |
|                                   |                                   | default value for the object.     |
+-----------------------------------+-----------------------------------+-----------------------------------+
| [[DefineOwnProperty]]             | SpecOp (propertyName,             | Creates or alters the named own   |
|                                   | PropertyDescriptor, Boolean) →    | property to have the state        |
|                                   | Boolean                           | described by a Property           |
|                                   |                                   | Descriptor. The flag controls     |
|                                   |                                   | failure handling.                 |
+-----------------------------------+-----------------------------------+-----------------------------------+

Every object (including host objects) must implement all of the internal properties listed in Table
8. However, the [[DefaultValue]] internal method may, for some objects, simply throw a TypeError
exception.

All objects have an internal property called [[Prototype]]. The value of this property is either
null or an object and is used for implementing inheritance. Whether or not a native object can have
a host object as its [[Prototype]] depends on the implementation. Every [[Prototype]] chain must
have finite length (that is, starting from any object, recursively accessing the [[Prototype]]
internal property must eventually lead to a null value). Named data properties of the [[Prototype]]
object are inherited (are visible as properties of the child object) for the purposes of get access,
but not for put access. Named accessor properties are inherited for both get access and put access.

Every ECMAScript object has a Boolean-valued [[Extensible]] internal property that controls whether
or not named properties may be added to the object. If the value of the [[Extensible]] internal
property is false then additional named properties may not be added to the object. In addition, if
[[Extensible]] is false the value of the [[Class]] and [[Prototype]] internal properties of the
object may not be modified. Once the value of an [[Extensible]] internal property has been set to
false it may not be subsequently changed to true.

NOTE This specification defines no ECMAScript language operators or built-in functions that permit a
program to modify an object’s [[Class]] or [[Prototype]] internal properties or to change the value
of [[Extensible]] from false to true. Implementation specific extensions that modify [[Class]],
[[Prototype]] or [[Extensible]] must not violate the invariants defined in the preceding paragraph.

The value of the [[Class]] internal property is defined by this specification for every kind of
built-in object. The value of the [[Class]] internal property of a host object may be any String
value except one of "Arguments", "Array", "Boolean", "Date", "Error", "Function", "JSON", "Math",
"Number", "Object", "RegExp", and "String". The value of a [[Class]] internal property is used
internally to distinguish different kinds of objects. Note that this specification does not provide
any means for a program to access that value except through Object.prototype.toString (see
15.2.4.2).

Unless otherwise specified, the common internal methods of native ECMAScript objects behave as
described in 8.12. Array objects have a slightly different implementation of the
[[DefineOwnProperty]] internal method (see 15.4.5.1) and String objects have a slightly different
implementation of the [[GetOwnProperty]] internal method (see 15.5.5.2). Arguments objects (10.6)
have different implementations of [[Get]], [[GetOwnProperty]], [[DefineOwnProperty]], and
[[Delete]]. Function objects (15.3) have a different implementation of [[Get]].

Host objects may implement these internal methods in any manner unless specified otherwise; for
example, one possibility is that [[Get]] and [[Put]] for a particular host object indeed fetch and
store property values but [[HasProperty]] always generates false. However, if any specified
manipulation of a host object’s internal properties is not supported by an implementation, that
manipulation must throw a TypeError exception when attempted.

The [[GetOwnProperty]] internal method of a host object must conform to the following invariants for
each property of the host object:

-   If a property is described as a data property and it may return different values over time, then
    either or both of the [[Writable]] and [[Configurable]] attributes must be true even if no
    mechanism to change the value is exposed via the other internal methods.

-   If a property is described as a data property and its [[Writable]] and [[Configurable]] are both
    false, then the SameValue (according to 9.12) must be returned for the [[Value]] attribute of
    the property on all calls to [[GetOwnProperty]].

-   If the attributes other than [[Writable]] may change over time or if the property might
    disappear, then the [[Configurable]] attribute must be true.

-   If the [[Writable]] attribute may change from false to true, then the [[Configurable]] attribute
    must be true.

-   If the value of the host object’s [[Extensible]] internal property has been observed by
    ECMAScript code to be false, then if a call to [[GetOwnProperty]] describes a property as
    non-existent all subsequent calls must also describe that property as non-existent.

The [[DefineOwnProperty]] internal method of a host object must not permit the addition of a new
property to a host object if the [[Extensible]] internal property of that host object has been
observed by ECMAScript code to be false.

If the [[Extensible]] internal property of that host object has been observed by ECMAScript code to
be false then it must not subsequently become true.

Table 9 — Internal Properties Only Defined for Some Objects

  Internal Property      Value Type Domain                               Description
  ---------------------- ----------------------------------------------- ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
  [[PrimitiveValue]]     primitive                                       Internal state information associated with this object. Of the standard built-in ECMAScript objects, only Boolean, Date, Number, and String objects implement [[PrimitiveValue]].
  [[Construct]]          SpecOp(a List of any) → Object                  Creates an object. Invoked via the new operator. The arguments to the SpecOp are the arguments passed to the new operator. Objects that implement this internal method are called constructors.
  [[Call]]               SpecOp(any, a List of any) → any or Reference   Executes code associated with the object. Invoked via a function call expression. The arguments to the SpecOp are this object and a list containing the arguments passed to the function call expression. Objects that implement this internal method are callable. Only callable objects that are host objects may return Reference values.
  [[HasInstance]]        SpecOp(any) → Boolean                           Returns a Boolean value indicating whether the argument is likely an Object that was constructed by this object. Of the standard built-in ECMAScript objects, only Function objects implement [[HasInstance]].
  [[Scope]]              Lexical Environment                             A lexical environment that defines the environment in which a Function object is executed. Of the standard built-in ECMAScript objects, only Function objects implement [[Scope]].
  [[FormalParameters]]   List of Strings                                 A possibly empty List containing the identifier Strings of a Function’s FormalParameterList. Of the standard built-in ECMAScript objects, only Function objects implement [[FormalParameterList]].
  [[Code]]               ECMAScript code                                 The ECMAScript code of a function. Of the standard built-in ECMAScript objects, only Function objects implement [[Code]].
  [[TargetFunction]]     Object                                          The target function of a function object created using the standard built-in Function.prototype.bind method. Only ECMAScript objects created using Function.prototype.bind have a [[TargetFunction]] internal property.
  [[BoundThis]]          any                                             The pre-bound this value of a function Object created using the standard built-in Function.prototype.bind method. Only ECMAScript objects created using Function.prototype.bind have a [[BoundThis]] internal property.
  [[BoundArguments]]     List of any                                     The pre-bound argument values of a function Object created using the standard built-in Function.prototype.bind method. Only ECMAScript objects created using Function.prototype.bind have a [[BoundArguments]] internal property.
  [[Match]]              SpecOp(String, index) → MatchResult             Tests for a regular expression match and returns a MatchResult value (see 15.10.2.1). Of the standard built-in ECMAScript objects, only RegExp objects implement [[Match]].
  [[ParameterMap]]       Object                                          Provides a mapping between the properties of an arguments object (see 10.6) and the formal parameters of the associated function. Only ECMAScript objects that are arguments objects have a [[ParameterMap]] internal property.

8.7 The Reference Specification Type
====================================

The Reference type is used to explain the behaviour of such operators as delete, typeof, and the
assignment operators. For example, the left-hand operand of an assignment is expected to produce a
reference. The behaviour of assignment could, instead, be explained entirely in terms of a case
analysis on the syntactic form of the left-hand operand of an assignment operator, but for one
difficulty: function calls are permitted to return references. This possibility is admitted purely
for the sake of host objects. No built-in ECMAScript function defined by this specification returns
a reference and there is no provision for a user-defined function to return a reference. (Another
reason not to use a syntactic case analysis is that it would be lengthy and awkward, affecting many
parts of the specification.)

A Reference is a resolved name binding. A Reference consists of three components, the base value,
the referenced name and the Boolean valued strict reference flag. The base value is either
undefined, an Object, a Boolean, a String, a Number, or an environment record (10.2.1). A base value
of undefined indicates that the reference could not be resolved to a binding. The referenced name is
a String.

The following abstract operations are used in this specification to access the components of
references:

-   GetBase(V). Returns the base value component of the reference V.

-   GetReferencedName(V). Returns the referenced name component of the reference V.

-   IsStrictReference(V). Returns the strict reference component of the reference V.

-   HasPrimitiveBase(V). Returns true if the base value is a Boolean, String, or Number.

-   IsPropertyReference(V). Returns true if either the base value is an object or
    HasPrimitiveBase(V) is true; otherwise returns false.

-   IsUnresolvableReference(V). Returns true if the base value is undefined and false otherwise.

The following abstract operations are used in this specification to operate on references:

8.7.1 GetValue (V)
==================

1.  If Type(V) is not Reference, return V.
2.  Let base be the result of calling GetBase(V).
3.  If IsUnresolvableReference(V), throw a ReferenceError exception.
4.  If IsPropertyReference(V), then
    1.  If HasPrimitiveBase(V) is false, then let get be the [[Get]] internal method of base,
        otherwise let get be the special [[Get]] internal method defined below.
    2.  Return the result of calling the get internal method using base as its this value, and
        passing GetReferencedName(V) for the argument.

5.  Else, base must be an environment record.
    1.  Return the result of calling the GetBindingValue (see 10.2.1) concrete method of base
        passing GetReferencedName(V) and IsStrictReference(V) as arguments.

The following [[Get]] internal method is used by GetValue when V is a property reference with a
primitive base value. It is called using base as its this value and with property P as its argument.
The following steps are taken:

1.  Let O be ToObject(base).
2.  Let desc be the result of calling the [[GetProperty]] internal method of O with property name P.
3.  If desc is undefined, return undefined.
4.  If IsDataDescriptor(desc) is true, return desc.[[Value]].
5.  Otherwise, IsAccessorDescriptor(desc) must be true so, let getter be desc.[[Get]] (see 8.10).
6.  If getter is undefined, return undefined.
7.  Return the result calling the [[Call]] internal method of getter providing base as the this
    value and providing no arguments.

NOTE The object that may be created in step 1 is not accessible outside of the above method. An
implementation might choose to avoid the actual creation of the object. The only situation where
such an actual property access that uses this internal method can have visible effect is when it
invokes an accessor function.

8.7.2 PutValue (V, W)
=====================

1.  If Type(V) is not Reference, throw a ReferenceError exception.
2.  Let base be the result of calling GetBase(V).
3.  If IsUnresolvableReference(V), then
    1.  If IsStrictReference(V) is true, then
        1.  Throw ReferenceError exception.

    2.  Call the [[Put]] internal method of the global object, passing GetReferencedName(V) for the
        property name, W for the value, and false for the Throw flag.

4.  Else if IsPropertyReference(V), then
    1.  If HasPrimitiveBase(V) is false, then let put be the [[Put]] internal method of base,
        otherwise let put be the special [[Put]] internal method defined below.
    2.  Call the put internal method using base as its this value, and passing GetReferencedName(V)
        for the property name, W for the value, and IsStrictReference(V) for the Throw flag.

5.  Else base must be a reference whose base is an environment record. So,
    1.  Call the SetMutableBinding (10.2.1) concrete method of base, passing GetReferencedName(V),
        W, and IsStrictReference(V) as arguments.

6.  Return.

The following [[Put]] internal method is used by PutValue when V is a property reference with a
primitive base value. It is called using base as its this value and with property P, value W, and
Boolean flag Throw as arguments. The following steps are taken:

1.  Let O be ToObject(base).
2.  If the result of calling the [[CanPut]] internal method of O with argument P is false, then
    1.  If Throw is true, then throw a TypeError exception.
    2.  Else return.

3.  Let ownDesc be the result of calling the [[GetOwnProperty]] internal method of O with argument
    P.
4.  If IsDataDescriptor(ownDesc) is true, then
    1.  If Throw is true, then throw a TypeError exception.
    2.  Else return.

5.  Let desc be the result of calling the [[GetProperty]] internal method of O with argument P. This
    may be either an own or inherited accessor property descriptor or an inherited data property
    descriptor.
6.  If IsAccessorDescriptor(desc) is true, then
    1.  Let setter be desc.[[Set]] (see 8.10) which cannot be undefined.
    2.  Call the [[Call]] internal method of setter providing base as the this value and an argument
        list containing only W.

7.  Else, this is a request to create an own property on the transient object O
    1.  If Throw is true, then throw a TypeError exception.

8.  Return.

NOTE The object that may be created in step 1 is not accessible outside of the above method. An
implementation might choose to avoid the actual creation of that transient object. The only
situations where such an actual property assignment that uses this internal method can have visible
effect are when it either invokes an accessor function or is in violation of a Throw predicated
error check. When Throw is true any property assignment that would create a new property on the
transient object throws an error.

8.8 The List Specification Type
===============================

The List type is used to explain the evaluation of argument lists (see 11.2.4) in new expressions,
in function calls, and in other algorithms where a simple list of values is needed. Values of the
List type are simply ordered sequences of values. These sequences may be of any length.

8.9 The Completion Specification Type
=====================================

The Completion type is used to explain the behaviour of statements (break, continue, return and
throw) that perform nonlocal transfers of control. Values of the Completion type are triples of the
form (type, value, target), where type is one of normal, break, continue, return, or throw, value is
any ECMAScript language value or empty, and target is any ECMAScript identifier or empty. If cv is a
completion value then cv.type, cv.value, and cv.target may be used to directly refer to its
constituent values.

The term “abrupt completion” refers to any completion with a type other than normal.

8.10 The Property Descriptor and Property Identifier Specification Types
========================================================================

The Property Descriptor type is used to explain the manipulation and reification of named property
attributes. Values of the Property Descriptor type are records composed of named fields where each
field’s name is an attribute name and its value is a corresponding attribute value as specified in
8.6.1. In addition, any field may be present or absent.

Property Descriptor values may be further classified as data property descriptors and accessor
property descriptors based upon the existence or use of certain fields. A data property descriptor
is one that includes any fields named either [[Value]] or [[Writable]]. An accessor property
descriptor is one that includes any fields named either [[Get]] or [[Set]]. Any property descriptor
may have fields named [[Enumerable]] and [[Configurable]]. A Property Descriptor value may not be
both a data property descriptor and an accessor property descriptor; however, it may be neither. A
generic property descriptor is a Property Descriptor value that is neither a data property
descriptor nor an accessor property descriptor. A fully populated property descriptor is one that is
either an accessor property descriptor or a data property descriptor and that has all of the fields
that correspond to the property attributes defined in either 8.6.1 Table 5 or Table 6.

For notational convenience within this specification, an object literal-like syntax can be used to
define a property descriptor value. For example, Property Descriptor {[[Value]]: 42, [[Writable]]:
false, [[Configurable]]: true} defines a data property descriptor. Field name order is not
significant. Any fields that are not explicitly listed are considered to be absent.

In specification text and algorithms, dot notation may be used to refer to a specific field of a
Property Descriptor. For example, if D is a property descriptor then D.[[Value]] is shorthand for
“the field of D named [[Value]]”.

The Property Identifier type is used to associate a property name with a Property Descriptor. Values
of the Property Identifier type are pairs of the form (name, descriptor), where name is a String and
descriptor is a Property Descriptor value.

The following abstract operations are used in this specification to operate upon Property Descriptor
values:

8.10.1 IsAccessorDescriptor ( Desc )
====================================

When the abstract operation IsAccessorDescriptor is called with property descriptor Desc, the
following steps are taken:

1.  If Desc is undefined, then return false.
2.  If both Desc.[[Get]] and Desc.[[Set]] are absent, then return false.
3.  Return true.

8.10.2 IsDataDescriptor ( Desc )
================================

When the abstract operation IsDataDescriptor is called with property descriptor Desc, the following
steps are taken:

1.  If Desc is undefined, then return false.
2.  If both Desc.[[Value]] and Desc.[[Writable]] are absent, then return false.
3.  Return true.

8.10.3 IsGenericDescriptor ( Desc )
===================================

When the abstract operation IsGenericDescriptor is called with property descriptor Desc, the
following steps are taken:

1.  If Desc is undefined, then return false.
2.  If IsAccessorDescriptor(Desc) and IsDataDescriptor(Desc) are both false, then return true.
3.  Return false.

8.10.4 FromPropertyDescriptor ( Desc )
======================================

When the abstract operation FromPropertyDescriptor is called with property descriptor Desc, the
following steps are taken:

The following algorithm assumes that Desc is a fully populated Property Descriptor, such as that
returned from [[GetOwnProperty]] (see 8.12.1).

1.  If Desc is undefined, then return undefined.
2.  Let obj be the result of creating a new object as if by the expression new Object() where Object
    is the standard built-in constructor with that name.
3.  If IsDataDescriptor(Desc) is true, then
    1.  Call the [[DefineOwnProperty]] internal method of obj with arguments "value", Property
        Descriptor {[[Value]]: Desc.[[Value]], [[Writable]]: true, [[Enumerable]]: true,
        [[Configurable]]: true}, and false.
    2.  Call the [[DefineOwnProperty]] internal method of obj with arguments "writable", Property
        Descriptor {[[Value]]: Desc.[[Writable]], [[Writable]]: true, [[Enumerable]]: true,
        [[Configurable]]: true}, and false.

4.  Else, IsAccessorDescriptor(Desc) must be true, so
    1.  Call the [[DefineOwnProperty]] internal method of obj with arguments "get", Property
        Descriptor {[[Value]]: Desc.[[Get]], [[Writable]]: true, [[Enumerable]]: true,
        [[Configurable]]: true}, and false.
    2.  Call the [[DefineOwnProperty]] internal method of obj with arguments "set", Property
        Descriptor {[[Value]]: Desc.[[Set]], [[Writable]]: true, [[Enumerable]]: true,
        [[Configurable]]: true}, and false.

5.  Call the [[DefineOwnProperty]] internal method of obj with arguments "enumerable", Property
    Descriptor {[[Value]]: Desc.[[Enumerable]], [[Writable]]: true, [[Enumerable]]: true,
    [[Configurable]]: true}, and false.
6.  Call the [[DefineOwnProperty]] internal method of obj with arguments "configurable", Property
    Descriptor {[[Value]]: Desc.[[Configurable]], [[Writable]]: true, [[Enumerable]]: true,
    [[Configurable]]: true}, and false.
7.  Return obj.

8.10.5 ToPropertyDescriptor ( Obj )
===================================

When the abstract operation ToPropertyDescriptor is called with object Obj, the following steps are
taken:

1.  If Type(Obj) is not Object throw a TypeError exception.
2.  Let desc be the result of creating a new Property Descriptor that initially has no fields.
3.  If the result of calling the [[HasProperty]] internal method of Obj with argument "enumerable"
    is true, then
    1.  Let enum be the result of calling the [[Get]] internal method of Obj with "enumerable".
    2.  Set the [[Enumerable]] field of desc to ToBoolean(enum).

4.  If the result of calling the [[HasProperty]] internal method of Obj with argument "configurable"
    is true, then
    1.  Let conf be the result of calling the [[Get]] internal method of Obj with argument
        "configurable".
    2.  Set the [[Configurable]] field of desc to ToBoolean(conf).

5.  If the result of calling the [[HasProperty]] internal method of Obj with argument "value" is
    true, then
    1.  Let value be the result of calling the [[Get]] internal method of Obj with argument “value”.
    2.  Set the [[Value]] field of desc to value.

6.  If the result of calling the [[HasProperty]] internal method of Obj with argument "writable" is
    true, then
    1.  Let writable be the result of calling the [[Get]] internal method of Obj with argument
        "writable".
    2.  Set the [[Writable]] field of desc to ToBoolean(writable).

7.  If the result of calling the [[HasProperty]] internal method of Obj with argument "get" is true,
    then
    1.  Let getter be the result of calling the [[Get]] internal method of Obj with argument "get".
    2.  If IsCallable(getter) is false and getter is not undefined, then throw a TypeError
        exception.
    3.  Set the [[Get]] field of desc to getter.

8.  If the result of calling the [[HasProperty]] internal method of Obj with argument "set" is true,
    then
    1.  Let setter be the result of calling the [[Get]] internal method of Obj with argument "set".
    2.  If IsCallable(setter) is false and setter is not undefined, then throw a TypeError
        exception.
    3.  Set the [[Set]] field of desc to setter.

9.  If either desc.[[Get]] or desc.[[Set]] are present, then
    1.  If either desc.[[Value]] or desc.[[Writable]] are present, then throw a TypeError exception.

10. Return desc.

8.11 The Lexical Environment and Environment Record Specification Types
=======================================================================

The Lexical Environment and Environment Record types are used to explain the behaviour of name
resolution in nested functions and blocks. These types and the operations upon them are defined in
Clause 10.

8.12 Algorithms for Object Internal Methods
===========================================

In the following algorithm descriptions, assume O is a native ECMAScript object, P is a String, Desc
is a Property Description record, and Throw is a Boolean flag.

8.12.1 [[GetOwnProperty]] (P)
=============================

When the [[GetOwnProperty]] internal method of O is called with property name P, the following steps
are taken:

1.  If O doesn’t have an own property with name P, return undefined.
2.  Let D be a newly created Property Descriptor with no fields.
3.  Let X be O’s own property named P.
4.  If X is a data property, then
    1.  Set D.[[Value]] to the value of X’s [[Value]] attribute.
    2.  Set D.[[Writable]] to the value of X’s [[Writable]] attribute

5.  Else X is an accessor property, so
    1.  Set D.[[Get]] to the value of X’s [[Get]] attribute.
    2.  Set D.[[Set]] to the value of X’s [[Set]] attribute.

6.  Set D.[[Enumerable]] to the value of X’s [[Enumerable]] attribute.
7.  Set D.[[Configurable]] to the value of X’s [[Configurable]] attribute.
8.  Return D.

However, if O is a String object it has a more elaborate [[GetOwnProperty]] internal method defined
in 15.5.5.2.

8.12.2 [[GetProperty]] (P)
==========================

When the [[GetProperty]] internal method of O is called with property name P, the following steps
are taken:

1.  Let prop be the result of calling the [[GetOwnProperty]] internal method of O with property name
    P.
2.  If prop is not undefined, return prop.
3.  Let proto be the value of the [[Prototype]] internal property of O.
4.  If proto is null, return undefined.
5.  Return the result of calling the [[GetProperty]] internal method of proto with argument P.

8.12.3 [[Get]] (P)
==================

When the [[Get]] internal method of O is called with property name P, the following steps are taken:

1.  Let desc be the result of calling the [[GetProperty]] internal method of O with property name P.
2.  If desc is undefined, return undefined.
3.  If IsDataDescriptor(desc) is true, return desc.[[Value]].
4.  Otherwise, IsAccessorDescriptor(desc) must be true so, let getter be desc.[[Get]].
5.  If getter is undefined, return undefined.
6.  Return the result calling the [[Call]] internal method of getter providing O as the this value
    and providing no arguments.

8.12.4 [[CanPut]] (P)
=====================

When the [[CanPut]] internal method of O is called with property name P, the following steps are
taken:

1.  Let desc be the result of calling the [[GetOwnProperty]] internal method of O with argument P.
2.  If desc is not undefined, then
    1.  If IsAccessorDescriptor(desc) is true, then
        1.  If desc.[[Set]] is undefined, then return false.
        2.  Else return true.

    2.  Else, desc must be a DataDescriptor so return the value of desc.[[Writable]].

3.  Let proto be the [[Prototype]] internal property of O.
4.  If proto is null, then return the value of the [[Extensible]] internal property of O.
5.  Let inherited be the result of calling the [[GetProperty]] internal method of proto with
    property name P.
6.  If inherited is undefined, return the value of the [[Extensible]] internal property of O.
7.  If IsAccessorDescriptor(inherited) is true, then
    1.  If inherited.[[Set]] is undefined, then return false.
    2.  Else return true.

8.  Else, inherited must be a DataDescriptor
    1.  If the [[Extensible]] internal property of O is false, return false.
    2.  Else return the value of inherited.[[Writable]].

Host objects may define additional constraints upon [[Put]] operations. If possible, host objects
should not allow [[Put]] operations in situations where this definition of [[CanPut]] returns false.

8.12.5 [[Put]] ( P, V, Throw )
==============================

When the [[Put]] internal method of O is called with property P, value V, and Boolean flag Throw,
the following steps are taken:

1.  If the result of calling the [[CanPut]] internal method of O with argument P is false, then
    1.  If Throw is true, then throw a TypeError exception.
    2.  Else return.

2.  Let ownDesc be the result of calling the [[GetOwnProperty]] internal method of O with argument
    P.
3.  If IsDataDescriptor(ownDesc) is true, then
    1.  Let valueDesc be the Property Descriptor {[[Value]]: V}.
    2.  Call the [[DefineOwnProperty]] internal method of O passing P, valueDesc, and Throw as
        arguments.
    3.  Return.

4.  Let desc be the result of calling the [[GetProperty]] internal method of O with argument P. This
    may be either an own or inherited accessor property descriptor or an inherited data property
    descriptor.
5.  If IsAccessorDescriptor(desc) is true, then
    1.  Let setter be desc.[[Set]] which cannot be undefined.
    2.  Call the [[Call]] internal method of setter providing O as the this value and providing V as
        the sole argument.

6.  Else, create a named data property named P on object O as follows
    1.  Let newDesc be the Property Descriptor  
        {[[Value]]: V, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}.
    2.  Call the [[DefineOwnProperty]] internal method of O passing P, newDesc, and Throw as
        arguments.

7.  Return.

8.12.6 [[HasProperty]] (P)
==========================

When the [[HasProperty]] internal method of O is called with property name P, the following steps
are taken:

1.  Let desc be the result of calling the [[GetProperty]] internal method of O with property name P.
2.  If desc is undefined, then return false.
3.  Else return true.

8.12.7 [[Delete]] (P, Throw)
============================

When the [[Delete]] internal method of O is called with property name P and the Boolean flag Throw,
the following steps are taken:

1.  Let desc be the result of calling the [[GetOwnProperty]] internal method of O with property name
    P.
2.  If desc is undefined, then return true.
3.  If desc.[[Configurable]] is true, then
    1.  Remove the own property with name P from O.
    2.  Return true.

4.  Else if Throw, then throw a TypeError exception.
5.  Return false.

8.12.8 [[DefaultValue]] (hint)
==============================

When the [[DefaultValue]] internal method of O is called with hint String, the following steps are
taken:

1.  Let toString be the result of calling the [[Get]] internal method of object O with argument
    "toString".
2.  If IsCallable(toString) is true then,
    1.  Let str be the result of calling the [[Call]] internal method of toString, with O as the
        this value and an empty argument list.
    2.  If str is a primitive value, return str.

3.  Let valueOf be the result of calling the [[Get]] internal method of object O with argument
    "valueOf".
4.  If IsCallable(valueOf) is true then,
    1.  Let val be the result of calling the [[Call]] internal method of valueOf, with O as the this
        value and an empty argument list.
    2.  If val is a primitive value, return val.

5.  Throw a TypeError exception.

When the [[DefaultValue]] internal method of O is called with hint Number, the following steps are
taken:

1.  Let valueOf be the result of calling the [[Get]] internal method of object O with argument
    "valueOf".
2.  If IsCallable(valueOf) is true then,
    1.  Let val be the result of calling the [[Call]] internal method of valueOf, with O as the this
        value and an empty argument list.
    2.  If val is a primitive value, return val.

3.  Let toString be the result of calling the [[Get]] internal method of object O with argument
    "toString".
4.  If IsCallable(toString) is true then,
    1.  Let str be the result of calling the [[Call]] internal method of toString, with O as the
        this value and an empty argument list.
    2.  If str is a primitive value, return str.

5.  Throw a TypeError exception.

When the [[DefaultValue]] internal method of O is called with no hint, then it behaves as if the
hint were Number, unless O is a Date object (see 15.9.6), in which case it behaves as if the hint
were String.

The above specification of [[DefaultValue]] for native objects can return only primitive values. If
a host object implements its own [[DefaultValue]] internal method, it must ensure that its
[[DefaultValue]] internal method can return only primitive values.

8.12.9 [[DefineOwnProperty]] (P, Desc, Throw)
=============================================

In the following algorithm, the term “Reject” means “If Throw is true, then throw a TypeError
exception, otherwise return false”. The algorithm contains steps that test various fields of the
Property Descriptor Desc for specific values. The fields that are tested in this manner need not
actually exist in Desc. If a field is absent then its value is considered to be false.

When the [[DefineOwnProperty]] internal method of O is called with property name P, property
descriptor Desc, and Boolean flag Throw, the following steps are taken:

1.  Let current be the result of calling the [[GetOwnProperty]] internal method of O with property
    name P.
2.  Let extensible be the value of the [[Extensible]] internal property of O.
3.  If current is undefined and extensible is false, then Reject.
4.  If current is undefined and extensible is true, then
    1.  If IsGenericDescriptor(Desc) or IsDataDescriptor(Desc) is true, then
        1.  Create an own data property named P of object O whose [[Value]], [[Writable]],
            [[Enumerable]] and [[Configurable]] attribute values are described by Desc. If the value
            of an attribute field of Desc is absent, the attribute of the newly created property is
            set to its default value.

    2.  Else, Desc must be an accessor Property Descriptor so,
        1.  Create an own accessor property named P of object O whose [[Get]], [[Set]],
            [[Enumerable]] and [[Configurable]] attribute values are described by Desc. If the value
            of an attribute field of Desc is absent, the attribute of the newly created property is
            set to its default value.

    3.  Return true.

5.  Return true, if every field in Desc is absent.
6.  Return true, if every field in Desc also occurs in current and the value of every field in Desc
    is the same value as the corresponding field in current when compared using the SameValue
    algorithm (9.12).
7.  If the [[Configurable]] field of current is false then
    1.  Reject, if the [[Configurable]] field of Desc is true.
    2.  Reject, if the [[Enumerable]] field of Desc is present and the [[Enumerable]] fields of
        current and Desc are the Boolean negation of each other.

8.  If IsGenericDescriptor(Desc) is true, then no further validation is required.
9.  Else, if IsDataDescriptor(current) and IsDataDescriptor(Desc) have different results, then
    1.  Reject, if the [[Configurable]] field of current is false.
    2.  If IsDataDescriptor(current) is true, then
        1.  Convert the property named P of object O from a data property to an accessor property.
            Preserve the existing values of the converted property’s [[Configurable]] and
            [[Enumerable]] attributes and set the rest of the property’s attributes to their default
            values.

    3.  Else,
        1.  Convert the property named P of object O from an accessor property to a data property.
            Preserve the existing values of the converted property’s [[Configurable]] and
            [[Enumerable]] attributes and set the rest of the property’s attributes to their default
            values.

10. Else, if IsDataDescriptor(current) and IsDataDescriptor(Desc) are both true, then
    1.  If the [[Configurable]] field of current is false, then
        1.  Reject, if the [[Writable]] field of current is false and the [[Writable]] field of Desc
            is true.
        2.  If the [[Writable]] field of current is false, then
            1.  Reject, if the [[Value]] field of Desc is present and SameValue(Desc.[[Value]],
                current.[[Value]]) is false.

    2.  else, the [[Configurable]] field of current is true, so any change is acceptable.

11. Else, IsAccessorDescriptor(current) and IsAccessorDescriptor(Desc) are both true so,
    1.  If the [[Configurable]] field of current is false, then
        1.  Reject, if the [[Set]] field of Desc is present and SameValue(Desc.[[Set]],
            current.[[Set]]) is false.
        2.  Reject, if the [[Get]] field of Desc is present and SameValue(Desc.[[Get]],
            current.[[Get]]) is false.

12. For each attribute field of Desc that is present, set the correspondingly named attribute of the
    property named P of object O to the value of the field.
13. Return true.

However, if O is an Array object, it has a more elaborate [[DefineOwnProperty]] internal method
defined in 15.4.5.1.

NOTE Step 10.b allows any field of Desc to be different from the corresponding field of current if
current’s [[Configurable]] field is true. This even permits changing the [[Value]] of a property
whose [[Writable]] attribute is false. This is allowed because a true [[Configurable]] attribute
would permit an equivalent sequence of calls where [[Writable]] is first set to true, a new
[[Value]] is set, and then [[Writable]] is set to false.

9 Type Conversion and Testing
=============================

The ECMAScript runtime system performs automatic type conversion as needed. To clarify the semantics
of certain constructs it is useful to define a set of conversion abstract operations. These abstract
operations are not a part of the language; they are defined here to aid the specification of the
semantics of the language. The conversion abstract operations are polymorphic; that is, they can
accept a value of any ECMAScript language type, but not of specification types.

9.1 ToPrimitive
===============

The abstract operation ToPrimitive takes an input argument and an optional argument PreferredType.
The abstract operation ToPrimitive converts its input argument to a non-Object type. If an object is
capable of converting to more than one primitive type, it may use the optional hint PreferredType to
favour that type. Conversion occurs according to Table 10:

Table 10 — ToPrimitive Conversions

  Input Type   Result
  ------------ --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
  Undefined    The result equals the input argument (no conversion).
  Null         The result equals the input argument (no conversion).
  Boolean      The result equals the input argument (no conversion).
  Number       The result equals the input argument (no conversion).
  String       The result equals the input argument (no conversion).
  Object       Return a default value for the Object. The default value of an object is retrieved by calling the [[DefaultValue]] internal method of the object, passing the optional hint PreferredType. The behaviour of the [[DefaultValue]] internal method is defined by this specification for all native ECMAScript objects in 8.12.8.

9.2 ToBoolean
=============

The abstract operation ToBoolean converts its argument to a value of type Boolean according to Table
11:

Table 11 — ToBoolean Conversions

  Argument Type   Result
  --------------- -------------------------------------------------------------------------------------------------------------
  Undefined       false
  Null            false
  Boolean         The result equals the input argument (no conversion).
  Number          The result is false if the argument is +0, −0, or NaN; otherwise the result is true.
  String          The result is false if the argument is the empty String (its length is zero); otherwise the result is true.
  Object          true

9.3 ToNumber
============

The abstract operation ToNumber converts its argument to a value of type Number according to Table
12:

Table 12 — To Number Conversions

+----------------------------------------------------+----------------------------------------------------+
| Argument Type                                      | Result                                             |
+====================================================+====================================================+
| Undefined                                          | NaN                                                |
+----------------------------------------------------+----------------------------------------------------+
| Null                                               | +0                                                 |
+----------------------------------------------------+----------------------------------------------------+
| Boolean                                            | The result is 1 if the argument is true. The       |
|                                                    | result is +0 if the argument is false.             |
+----------------------------------------------------+----------------------------------------------------+
| Number                                             | The result equals the input argument (no           |
|                                                    | conversion).                                       |
+----------------------------------------------------+----------------------------------------------------+
| String                                             | See grammar and note below.                        |
+----------------------------------------------------+----------------------------------------------------+
| Object                                             | Apply the following steps:                         |
|                                                    |                                                    |
|                                                    | 1.  Let primValue be ToPrimitive(input argument,   |
|                                                    |     hint Number).                                  |
|                                                    | 2.  Return ToNumber(primValue).                    |
+----------------------------------------------------+----------------------------------------------------+

9.3.1 ToNumber Applied to the String Type
=========================================

ToNumber applied to Strings applies the following grammar to the input String. If the grammar cannot
interpret the String as an expansion of StringNumericLiteral, then the result of ToNumber is NaN.

Syntax
------

StringNumericLiteral :::

StrWhiteSpaceopt

StrWhiteSpaceopt StrNumericLiteral StrWhiteSpaceopt

StrWhiteSpace :::

StrWhiteSpaceChar StrWhiteSpaceopt

StrWhiteSpaceChar :::

WhiteSpace

LineTerminator

StrNumericLiteral :::

StrDecimalLiteral

HexIntegerLiteral

StrDecimalLiteral :::

StrUnsignedDecimalLiteral

+ StrUnsignedDecimalLiteral

- StrUnsignedDecimalLiteral

StrUnsignedDecimalLiteral :::

Infinity

DecimalDigits . DecimalDigitsopt ExponentPartopt

. DecimalDigits ExponentPartopt

DecimalDigits ExponentPartopt

DecimalDigits :::

DecimalDigit

DecimalDigits DecimalDigit

DecimalDigit ::: one of

0 1 2 3 4 5 6 7 8 9

ExponentPart :::

ExponentIndicator SignedInteger

ExponentIndicator ::: one of

e E

SignedInteger :::

DecimalDigits

+ DecimalDigits

- DecimalDigits

HexIntegerLiteral :::

0x HexDigit

0X HexDigit

HexIntegerLiteral HexDigit

HexDigit ::: one of

0 1 2 3 4 5 6 7 8 9 a b c d e f A B C D E F

Some differences should be noted between the syntax of a StringNumericLiteral and a NumericLiteral
(see 7.8.3):

-   A StringNumericLiteral may be preceded and/or followed by white space and/or line terminators.

-   A StringNumericLiteral that is decimal may have any number of leading 0 digits.

-   A StringNumericLiteral that is decimal may be preceded by + or - to indicate its sign.

-   A StringNumericLiteral that is empty or contains only white space is converted to +0.

The conversion of a String to a Number value is similar overall to the determination of the Number
value for a numeric literal (see 7.8.3), but some of the details are different, so the process for
converting a String numeric literal to a value of Number type is given here in full. This value is
determined in two steps: first, a mathematical value (MV) is derived from the String numeric
literal; second, this mathematical value is rounded as described below.

-   The MV of StringNumericLiteral ::: [empty] is 0.

-   The MV of StringNumericLiteral ::: StrWhiteSpace is 0.

-   The MV of StringNumericLiteral ::: StrWhiteSpaceopt StrNumericLiteral StrWhiteSpaceopt is the MV
    of StrNumericLiteral, no matter whether white space is present or not.

-   The MV of StrNumericLiteral ::: StrDecimalLiteral is the MV of StrDecimalLiteral.

-   The MV of StrNumericLiteral ::: HexIntegerLiteral is the MV of HexIntegerLiteral.

-   The MV of StrDecimalLiteral ::: StrUnsignedDecimalLiteral is the MV of
    StrUnsignedDecimalLiteral.

-   The MV of StrDecimalLiteral ::: + StrUnsignedDecimalLiteral is the MV of
    StrUnsignedDecimalLiteral.

-   The MV of StrDecimalLiteral ::: - StrUnsignedDecimalLiteral is the negative of the MV of
    StrUnsignedDecimalLiteral. (Note that if the MV of StrUnsignedDecimalLiteral is 0, the negative
    of this MV is also 0. The rounding rule described below handles the conversion of this signless
    mathematical zero to a floating-point +0 or −0 as appropriate.)

-   The MV of StrUnsignedDecimalLiteral ::: Infinity is 10^10000 (a value so large that it will
    round to +∞).

-   The MV of StrUnsignedDecimalLiteral ::: DecimalDigits . is the MV of DecimalDigits.

-   The MV of StrUnsignedDecimalLiteral ::: DecimalDigits . DecimalDigits is the MV of the first
    DecimalDigits plus (the MV of the second DecimalDigits times 10−n), where n is the number of
    characters in the second DecimalDigits.

-   The MV of StrUnsignedDecimalLiteral ::: DecimalDigits . ExponentPart is the MV of DecimalDigits
    times 10e, where e is the MV of ExponentPart.

-   The MV of StrUnsignedDecimalLiteral ::: DecimalDigits . DecimalDigits ExponentPart is (the MV of
    the first DecimalDigits plus (the MV of the second DecimalDigits times 10−n)) times 10e, where n
    is the number of characters in the second DecimalDigits and e is the MV of ExponentPart.

-   The MV of StrUnsignedDecimalLiteral ::: . DecimalDigits is the MV of DecimalDigits times 10−n,
    where n is the number of characters in DecimalDigits.

-   The MV of StrUnsignedDecimalLiteral ::: . DecimalDigits ExponentPart is the MV of DecimalDigits
    times 10e−n, where n is the number of characters in DecimalDigits and e is the MV of
    ExponentPart.

-   The MV of StrUnsignedDecimalLiteral ::: DecimalDigits is the MV of DecimalDigits.

-   The MV of StrUnsignedDecimalLiteral ::: DecimalDigits ExponentPart is the MV of DecimalDigits
    times 10e, where e is the MV of ExponentPart.

-   The MV of DecimalDigits ::: DecimalDigit is the MV of DecimalDigit.

-   The MV of DecimalDigits ::: DecimalDigits DecimalDigit is (the MV of DecimalDigits times 10)
    plus the MV of DecimalDigit.

-   The MV of ExponentPart ::: ExponentIndicator SignedInteger is the MV of SignedInteger.

-   The MV of SignedInteger ::: DecimalDigits is the MV of DecimalDigits.

-   The MV of SignedInteger ::: + DecimalDigits is the MV of DecimalDigits.

-   The MV of SignedInteger ::: - DecimalDigits is the negative of the MV of DecimalDigits.

-   The MV of DecimalDigit ::: 0 or of HexDigit ::: 0 is 0.

-   The MV of DecimalDigit ::: 1 or of HexDigit ::: 1 is 1.

-   The MV of DecimalDigit ::: 2 or of HexDigit ::: 2 is 2.

-   The MV of DecimalDigit ::: 3 or of HexDigit ::: 3 is 3.

-   The MV of DecimalDigit ::: 4 or of HexDigit ::: 4 is 4.

-   The MV of DecimalDigit ::: 5 or of HexDigit ::: 5 is 5.

-   The MV of DecimalDigit ::: 6 or of HexDigit ::: 6 is 6.

-   The MV of DecimalDigit ::: 7 or of HexDigit ::: 7 is 7.

-   The MV of DecimalDigit ::: 8 or of HexDigit ::: 8 is 8.

-   The MV of DecimalDigit ::: 9 or of HexDigit ::: 9 is 9.

-   The MV of HexDigit ::: a or of HexDigit ::: A is 10.

-   The MV of HexDigit ::: b or of HexDigit ::: B is 11.

-   The MV of HexDigit ::: c or of HexDigit ::: C is 12.

-   The MV of HexDigit ::: d or of HexDigit ::: D is 13.

-   The MV of HexDigit ::: e or of HexDigit ::: E is 14.

-   The MV of HexDigit ::: f or of HexDigit ::: F is 15.

-   The MV of HexIntegerLiteral ::: 0x HexDigit is the MV of HexDigit.

-   The MV of HexIntegerLiteral ::: 0X HexDigit is the MV of HexDigit.

-   The MV of HexIntegerLiteral ::: HexIntegerLiteral HexDigit is (the MV of HexIntegerLiteral times
    16) plus the MV of HexDigit.

Once the exact MV for a String numeric literal has been determined, it is then rounded to a value of
the Number type. If the MV is 0, then the rounded value is +0 unless the first non white space
character in the String numeric literal is ‘-’, in which case the rounded value is −0. Otherwise,
the rounded value must be the Number value for the MV (in the sense defined in 8.5), unless the
literal includes a StrUnsignedDecimalLiteral and the literal has more than 20 significant digits, in
which case the Number value may be either the Number value for the MV of a literal produced by
replacing each significant digit after the 20th with a 0 digit or the Number value for the MV of a
literal produced by replacing each significant digit after the 20th with a 0 digit and then
incrementing the literal at the 20th digit position. A digit is significant if it is not part of an
ExponentPart and

-   it is not 0; or
-   there is a nonzero digit to its left and there is a nonzero digit, not in the ExponentPart, to
    its right.

9.4 ToInteger
=============

The abstract operation ToInteger converts its argument to an integral numeric value. This abstract
operation functions as follows:

1.  Let number be the result of calling ToNumber on the input argument.
2.  If number is NaN, return +0.
3.  If number is +0, −0, +∞, or −∞, return number.
4.  Return the result of computing sign(number) × floor(abs(number)).

9.5 ToInt32: (Signed 32 Bit Integer)
====================================

The abstract operation ToInt32 converts its argument to one of 2^32 integer values in the range
−2^31 through 2^31−1, inclusive. This abstract operation functions as follows:

1.  Let number be the result of calling ToNumber on the input argument.
2.  If number is NaN, +0, −0, +∞, or −∞, return +0.
3.  Let posInt be sign(number) * floor(abs(number)).
4.  Let int32bit be posInt modulo 2^32; that is, a finite integer value k of Number type with
    positive sign and less than 2^32 in magnitude such that the mathematical difference of posInt
    and k is mathematically an integer multiple of 2^32.
5.  If int32bit is greater than or equal to 2^31, return int32bit − 2^32, otherwise return int32bit.

NOTE Given the above definition of ToInt32:

-   The ToInt32 abstract operation is idempotent: if applied to a result that it produced, the
    second application leaves that value unchanged.

-   ToInt32(ToUint32(x)) is equal to ToInt32(x) for all values of x. (It is to preserve this latter
    property that +∞ and −∞ are mapped to +0.)

-   ToInt32 maps −0 to +0.

9.6 ToUint32: (Unsigned 32 Bit Integer)
=======================================

The abstract operation ToUint32 converts its argument to one of 2^32 integer values in the range 0
through 2^32−1, inclusive. This abstraction operation functions as follows:

1.  Let number be the result of calling ToNumber on the input argument.
2.  If number is NaN, +0, −0, +∞, or −∞, return +0.
3.  Let posInt be sign(number) × floor(abs(number)).
4.  Let int32bit be posInt modulo 2^32; that is, a finite integer value k of Number type with
    positive sign and less than 2^32 in magnitude such that the mathematical difference of posInt
    and k is mathematically an integer multiple of 2^32.
5.  Return int32bit.

NOTE Given the above definition of ToUInt32:

-   Step 5 is the only difference between ToUint32 and ToInt32.

-   The ToUint32 abstract operation is idempotent: if applied to a result that it produced, the
    second application leaves that value unchanged.

-   ToUint32(ToInt32(x)) is equal to ToUint32(x) for all values of x. (It is to preserve this latter
    property that +∞ and −∞ are mapped to +0.)

-   ToUint32 maps −0 to +0.

9.7 ToUint16: (Unsigned 16 Bit Integer)
=======================================

The abstract operation ToUint16 converts its argument to one of 2^16 integer values in the range 0
through 2^16−1, inclusive. This abstract operation functions as follows:

1.  Let number be the result of calling ToNumber on the input argument.
2.  If number is NaN, +0, −0, +∞, or −∞, return +0.
3.  Let posInt be sign(number) × floor(abs(number)).
4.  Let int16bit be posInt modulo 2^16; that is, a finite integer value k of Number type with
    positive sign and less than 2^16 in magnitude such that the mathematical difference of posInt
    and k is mathematically an integer multiple of 2^16.
5.  Return int16bit.

NOTE Given the above definition of ToUint16:

-   The substitution of 2^16 for 2^32 in step 4 is the only difference between ToUint32 and
    ToUint16.
-   ToUint16 maps −0 to +0.

9.8 ToString
============

The abstract operation ToString converts its argument to a value of type String according to Table
13:

Table 13 — ToString Conversions

+----------------------------------------------------+----------------------------------------------------+
| Argument Type                                      | Result                                             |
+====================================================+====================================================+
| Undefined                                          | "undefined"                                        |
+----------------------------------------------------+----------------------------------------------------+
| Null                                               | "null"                                             |
+----------------------------------------------------+----------------------------------------------------+
| Boolean                                            | If the argument is true, then the result is        |
|                                                    | "true".                                            |
|                                                    |                                                    |
|                                                    | If the argument is false, then the result is       |
|                                                    | “false”.                                           |
+----------------------------------------------------+----------------------------------------------------+
| Number                                             | See 9.8.1.                                         |
+----------------------------------------------------+----------------------------------------------------+
| String                                             | Return the input argument (no conversion)          |
+----------------------------------------------------+----------------------------------------------------+
| Object                                             | Apply the following steps:                         |
|                                                    |                                                    |
|                                                    | 1. Let primValue be ToPrimitive(input argument,    |
|                                                    | hint String).                                      |
|                                                    |                                                    |
|                                                    | 2. Return ToString(primValue).                     |
+----------------------------------------------------+----------------------------------------------------+

9.8.1 ToString Applied to the Number Type
=========================================

The abstract operation ToString converts a Number m to String format as follows:

1.  If m is NaN, return the String "NaN".
2.  If m is +0 or −0, return the String "0".
3.  If m is less than zero, return the String concatenation of the String "-" and ToString(−m).
4.  If m is infinity, return the String "Infinity".
5.  Otherwise, let n, k, and s be integers such that k ≥ 1, 10k−1 ≤ s < 10k, the Number value for s
    × 10n−k is m, and k is as small as possible. Note that k is the number of digits in the decimal
    representation of s, that s is not divisible by 10, and that the least significant digit of s is
    not necessarily uniquely determined by these criteria.
6.  If k ≤ n ≤ 21, return the String consisting of the k digits of the decimal representation of s
    (in order, with no leading zeroes), followed by n−k occurrences of the character ‘0’.
7.  If 0 < n ≤ 21, return the String consisting of the most significant n digits of the decimal
    representation of s, followed by a decimal point ‘.’, followed by the remaining k−n digits of
    the decimal representation of s.
8.  If −6 < n ≤ 0, return the String consisting of the character ‘0’, followed by a decimal point
    ‘.’, followed by −n occurrences of the character ‘0’, followed by the k digits of the decimal
    representation of s.
9.  Otherwise, if k = 1, return the String consisting of the single digit of s, followed by
    lowercase character ‘e’, followed by a plus sign ‘+’ or minus sign ‘−’ according to whether n−1
    is positive or negative, followed by the decimal representation of the integer abs(n−1) (with no
    leading zeroes).
10. Return the String consisting of the most significant digit of the decimal representation of s,
    followed by a decimal point ‘.’, followed by the remaining k−1 digits of the decimal
    representation of s, followed by the lowercase character ‘e’, followed by a plus sign ‘+’ or
    minus sign ‘−’ according to whether n−1 is positive or negative, followed by the decimal
    representation of the integer abs(n−1) (with no leading zeroes).

NOTE 1 The following observations may be useful as guidelines for implementations, but are not part
of the normative requirements of this Standard:

-   If x is any Number value other than −0, then ToNumber(ToString(x)) is exactly the same Number
    value as x.

-   The least significant digit of s is not always uniquely determined by the requirements listed in
    step 5.

NOTE 2 For implementations that provide more accurate conversions than required by the rules above,
it is recommended that the following alternative version of step 5 be used as a guideline:

Otherwise, let n, k, and s be integers such that k ≥ 1, 10k−1 ≤ s < 10k, the Number value for s ×
10n−k is m, and k is as small as possible. If there are multiple possibilities for s, choose the
value of s for which s × 10n−k is closest in value to m. If there are two such possible values of s,
choose the one that is even. Note that k is the number of digits in the decimal representation of s
and that s is not divisible by 10.

NOTE 3 Implementers of ECMAScript may find useful the paper and code written by David M. Gay for
binary-to-decimal conversion of floating-point numbers:

Gay, David M. Correctly Rounded Binary-Decimal and Decimal-Binary Conversions. Numerical Analysis,
Manuscript 90-10. AT&T Bell Laboratories (Murray Hill, New Jersey). November 30, 1990. Available
as  
http://cm.bell-labs.com/cm/cs/doc/90/4-10.ps.gz. Associated code available as  
http://cm.bell-labs.com/netlib/fp/dtoa.c.gz and as  
http://cm.bell-labs.com/netlib/fp/g_fmt.c.gz and may also be found at the various netlib mirror
sites.

9.9 ToObject
============

The abstract operation ToObject converts its argument to a value of type Object according to Table
14:

Table 14 — ToObject

  Argument Type   Result
  --------------- ------------------------------------------------------------------------------------------------------------------------------------------------------------
  Undefined       Throw a TypeError exception.
  Null            Throw a TypeError exception.
  Boolean         Create a new Boolean object whose [[PrimitiveValue]] internal property is set to the value of the argument. See 15.6 for a description of Boolean objects.
  Number          Create a new Number object whose [[PrimitiveValue]] internal property is set to the value of the argument. See 15.7 for a description of Number objects.
  String          Create a new String object whose [[PrimitiveValue]] internal property is set to the value of the argument. See 15.5 for a description of String objects.
  Object          The result is the input argument (no conversion).

9.10 CheckObjectCoercible
=========================

The abstract operation CheckObjectCoercible throws an error if its argument is a value that cannot
be converted to an Object using ToObject. It is defined by Table 15:

Table 15 — CheckObjectCoercible Results

  Argument Type   Result
  --------------- ------------------------------
  Undefined       Throw a TypeError exception.
  Null            Throw a TypeError exception.
  Boolean         Return
  Number          Return
  String          Return
  Object          Return

9.11 IsCallable
===============

The abstract operation IsCallable determines if its argument, which must be an ECMAScript language
value, is a callable function Object according to Table 16:

Table 16 — IsCallable Results

  Argument Type   Result
  --------------- --------------------------------------------------------------------------------------------------
  Undefined       Return false.
  Null            Return false.
  Boolean         Return false.
  Number          Return false.
  String          Return false.
  Object          If the argument object has a [[Call]] internal method, then return true, otherwise return false.

9.12 The SameValue Algorithm
============================

The internal comparison abstract operation SameValue(x, y), where x and y are ECMAScript language
values, produces true or false. Such a comparison is performed as follows:

1.  If Type(x) is different from Type(y), return false.
2.  If Type(x) is Undefined, return true.
3.  If Type(x) is Null, return true.
4.  If Type(x) is Number, then.
    1.  If x is NaN and y is NaN, return true.
    2.  If x is +0 and y is -0, return false.
    3.  If x is -0 and y is +0, return false.
    4.  If x is the same Number value as y, return true.
    5.  Return false.

5.  If Type(x) is String, then return true if x and y are exactly the same sequence of characters
    (same length and same characters in corresponding positions); otherwise, return false.
6.  If Type(x) is Boolean, return true if x and y are both true or both false; otherwise, return
    false.
7.  Return true if x and y refer to the same object. Otherwise, return false.

10 Executable Code and Execution Contexts
=========================================

10.1 Types of Executable Code
=============================

There are three types of ECMAScript executable code:

-   Global code is source text that is treated as an ECMAScript Program. The global code of a
    particular Program does not include any source text that is parsed as part of a FunctionBody.

-   Eval code is the source text supplied to the built-in eval function. More precisely, if the
    parameter to the built-in eval function is a String, it is treated as an ECMAScript Program. The
    eval code for a particular invocation of eval is the global code portion of that Program.

-   Function code is source text that is parsed as part of a FunctionBody. The function code of a
    particular FunctionBody does not include any source text that is parsed as part of a nested
    FunctionBody. Function code also denotes the source text supplied when using the built-in
    Function object as a constructor. More precisely, the last parameter provided to the Function
    constructor is converted to a String and treated as the FunctionBody. If more than one parameter
    is provided to the Function constructor, all parameters except the last one are converted to
    Strings and concatenated together, separated by commas. The resulting String is interpreted as
    the FormalParameterList for the FunctionBody defined by the last parameter. The function code
    for a particular instantiation of a Function does not include any source text that is parsed as
    part of a nested FunctionBody.

10.1.1 Strict Mode Code
=======================

An ECMAScript Program syntactic unit may be processed using either unrestricted or strict mode
syntax and semantics. When processed using strict mode the three types of ECMAScript code are
referred to as strict global code, strict eval code, and strict function code. Code is interpreted
as strict mode code in the following situations:

-   Global code is strict global code if it begins with a Directive Prologue that contains a Use
    Strict Directive (see 14.1).

-   Eval code is strict eval code if it begins with a Directive Prologue that contains a Use Strict
    Directive or if the call to eval is a direct call (see 15.1.2.1.1) to the eval function that is
    contained in strict mode code.

-   Function code that is part of a FunctionDeclaration, FunctionExpression, or accessor
    PropertyAssignment is strict function code if its FunctionDeclaration, FunctionExpression, or
    PropertyAssignment is contained in strict mode code or if the function code begins with a
    Directive Prologue that contains a Use Strict Directive.

-   Function code that is supplied as the last argument to the built-in Function constructor is
    strict function code if the last argument is a String that when processed as a FunctionBody
    begins with a Directive Prologue that contains a Use Strict Directive.

10.2 Lexical Environments
=========================

A Lexical Environment is a specification type used to define the association of Identifiers to
specific variables and functions based upon the lexical nesting structure of ECMAScript code. A
Lexical Environment consists of an Environment Record and a possibly null reference to an outer
Lexical Environment. Usually a Lexical Environment is associated with some specific syntactic
structure of ECMAScript code such as a FunctionDeclaration, a WithStatement, or a Catch clause of a
TryStatement and a new Lexical Environment is created each time such code is evaluated.

An Environment Record records the identifier bindings that are created within the scope of its
associated Lexical Environment.

The outer environment reference is used to model the logical nesting of Lexical Environment values.
The outer reference of a (inner) Lexical Environment is a reference to the Lexical Environment that
logically surrounds the inner Lexical Environment. An outer Lexical Environment may, of course, have
its own outer Lexical Environment. A Lexical Environment may serve as the outer environment for
multiple inner Lexical Environments. For example, if a FunctionDeclaration contains two nested
FunctionDeclarations then the Lexical Environments of each of the nested functions will have as
their outer Lexical Environment the Lexical Environment of the current execution of the surrounding
function.

Lexical Environments and Environment Record values are purely specification mechanisms and need not
correspond to any specific artefact of an ECMAScript implementation. It is impossible for an
ECMAScript program to directly access or manipulate such values.

10.2.1 Environment Records
==========================

There are two kinds of Environment Record values used in this specification: declarative environment
records and object environment records. Declarative environment records are used to define the
effect of ECMAScript language syntactic elements such as FunctionDeclarations, VariableDeclarations,
and Catch clauses that directly associate identifier bindings with ECMAScript language values.
Object environment records are used to define the effect of ECMAScript elements such as Program and
WithStatement that associate identifier bindings with the properties of some object.

For specification purposes Environment Record values can be thought of as existing in a simple
object-oriented hierarchy where Environment Record is an abstract class with two concrete
subclasses, declarative environment record and object environment record. The abstract class
includes the abstract specification methods defined in Table 17. These abstract methods have
distinct concrete algorithms for each of the concrete subclasses.

Table 17 — Abstract Methods of Environment Records

  Method                       Purpose
  ---------------------------- ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
  HasBinding(N)                Determine if an environment record has a binding for an identifier. Return true if it does and false if it does not. The String value N is the text of the identifier.
  CreateMutableBinding(N, D)   Create a new mutable binding in an environment record. The String value N is the text of the bound name. If the optional Boolean argument D is true the binding is may be subsequently deleted.
  SetMutableBinding(N,V, S)    Set the value of an already existing mutable binding in an environment record. The String value N is the text of the bound name. V is the value for the binding and may be a value of any ECMAScript language type. S is a Boolean flag. If S is true and the binding cannot be set throw a TypeError exception. S is used to identify strict mode references.
  GetBindingValue(N,S)         Returns the value of an already existing binding from an environment record. The String value N is the text of the bound name. S is used to identify strict mode references. If S is true and the binding does not exist or is uninitialised throw a ReferenceError exception.
  DeleteBinding(N)             Delete a binding from an environment record. The String value N is the text of the bound name If a binding for N exists, remove the binding and return true. If the binding exists but cannot be removed return false. If the binding does not exist return true.
  ImplicitThisValue()          Returns the value to use as the this value on calls to function objects that are obtained as binding values from this environment record.

10.2.1.1 Declarative Environment Records
========================================

Each declarative environment record is associated with an ECMAScript program scope containing
variable and/or function declarations. A declarative environment record binds the set of identifiers
defined by the declarations contained within its scope.

In addition to the mutable bindings supported by all Environment Records, declarative environment
records also provide for immutable bindings. An immutable binding is one where the association
between an identifier and a value may not be modified once it has been established. Creation and
initialisation of immutable binding are distinct steps so it is possible for such bindings to exist
in either an initialised or uninitialised state. Declarative environment records support the methods
listed in Table 18 in addition to the Environment Record abstract specification methods:

Table 18 — Additional Methods of Declarative Environment Records

  Method                            Purpose
  --------------------------------- -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
  CreateImmutableBinding(N)         Create a new but uninitialised immutable binding in an environment record. The String value N is the text of the bound name.
  InitializeImmutableBinding(N,V)   Set the value of an already existing but uninitialised immutable binding in an environment record. The String value N is the text of the bound name. V is the value for the binding and is a value of any ECMAScript language type.

The behaviour of the concrete specification methods for Declarative Environment Records is defined
by the following algorithms.

10.2.1.1.1 HasBinding(N)
========================

The concrete environment record method HasBinding for declarative environment records simply
determines if the argument identifier is one of the identifiers bound by the record:

1.  Let envRec be the declarative environment record for which the method was invoked.
2.  If envRec has a binding for the name that is the value of N, return true.
3.  If it does not have such a binding, return false.

10.2.1.1.2 CreateMutableBinding (N, D)
======================================

The concrete Environment Record method CreateMutableBinding for declarative environment records
creates a new mutable binding for the name N that is initialised to the value undefined. A binding
must not already exist in this Environment Record for N. If Boolean argument D is provided and has
the value true the new binding is marked as being subject to deletion.

1.  Let envRec be the declarative environment record for which the method was invoked.
2.  Assert: envRec does not already have a binding for N.
3.  Create a mutable binding in envRec for N and set its bound value to undefined. If D is true
    record that the newly created binding may be deleted by a subsequent DeleteBinding call.

10.2.1.1.3 SetMutableBinding (N,V,S)
====================================

The concrete Environment Record method SetMutableBinding for declarative environment records
attempts to change the bound value of the current binding of the identifier whose name is the value
of the argument N to the value of argument V. A binding for N must already exist. If the binding is
an immutable binding, a TypeError is thrown if S is true.

1.  Let envRec be the declarative environment record for which the method was invoked.
2.  Assert: envRec must have a binding for N.
3.  If the binding for N in envRec is a mutable binding, change its bound value to V.
4.  Else this must be an attempt to change the value of an immutable binding so if S if true throw a
    TypeError exception.

10.2.1.1.4 GetBindingValue(N,S)
===============================

The concrete Environment Record method GetBindingValue for declarative environment records simply
returns the value of its bound identifier whose name is the value of the argument N. The binding
must already exist. If S is true and the binding is an uninitialised immutable binding throw a
ReferenceError exception.

1.  Let envRec be the declarative environment record for which the method was invoked.
2.  Assert: envRec has a binding for N.
3.  If the binding for N in envRec is an uninitialised immutable binding, then
    1.  If S is false, return the value undefined, otherwise throw a ReferenceError exception.

4.  Else, return the value currently bound to N in envRec.

10.2.1.1.5 DeleteBinding (N)
============================

The concrete Environment Record method DeleteBinding for declarative environment records can only
delete bindings that have been explicitly designated as being subject to deletion.

1.  Let envRec be the declarative environment record for which the method was invoked.
2.  If envRec does not have a binding for the name that is the value of N, return true.
3.  If the binding for N in envRec is cannot be deleted, return false.
4.  Remove the binding for N from envRec.
5.  Return true.

10.2.1.1.6 ImplicitThisValue()
==============================

Declarative Environment Records always return undefined as their ImplicitThisValue.

1.  Return undefined.

10.2.1.1.7 CreateImmutableBinding (N)
=====================================

The concrete Environment Record method CreateImmutableBinding for declarative environment records
creates a new immutable binding for the name N that is initialised to the value undefined. A binding
must not already exist in this environment record for N.

1.  Let envRec be the declarative environment record for which the method was invoked.
2.  Assert: envRec does not already have a binding for N.
3.  Create an immutable binding in envRec for N and record that it is uninitialised.

10.2.1.1.8 InitializeImmutableBinding (N,V)
===========================================

The concrete Environment Record method InitializeImmutableBinding for declarative environment
records is used to set the bound value of the current binding of the identifier whose name is the
value of the argument N to the value of argument V. An uninitialised immutable binding for N must
already exist.

1.  Let envRec be the declarative environment record for which the method was invoked.
2.  Assert: envRec must have an uninitialised immutable binding for N.
3.  Set the bound value for N in envRec to V.
4.  Record that the immutable binding for N in envRec has been initialised.

10.2.1.2 Object Environment Records
===================================

Each object environment record is associated with an object called its binding object. An object
environment record binds the set of identifier names that directly correspond to the property names
of its binding object. Property names that are not an IdentifierName are not included in the set of
bound identifiers. Both own and inherited properties are included in the set regardless of the
setting of their [[Enumerable]] attribute. Because properties can be dynamically added and deleted
from objects, the set of identifiers bound by an object environment record may potentially change as
a side-effect of any operation that adds or deletes properties. Any bindings that are created as a
result of such a side-effect are considered to be a mutable binding even if the Writable attribute
of the corresponding property has the value false. Immutable bindings do not exist for object
environment records.

Object environment records can be configured to provide their binding object as an implicit this
value for use in function calls. This capability is used to specify the behaviour of With Statement
(12.10) induced bindings. The capability is controlled by a provideThis Boolean value that is
associated with each object environment record. By default, the value of provideThis is false for
any object environment record.

The behaviour of the concrete specification methods for Object Environment Records is defined by the
following algorithms.

10.2.1.2.1 HasBinding(N)
========================

The concrete Environment Record method HasBinding for object environment records determines if its
associated binding object has a property whose name is the value of the argument N:

1.  Let envRec be the object environment record for which the method was invoked.
2.  Let bindings be the binding object for envRec.
3.  Return the result of calling the [[HasProperty]] internal method of bindings, passing N as the
    property name.

10.2.1.2.2 CreateMutableBinding (N, D)
======================================

The concrete Environment Record method CreateMutableBinding for object environment records creates
in an environment record’s associated binding object a property whose name is the String value and
initialises it to the value undefined. A property named N must not already exist in the binding
object. If Boolean argument D is provided and has the value true the new property’s [[Configurable]]
attribute is set to true, otherwise it is set to false.

1.  Let envRec be the object environment record for which the method was invoked.
2.  Let bindings be the binding object for envRec.
3.  Assert: The result of calling the [[HasProperty]] internal method of bindings, passing N as the
    property name, is false.
4.  If D is true then let configValue be true otherwise let configValue be false.
5.  Call the [[DefineOwnProperty]] internal method of bindings, passing N, Property Descriptor
    {[[Value]]:undefined, [[Writable]]: true, [[Enumerable]]: true , [[Configurable]]: configValue},
    and true as arguments.

10.2.1.2.3 SetMutableBinding (N,V,S)
====================================

The concrete Environment Record method SetMutableBinding for object environment records attempts to
set the value of the environment record’s associated binding object’s property whose name is the
value of the argument N to the value of argument V. A property named N should already exist but if
it does not or is not currently writable, error handling is determined by the value of the Boolean
argument S.

1.  Let envRec be the object environment record for which the method was invoked.
2.  Let bindings be the binding object for envRec.
3.  Call the [[Put]] internal method of bindings with arguments N, V, and S.

10.2.1.2.4 GetBindingValue(N,S)
===============================

The concrete Environment Record method GetBindingValue for object environment records returns the
value of its associated binding object’s property whose name is the String value of the argument
identifier N. The property should already exist but if it does not the result depends upon the value
of the S argument:

1.  Let envRec be the object environment record for which the method was invoked.
2.  Let bindings be the binding object for envRec.
3.  Let value be the result of calling the [[HasProperty]] internal method of bindings, passing N as
    the property name.
4.  If value is false, then
    1.  If S is false, return the value undefined, otherwise throw a ReferenceError exception.

5.  Return the result of calling the [[Get]] internal method of bindings, passing N for the
    argument.

10.2.1.2.5 DeleteBinding (N)
============================

The concrete Environment Record method DeleteBinding for object environment records can only delete
bindings that correspond to properties of the environment object whose [[Configurable]] attribute
have the value true.

1.  Let envRec be the object environment record for which the method was invoked.
2.  Let bindings be the binding object for envRec.
3.  Return the result of calling the [[Delete]] internal method of bindings, passing N and false as
    arguments.

10.2.1.2.6 ImplicitThisValue()
==============================

Object Environment Records return undefined as their ImplicitThisValue unless their provideThis flag
is true.

1.  Let envRec be the object environment record for which the method was invoked.
2.  If the provideThis flag of envRec is true, return the binding object for envRec.
3.  Otherwise, return undefined.

10.2.2 Lexical Environment Operations
=====================================

The following abstract operations are used in this specification to operate upon lexical
environments:

10.2.2.1 GetIdentifierReference (lex, name, strict)
===================================================

The abstract operation GetIdentifierReference is called with a Lexical Environment lex, an
identifier String name, and a Boolean flag strict. The value of lex may be null. When called, the
following steps are performed:

1.  If lex is the value null, then
    1.  Return a value of type Reference whose base value is undefined, whose referenced name is
        name, and whose strict mode flag is strict.

2.  Let envRec be lex’s environment record.
3.  Let exists be the result of calling the HasBinding(N) concrete method of envRec passing name as
    the argument N.
4.  If exists is true, then
    1.  Return a value of type Reference whose base value is envRec, whose referenced name is name,
        and whose strict mode flag is strict.

5.  Else
    1.  Let outer be the value of lex’s outer environment reference.
    2.  Return the result of calling GetIdentifierReference passing outer, name, and strict as
        arguments.

10.2.2.2 NewDeclarativeEnvironment (E)
======================================

When the abstract operation NewDeclarativeEnvironment is called with either a Lexical Environment or
null as argument E the following steps are performed:

1.  Let env be a new Lexical Environment.
2.  Let envRec be a new declarative environment record containing no bindings.
3.  Set env’s environment record to be envRec.
4.  Set the outer lexical environment reference of env to E.
5.  Return env.

10.2.2.3 NewObjectEnvironment (O, E)
====================================

When the abstract operation NewObjectEnvironment is called with an Object O and a Lexical
Environment E (or null) as arguments, the following steps are performed:

1.  Let env be a new Lexical Environment.
2.  Let envRec be a new object environment record containing O as the binding object.
3.  Set env’s environment record to be envRec.
4.  Set the outer lexical environment reference of env to E.
5.  Return env.

10.2.3 The Global Environment
=============================

The global environment is a unique Lexical Environment which is created before any ECMAScript code
is executed. The global environment’s Environment Record is an object environment record whose
binding object is the global object (15.1). The global environment’s outer environment reference is
null.

As ECMAScript code is executed, additional properties may be added to the global object and the
initial properties may be modified.

10.3 Execution Contexts
=======================

When control is transferred to ECMAScript executable code, control is entering an execution context.
Active execution contexts logically form a stack. The top execution context on this logical stack is
the running execution context. A new execution context is created whenever control is transferred
from the executable code associated with the currently running execution context to executable code
that is not associated with that execution context. The newly created execution context is pushed
onto the stack and becomes the running execution context.

An execution context contains whatever state is necessary to track the execution progress of its
associated code. In addition, each execution context has the state components listed in Table 19.

Table 19 —Execution Context State Components

  Component             Purpose
  --------------------- ------------------------------------------------------------------------------------------------------------------------------------------------------------------
  LexicalEnvironment    Identifies the Lexical Environment used to resolve identifier references made by code within this execution context.
  VariableEnvironment   Identifies the Lexical Environment whose environment record holds bindings created by VariableStatements and FunctionDeclarations within this execution context.
  ThisBinding           The value associated with the this keyword within ECMAScript code associated with this execution context.

The LexicalEnvironment and VariableEnvironment components of an execution context are always Lexical
Environments. When an execution context is created its LexicalEnvironment and VariableEnvironment
components initially have the same value. The value of the VariableEnvironment component never
changes while the value of the LexicalEnvironment component may change during execution of code
within an execution context.

In most situations only the running execution context (the top of the execution context stack) is
directly manipulated by algorithms within this specification. Hence when the terms
“LexicalEnvironment”, “VariableEnvironment” and “ThisBinding” are used without qualification they
are in reference to those components of the running execution context.

An execution context is purely a specification mechanism and need not correspond to any particular
artefact of an ECMAScript implementation. It is impossible for an ECMAScript program to access an
execution context.

10.3.1 Identifier Resolution
============================

Identifier resolution is the process of determining the binding of an Identifier using the
LexicalEnvironment of the running execution context. During execution of ECMAScript code, the
syntactic production PrimaryExpression : Identifier is evaluated using the following algorithm:

1.  Let env be the running execution context’s LexicalEnvironment.
2.  If the syntactic production that is being evaluated is contained in a strict mode code, then let
    strict be true, else let strict be false.
3.  Return the result of calling GetIdentifierReference function passing env, Identifier, and strict
    as arguments.

The result of evaluating an identifier is always a value of type Reference with its referenced name
component equal to the Identifier String.

10.4 Establishing an Execution Context
======================================

Evaluation of global code or code using the eval function (15.1.2.1) establishes and enters a new
execution context. Every invocation of an ECMAScript code function (13.2.1) also establishes and
enters a new execution context, even if a function is calling itself recursively. Every return exits
an execution context. A thrown exception may also exit one or more execution contexts.

When control enters an execution context, the execution context’s ThisBinding is set, its
VariableEnvironment and initial LexicalEnvironment are defined, and declaration binding
instantiation (10.5) is performed. The exact manner in which these actions occur depend on the type
of code being entered.

10.4.1 Entering Global Code
===========================

The following steps are performed when control enters the execution context for global code:

1.  Initialise the execution context using the global code as described in 10.4.1.1.
2.  Perform Declaration Binding Instantiation as described in 10.5 using the global code.

10.4.1.1 Initial Global Execution Context
=========================================

The following steps are performed to initialise a global execution context for ECMAScript code C:

1.  Set the VariableEnvironment to the Global Environment.
2.  Set the LexicalEnvironment to the Global Environment.
3.  Set the ThisBinding to the global object.

10.4.2 Entering Eval Code
=========================

The following steps are performed when control enters the execution context for eval code:

1.  If there is no calling context or if the eval code is not being evaluated by a direct call
    (15.1.2.1.1) to the eval function then,
    1.  Initialise the execution context as if it was a global execution context using the eval code
        as C as described in 10.4.1.1.

2.  Else,
    1.  Set the ThisBinding to the same value as the ThisBinding of the calling execution context.
    2.  Set the LexicalEnvironment to the same value as the LexicalEnvironment of the calling
        execution context.
    3.  Set the VariableEnvironment to the same value as the VariableEnvironment of the calling
        execution context.

3.  If the eval code is strict code, then
    1.  Let strictVarEnv be the result of calling NewDeclarativeEnvironment passing the
        LexicalEnvironment as the argument.
    2.  Set the LexicalEnvironment to strictVarEnv.
    3.  Set the VariableEnvironment to strictVarEnv.

4.  Perform Declaration Binding Instantiation as described in 10.5 using the eval code.

10.4.2.1 Strict Mode Restrictions
=================================

The eval code cannot instantiate variable or function bindings in the variable environment of the
calling context that invoked the eval if either the code of the calling context or the eval code is
strict code. Instead such bindings are instantiated in a new VariableEnvironment that is only
accessible to the eval code.

10.4.3 Entering Function Code
=============================

The following steps are performed when control enters the execution context for function code
contained in function object F, a caller provided thisArg, and a caller provided argumentsList:

1.  If the function code is strict code, set the ThisBinding to thisArg.
2.  Else if thisArg is null or undefined, set the ThisBinding to the global object.
3.  Else if Type(thisArg) is not Object, set the ThisBinding to ToObject(thisArg).
4.  Else set the ThisBinding to thisArg.
5.  Let localEnv be the result of calling NewDeclarativeEnvironment passing the value of the
    [[Scope]] internal property of F as the argument.
6.  Set the LexicalEnvironment to localEnv.
7.  Set the VariableEnvironment to localEnv.
8.  Let code be the value of F’s [[Code]] internal property.
9.  Perform Declaration Binding Instantiation using the function code code and argumentsList as
    described in 10.5.

10.5 Declaration Binding Instantiation
======================================

Every execution context has an associated VariableEnvironment. Variables and functions declared in
ECMAScript code evaluated in an execution context are added as bindings in that
VariableEnvironment’s Environment Record. For function code, parameters are also added as bindings
to that Environment Record.

Which Environment Record is used to bind a declaration and its kind depends upon the type of
ECMAScript code executed by the execution context, but the remainder of the behaviour is generic. On
entering an execution context, bindings are created in the VariableEnvironment as follows using the
caller provided code and, if it is function code, argument List args:

1.  Let env be the environment record component of the running execution context’s
    VariableEnvironment.
2.  If code is eval code, then let configurableBindings be true else let configurableBindings be
    false.
3.  If code is strict mode code, then let strict be true else let strict be false.
4.  If code is function code, then
    1.  Let func be the function whose [[Call]] internal method initiated execution of code. Let
        names be the value of func’s [[FormalParameters]] internal property.
    2.  Let argCount be the number of elements in args.
    3.  Let n be the number 0.
    4.  For each String argName in names, in list order do
        1.  Let n be the current value of n plus 1.
        2.  If n is greater than argCount, let v be undefined otherwise let v be the value of the
            n’th element of args.
        3.  Let argAlreadyDeclared be the result of calling env’s HasBinding concrete method passing
            argName as the argument.
        4.  If argAlreadyDeclared is false, call env’s CreateMutableBinding concrete method passing
            argName as the argument.
        5.  Call env’s SetMutableBinding concrete method passing argName, v, and strict as the
            arguments.

5.  For each FunctionDeclaration f in code, in source text order do
    1.  Let fn be the Identifier in FunctionDeclaration f.
    2.  Let fo be the result of instantiating FunctionDeclaration f as described in Clause 13.
    3.  Let funcAlreadyDeclared be the result of calling env’s HasBinding concrete method passing fn
        as the argument.
    4.  If funcAlreadyDeclared is false, call env’s CreateMutableBinding concrete method passing fn
        and configurableBindings as the arguments.
    5.  Else if env is the environment record component of the global environment then
        1.  Let go be the global object.
        2.  Let existingProp be the resulting of calling the [[GetProperty]] internal method of go
            with argument fn.
        3.  If existingProp .[[Configurable]] is true, then
            1.  Call the [[DefineOwnProperty]] internal method of go, passing fn, Property
                Descriptor {[[Value]]: undefined, [[Writable]]: true, [[Enumerable]]: true ,
                [[Configurable]]: configurableBindings }, and true as arguments.

        4.  Else if IsAccessorDescriptor(existingProp) or existingProp does not have attribute
            values {[[Writable]]: true, [[Enumerable]]: true}, then
            1.  Throw a TypeError exception.

    6.  Call env’s SetMutableBinding concrete method passing fn, fo, and strict as the arguments.

6.  Let argumentsAlreadyDeclared be the result of calling env’s HasBinding concrete method passing
    "arguments" as the argument.
7.  If code is function code and argumentsAlreadyDeclared is false, then
    1.  Let argsObj be the result of calling the abstract operation CreateArgumentsObject (10.6)
        passing func, names, args, env and strict as arguments.
    2.  If strict is true, then
        1.  Call env’s CreateImmutableBinding concrete method passing the String "arguments" as the
            argument.
        2.  Call env’s InitializeImmutableBinding concrete method passing "arguments" and argsObj as
            arguments.

    3.  Else,
        1.  Call env’s CreateMutableBinding concrete method passing the String "arguments" as the
            argument.
        2.  Call env’s SetMutableBinding concrete method passing "arguments", argsObj, and false as
            arguments.

8.  For each VariableDeclaration and VariableDeclarationNoIn d in code, in source text order do
    1.  Let dn be the Identifier in d.
    2.  Let varAlreadyDeclared be the result of calling env’s HasBinding concrete method passing dn
        as the argument.
    3.  If varAlreadyDeclared is false, then
        1.  Call env’s CreateMutableBinding concrete method passing dn and configurableBindings as
            the arguments.
        2.  Call env’s SetMutableBinding concrete method passing dn, undefined, and strict as the
            arguments.

10.6 Arguments Object
=====================

When control enters an execution context for function code, an arguments object is created unless
(as specified in 10.5) the identifier arguments occurs as an Identifier in the function’s
FormalParameterList or occurs as the Identifier of a VariableDeclaration or FunctionDeclaration
contained in the function code.

The arguments object is created by calling the abstract operation CreateArgumentsObject with
arguments func the function object whose code is to be evaluated, names a List containing the
function’s formal parameter names, args the actual arguments passed to the [[Call]] internal method,
env the variable environment for the function code, and strict a Boolean that indicates whether or
not the function code is strict code. When CreateArgumentsObject is called the following steps are
performed:

1.  Let len be the number of elements in args.
2.  Let obj be the result of creating a new ECMAScript object.
3.  Set all the internal methods of obj as specified in 8.12.
4.  Set the [[Class]] internal property of obj to "Arguments".
5.  Let Object be the standard built-in Object constructor (15.2.2).
6.  Set the [[Prototype]] internal property of obj to the standard built-in Object prototype object
    (15.2.4).
7.  Call the [[DefineOwnProperty]] internal method on obj passing "length", the Property Descriptor
    {[[Value]]: len, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true}, and false
    as arguments.
8.  Let map be the result of creating a new object as if by the expression new Object() where Object
    is the standard built-in constructor with that name
9.  Let mappedNames be an empty List.
10. Let indx = len - 1.
11. Repeat while indx >= 0,
    1.  Let val be the element of args at 0-origined list position indx.
    2.  Call the [[DefineOwnProperty]] internal method on obj passing ToString(indx), the property
        descriptor {[[Value]]: val, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]:
        true}, and false as arguments.
    3.  If indx is less than the number of elements in names, then
        1.  Let name be the element of names at 0-origined list position indx.
        2.  If strict is false and name is not an element of mappedNames, then
            1.  Add name as an element of the list mappedNames.
            2.  Let g be the result of calling the MakeArgGetter abstract operation with arguments
                name and env.
            3.  Let p be the result of calling the MakeArgSetter abstract operation with arguments
                name and env.
            4.  Call the [[DefineOwnProperty]] internal method of map passing ToString(indx), the
                Property Descriptor {[[Set]]: p, [[Get]]: g, [[Configurable]]: true}, and false as
                arguments.

    4.  Let indx = indx - 1

12. If mappedNames is not empty, then
    1.  Set the [[ParameterMap]] internal property of obj to map.
    2.  Set the [[Get]], [[GetOwnProperty]], [[DefineOwnProperty]], and [[Delete]] internal methods
        of obj to the definitions provided below.

13. If strict is false, then
    1.  Call the [[DefineOwnProperty]] internal method on obj passing "callee", the property
        descriptor {[[Value]]: func, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]:
        true}, and false as arguments.

14. Else, strict is true so
    1.  Let thrower be the [[ThrowTypeError]] function Object (13.2.3).
    2.  Call the [[DefineOwnProperty]] internal method of obj with arguments "caller",
        PropertyDescriptor {[[Get]]: thrower, [[Set]]: thrower, [[Enumerable]]: false,
        [[Configurable]]: false}, and false.
    3.  Call the [[DefineOwnProperty]] internal method of obj with arguments "callee",
        PropertyDescriptor {[[Get]]: thrower, [[Set]]: thrower, [[Enumerable]]: false,
        [[Configurable]]: false}, and false.

15. Return obj

The abstract operation MakeArgGetter called with String name and environment record env creates a
function object that when executed returns the value bound for name in env. It performs the
following steps:

1.  Let body be the result of concatenating the Strings "return ", name, and ";".
2.  Return the result of creating a function object as described in 13.2 using no
    FormalParameterList, body for FunctionBody, env as Scope, and true for Strict.

The abstract operation MakeArgSetter called with String name and environment record env creates a
function object that when executed sets the value bound for name in env. It performs the following
steps:

1.  Let param be the String name concatenated with the String "_arg".
2.  Let body be the String "<name> = <param>;" with <name> replaced by the value of name and <param>
    replaced by the value of param.
3.  Return the result of creating a function object as described in 13.2 using a List containing the
    single String param as FormalParameterList, body for FunctionBody, env as Scope, and true for
    Strict.

The [[Get]] internal method of an arguments object for a non-strict mode function with formal
parameters when called with a property name P performs the following steps:

1.  Let map be the value of the [[ParameterMap]] internal property of the arguments object.
2.  Let isMapped be the result of calling the [[GetOwnProperty]] internal method of map passing P as
    the argument.
3.  If the value of isMapped is undefined, then
    1.  Let v be the result of calling the default [[Get]] internal method (8.12.3) on the arguments
        object passing P as the argument.
    2.  If P is "caller" and v is a strict mode Function object, throw a TypeError exception.
    3.  Return v.

4.  Else, map contains a formal parameter mapping for P so,
    1.  Return the result of calling the [[Get]] internal method of map passing P as the argument.

The [[GetOwnProperty]] internal method of an arguments object for a non-strict mode function with
formal parameters when called with a property name P performs the following steps:

1.  Let desc be the result of calling the default [[GetOwnProperty]] internal method (8.12.1) on the
    arguments object passing P as the argument.
2.  If desc is undefined then return desc.
3.  Let map be the value of the [[ParameterMap]] internal property of the arguments object.
4.  Let isMapped be the result of calling the [[GetOwnProperty]] internal method of map passing P as
    the argument.
5.  If the value of isMapped is not undefined, then
    1.  Set desc.[[Value]] to the result of calling the [[Get]] internal method of map passing P as
        the argument.

6.  Return desc.

The [[DefineOwnProperty]] internal method of an arguments object for a non-strict mode function with
formal parameters when called with a property name P, Property Descriptor Desc, and Boolean flag
Throw performs the following steps:

1.  Let map be the value of the [[ParameterMap]] internal property of the arguments object.
2.  Let isMapped be the result of calling the [[GetOwnProperty]] internal method of map passing P as
    the argument.
3.  Let allowed be the result of calling the default [[DefineOwnProperty]] internal method (8.12.9)
    on the arguments object passing P, Desc, and false as the arguments.
4.  If allowed is false, then
    1.  If Throw is true then throw a TypeError exception, otherwise return false.

5.  If the value of isMapped is not undefined, then
    1.  If IsAccessorDescriptor(Desc) is true, then
        1.  Call the [[Delete]] internal method of map passing P, and false as the arguments.

    2.  Else
        1.  If Desc.[[Value]] is present, then
            1.  Call the [[Put]] internal method of map passing P, Desc.[[Value]], and Throw as the
                arguments.

        2.  If Desc.[[Writable]] is present and its value is false, then
            1.  Call the [[Delete]] internal method of map passing P and false as arguments.

6.  Return true.

The [[Delete]] internal method of an arguments object for a non-strict mode function with formal
parameters when called with a property name P and Boolean flag Throw performs the following steps:

1.  Let map be the value of the [[ParameterMap]] internal property of the arguments object.
2.  Let isMapped be the result of calling the [[GetOwnProperty]] internal method of map passing P as
    the argument.
3.  Let result be the result of calling the default [[Delete]] internal method (8.12.7) on the
    arguments object passing P and Throw as the arguments.
4.  If result is true and the value of isMapped is not undefined, then
    1.  Call the [[Delete]] internal method of map passing P, and false as the arguments.

5.  Return result.

NOTE 1 For non-strict mode functions the array index (defined in 15.4) named data properties of an
arguments object whose numeric name values are less than the number of formal parameters of the
corresponding function object initially share their values with the corresponding argument bindings
in the function’s execution context. This means that changing the property changes the corresponding
value of the argument binding and vice-versa. This correspondence is broken if such a property is
deleted and then redefined or if the property is changed into an accessor property. For strict mode
functions, the values of the arguments object’s properties are simply a copy of the arguments passed
to the function and there is no dynamic linkage between the property values and the formal parameter
values.

NOTE 2 The ParameterMap object and its property values are used as a device for specifying the
arguments object correspondence to argument bindings. The ParameterMap object and the objects that
are the values of its properties are not directly accessible from ECMAScript code. An ECMAScript
implementation does not need to actually create or use such objects to implement the specified
semantics.

NOTE 3 Arguments objects for strict mode functions define non-configurable accessor properties named
"caller" and "callee" which throw a TypeError exception on access. The "callee" property has a more
specific meaning for non-strict mode functions and a "caller" property has historically been
provided as an implementation-defined extension by some ECMAScript implementations. The strict mode
definition of these properties exists to ensure that neither of them is defined in any other manner
by conforming ECMAScript implementations.

11 Expressions
==============

11.1 Primary Expressions
========================

Syntax
------

PrimaryExpression :

this

Identifier

Literal

ArrayLiteral

ObjectLiteral

( Expression )

11.1.1 The this Keyword
=======================

The this keyword evaluates to the value of the ThisBinding of the current execution context.

11.1.2 Identifier Reference
===========================

An Identifier is evaluated by performing Identifier Resolution as specified in 10.3.1. The result of
evaluating an Identifier is always a value of type Reference.

11.1.3 Literal Reference
========================

A Literal is evaluated as described in 7.8.

11.1.4 Array Initialiser
========================

An array initialiser is an expression describing the initialisation of an Array object, written in a
form of a literal. It is a list of zero or more expressions, each of which represents an array
element, enclosed in square brackets. The elements need not be literals; they are evaluated each
time the array initialiser is evaluated.

Array elements may be elided at the beginning, middle or end of the element list. Whenever a comma
in the element list is not preceded by an AssignmentExpression (i.e., a comma at the beginning or
after another comma), the missing array element contributes to the length of the Array and increases
the index of subsequent elements. Elided array elements are not defined. If an element is elided at
the end of an array, that element does not contribute to the length of the Array.

Syntax
------

ArrayLiteral :

[ Elisionopt ]

[ ElementList ]

[ ElementList , Elisionopt ]

ElementList :

Elisionopt AssignmentExpression

ElementList , Elisionopt AssignmentExpression

Elision :

,

Elision ,

Semantics
---------

The production ArrayLiteral : [ Elisionopt ] is evaluated as follows:

1.  Let array be the result of creating a new object as if by the expression new Array() where Array
    is the standard built-in constructor with that name.
2.  Let pad be the result of evaluating Elision; if not present, use the numeric value zero.
3.  Call the [[Put]] internal method of array with arguments "length", pad, and false.
4.  Return array.

The production ArrayLiteral : [ ElementList ] is evaluated as follows:

1.  Return the result of evaluating ElementList.

The production ArrayLiteral : [ ElementList , Elisionopt ] is evaluated as follows:

1.  Let array be the result of evaluating ElementList.
2.  Let pad be the result of evaluating Elision; if not present, use the numeric value zero.
3.  Let len be the result of calling the [[Get]] internal method of array with argument "length".
4.  Call the [[Put]] internal method of array with arguments "length", ToUint32(pad+len), and false.
5.  Return array.

The production ElementList : Elisionopt AssignmentExpression is evaluated as follows:

1.  Let array be the result of creating a new object as if by the expression new Array() where Array
    is the standard built-in constructor with that name.
2.  Let firstIndex be the result of evaluating Elision; if not present, use the numeric value zero.
3.  Let initResult be the result of evaluating AssignmentExpression.
4.  Let initValue be GetValue(initResult).
5.  Call the [[DefineOwnProperty]] internal method of array with arguments ToString(firstIndex), the
    Property Descriptor { [[Value]]: initValue, [[Writable]]: true, [[Enumerable]]: true,
    [[Configurable]]: true}, and false.
6.  Return array.

The production ElementList : ElementList , Elisionopt AssignmentExpression is evaluated as follows:

1.  Let array be the result of evaluating ElementList.
2.  Let pad be the result of evaluating Elision; if not present, use the numeric value zero.
3.  Let initResult be the result of evaluating AssignmentExpression.
4.  Let initValue be GetValue(initResult).
5.  Let len be the result of calling the [[Get]] internal method of array with argument "length".
6.  Call the [[DefineOwnProperty]] internal method of array with arguments
    ToString(ToUint32((pad+len)) and the Property Descriptor { [[Value]]: initValue, [[Writable]]:
    true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
7.  Return array.

The production Elision : , is evaluated as follows:

1.  Return the numeric value 1.

The production Elision : Elision , is evaluated as follows:

1.  Let preceding be the result of evaluating Elision.
2.  Return preceding+1.

NOTE [[DefineOwnProperty]] is used to ensure that own properties are defined for the array even if
the standard built-in Array prototype object has been modified in a manner that would preclude the
creation of new own properties using [[Put]].

11.1.5 Object Initialiser
=========================

An object initialiser is an expression describing the initialisation of an Object, written in a form
resembling a literal. It is a list of zero or more pairs of property names and associated values,
enclosed in curly braces. The values need not be literals; they are evaluated each time the object
initialiser is evaluated.

Syntax
------

ObjectLiteral :

{ }

{ PropertyNameAndValueList }

{ PropertyNameAndValueList , }

PropertyNameAndValueList :

PropertyAssignment

PropertyNameAndValueList , PropertyAssignment

PropertyAssignment :

PropertyName : AssignmentExpression

get PropertyName ( ) { FunctionBody }

set PropertyName ( PropertySetParameterList ) { FunctionBody }

PropertyName :

IdentifierName

StringLiteral

NumericLiteral

PropertySetParameterList :

Identifier

Semantics
---------

The production ObjectLiteral : { } is evaluated as follows:

1.  Return a new object created as if by the expression new Object() where Object is the standard
    built-in constructor with that name.

The productions ObjectLiteral : { PropertyNameAndValueList } and  
ObjectLiteral : { PropertyNameAndValueList ,} are evaluated as follows:

1.  Return the result of evaluating PropertyNameAndValueList.

The production PropertyNameAndValueList : PropertyAssignment is evaluated as follows:

1.  Let obj be the result of creating a new object as if by the expression new Object() where Object
    is the standard built-in constructor with that name.
2.  Let propId be the result of evaluating PropertyAssignment.
3.  Call the [[DefineOwnProperty]] internal method of obj with arguments propId.name,
    propId.descriptor, and false.
4.  Return obj.

The production  
 PropertyNameAndValueList : PropertyNameAndValueList , PropertyAssignment  
is evaluated as follows:

1.  Let obj be the result of evaluating PropertyNameAndValueList.
2.  Let propId be the result of evaluating PropertyAssignment.
3.  Let previous be the result of calling the [[GetOwnProperty]] internal method of obj with
    argument propId.name.
4.  If previous is not undefined then throw a SyntaxError exception if any of the following
    conditions are true
    1.  This production is contained in strict code and IsDataDescriptor(previous) is true and
        IsDataDescriptor(propId.descriptor) is true.
    2.  IsDataDescriptor(previous) is true and IsAccessorDescriptor(propId.descriptor) is true.
    3.  IsAccessorDescriptor(previous) is true and IsDataDescriptor(propId.descriptor) is true.
    4.  IsAccessorDescriptor(previous) is true and IsAccessorDescriptor(propId.descriptor) is true
        and either both previous and propId.descriptor have [[Get]] fields or both previous and
        propId.descriptor have [[Set]] fields

5.  Call the [[DefineOwnProperty]] internal method of obj with arguments propId.name,
    propId.descriptor, and false.
6.  Return obj.

If the above steps would throw a SyntaxError then an implementation must treat the error as an early
error (Clause 16).

The production PropertyAssignment : PropertyName : AssignmentExpression is evaluated as follows:

1.  Let propName be the result of evaluating PropertyName.
2.  Let exprValue be the result of evaluating AssignmentExpression.
3.  Let propValue be GetValue(exprValue).
4.  Let desc be the Property Descriptor{[[Value]]: propValue, [[Writable]]: true, [[Enumerable]]:
    true, [[Configurable]]: true}
5.  Return Property Identifier (propName, desc).

The production PropertyAssignment : get PropertyName ( ) { FunctionBody } is evaluated as follows:

1.  Let propName be the result of evaluating PropertyName.
2.  Let closure be the result of creating a new Function object as specified in 13.2 with an empty
    parameter list and body specified by FunctionBody. Pass in the LexicalEnvironment of the running
    execution context as the Scope. Pass in true as the Strict flag if the PropertyAssignment is
    contained in strict code or if its FunctionBody is strict code.
3.  Let desc be the Property Descriptor{[[Get]]: closure, [[Enumerable]]: true, [[Configurable]]:
    true}
4.  Return Property Identifier (propName, desc).

The production PropertyAssignment : set PropertyName ( PropertySetParameterList ) { FunctionBody }
is evaluated as follows:

1.  Let propName be the result of evaluating PropertyName.
2.  Let closure be the result of creating a new Function object as specified in 13.2 with parameters
    specified by PropertySetParameterList and body specified by FunctionBody. Pass in the
    LexicalEnvironment of the running execution context as the Scope. Pass in true as the Strict
    flag if the PropertyAssignment is contained in strict code or if its FunctionBody is strict
    code.
3.  Let desc be the Property Descriptor{[[Set]]: closure, [[Enumerable]]: true, [[Configurable]]:
    true}
4.  Return Property Identifier (propName, desc).

It is a SyntaxError if the Identifier "eval" or the Identifier "arguments" occurs as the Identifier
in a PropertySetParameterList of a PropertyAssignment that is contained in strict code or if its
FunctionBody is strict code.

The production PropertyName : IdentifierName is evaluated as follows:

1.  Return the String value containing the same sequence of characters as the IdentifierName.

The production PropertyName : StringLiteral is evaluated as follows:

1.  Return the SV of the StringLiteral.

The production PropertyName : NumericLiteral is evaluated as follows:

1.  Let nbr be the result of forming the value of the NumericLiteral.
2.  Return ToString(nbr).

11.1.6 The Grouping Operator
============================

The production PrimaryExpression : ( Expression ) is evaluated as follows:

1.  Return the result of evaluating Expression. This may be of type Reference.

NOTE This algorithm does not apply GetValue to the result of evaluating Expression. The principal
motivation for this is so that operators such as delete and typeof may be applied to parenthesised
expressions.

11.2 Left-Hand-Side Expressions
===============================

Syntax
------

MemberExpression :

PrimaryExpression

FunctionExpression

MemberExpression [ Expression ]

MemberExpression . IdentifierName

new MemberExpression Arguments

NewExpression :

MemberExpression

new NewExpression

CallExpression :

MemberExpression Arguments

CallExpression Arguments

CallExpression [ Expression ]

CallExpression . IdentifierName

Arguments :

( )

( ArgumentList )

ArgumentList :

AssignmentExpression

ArgumentList , AssignmentExpression

LeftHandSideExpression :

NewExpression

CallExpression

11.2.1 Property Accessors
=========================

Properties are accessed by name, using either the dot notation:

MemberExpression . IdentifierName  
CallExpression . IdentifierName

or the bracket notation:

MemberExpression [ Expression ]  
CallExpression [ Expression ]

The dot notation is explained by the following syntactic conversion:

MemberExpression . IdentifierName

is identical in its behaviour to

MemberExpression [ <identifier-name-string> ]

and similarly

CallExpression . IdentifierName

is identical in its behaviour to

CallExpression [ <identifier-name-string> ]

where <identifier-name-string> is a string literal containing the same sequence of characters after
processing of Unicode escape sequences as the IdentifierName.

The production MemberExpression : MemberExpression [ Expression ] is evaluated as follows:

1.  Let baseReference be the result of evaluating MemberExpression.
2.  Let baseValue be GetValue(baseReference).
3.  Let propertyNameReference be the result of evaluating Expression.
4.  Let propertyNameValue be GetValue(propertyNameReference).
5.  Call CheckObjectCoercible(baseValue).
6.  Let propertyNameString be ToString(propertyNameValue).
7.  If the syntactic production that is being evaluated is contained in strict mode code, let strict
    be true, else let strict be false.
8.  Return a value of type Reference whose base value is baseValue and whose referenced name is
    propertyNameString, and whose strict mode flag is strict.

The production CallExpression : CallExpression [ Expression ] is evaluated in exactly the same
manner, except that the contained CallExpression is evaluated in step 1.

11.2.2 The new Operator
=======================

The production NewExpression : new NewExpression is evaluated as follows:

1.  Let ref be the result of evaluating NewExpression.
2.  Let constructor be GetValue(ref).
3.  If Type(constructor) is not Object, throw a TypeError exception.
4.  If constructor does not implement the [[Construct]] internal method, throw a TypeError
    exception.
5.  Return the result of calling the [[Construct]] internal method on constructor, providing no
    arguments (that is, an empty list of arguments).

The production MemberExpression : new MemberExpression Arguments is evaluated as follows:

1.  Let ref be the result of evaluating MemberExpression.
2.  Let constructor be GetValue(ref).
3.  Let argList be the result of evaluating Arguments, producing an internal list of argument values
    (11.2.4).
4.  If Type(constructor) is not Object, throw a TypeError exception.
5.  If constructor does not implement the [[Construct]] internal method, throw a TypeError
    exception.
6.  Return the result of calling the [[Construct]] internal method on constructor, providing the
    list argList as the argument values.

11.2.3 Function Calls
=====================

The production CallExpression : MemberExpression Arguments is evaluated as follows:

1.  Let ref be the result of evaluating MemberExpression.
2.  Let func be GetValue(ref).
3.  Let argList be the result of evaluating Arguments, producing an internal list of argument values
    (see 11.2.4).
4.  If Type(func) is not Object, throw a TypeError exception.
5.  If IsCallable(func) is false, throw a TypeError exception.
6.  If Type(ref) is Reference, then
    1.  If IsPropertyReference(ref) is true, then
        1.  Let thisValue be GetBase(ref).

    2.  Else, the base of ref is an Environment Record
        1.  Let thisValue be the result of calling the ImplicitThisValue concrete method of
            GetBase(ref).

7.  Else, Type(ref) is not Reference.
    1.  Let thisValue be undefined.

8.  Return the result of calling the [[Call]] internal method on func, providing thisValue as the
    this value and providing the list argList as the argument values.

The production CallExpression : CallExpression Arguments is evaluated in exactly the same manner,
except that the contained CallExpression is evaluated in step 1.

NOTE The returned result will never be of type Reference if func is a native ECMAScript object.
Whether calling a host object can return a value of type Reference is implementation-dependent. If a
value of type Reference is returned, it must be a non-strict Property Reference.

11.2.4 Argument Lists
=====================

The evaluation of an argument list produces a List of values (see 8.8).

The production Arguments : ( ) is evaluated as follows:

1.  Return an empty List.

The production Arguments : ( ArgumentList ) is evaluated as follows:

1.  Return the result of evaluating ArgumentList.

The production ArgumentList : AssignmentExpression is evaluated as follows:

1.  Let ref be the result of evaluating AssignmentExpression.
2.  Let arg be GetValue(ref).
3.  Return a List whose sole item is arg.

The production ArgumentList : ArgumentList , AssignmentExpression is evaluated as follows:

1.  Let precedingArgs be the result of evaluating ArgumentList.
2.  Let ref be the result of evaluating AssignmentExpression.
3.  Let arg be GetValue(ref).
4.  Return a List whose length is one greater than the length of precedingArgs and whose items are
    the items of precedingArgs, in order, followed at the end by arg which is the last item of the
    new list.

11.2.5 Function Expressions
===========================

The production MemberExpression : FunctionExpression is evaluated as follows:

1.  Return the result of evaluating FunctionExpression.

11.3 Postfix Expressions
========================

Syntax
------

PostfixExpression :

LeftHandSideExpression

LeftHandSideExpression [no LineTerminator here] ++

LeftHandSideExpression [no LineTerminator here] --

11.3.1 Postfix Increment Operator
=================================

The production PostfixExpression : LeftHandSideExpression [no LineTerminator here] ++ is evaluated
as follows:

1.  Let lhs be the result of evaluating LeftHandSideExpression.
2.  Throw a SyntaxError exception if the following conditions are all true:
    -   Type(lhs) is Reference is true
    -   IsStrictReference(lhs) is true
    -   Type(GetBase(lhs)) is Environment Record
    -   GetReferencedName(lhs) is either "eval" or "arguments"

3.  Let oldValue be ToNumber(GetValue(lhs)).
4.  Let newValue be the result of adding the value 1 to oldValue, using the same rules as for the +
    operator (see 11.6.3).
5.  Call PutValue(lhs, newValue).
6.  Return oldValue.

11.3.2 Postfix Decrement Operator
=================================

The production PostfixExpression : LeftHandSideExpression [no LineTerminator here] -- is evaluated
as follows:

1.  Let lhs be the result of evaluating LeftHandSideExpression.
2.  Throw a SyntaxError exception if the following conditions are all true:
    -   Type(lhs) is Reference is true
    -   IsStrictReference(lhs) is true
    -   Type(GetBase(lhs)) is Environment Record
    -   GetReferencedName(lhs) is either "eval" or "arguments"

3.  Let oldValue be ToNumber(GetValue(lhs)).
4.  Let newValue be the result of subtracting the value 1 from oldValue, using the same rules as for
    the - operator (11.6.3).
5.  Call PutValue(lhs, newValue).
6.  Return oldValue.

11.4 Unary Operators
====================

Syntax
------

UnaryExpression :

PostfixExpression

delete UnaryExpression

void UnaryExpression

typeof UnaryExpression

++ UnaryExpression

-- UnaryExpression

+ UnaryExpression

- UnaryExpression

~ UnaryExpression

! UnaryExpression

11.4.1 The delete Operator
==========================

The production UnaryExpression : delete UnaryExpression is evaluated as follows:

1.  Let ref be the result of evaluating UnaryExpression.
2.  If Type(ref) is not Reference, return true.
3.  If IsUnresolvableReference(ref) then,
    1.  If IsStrictReference(ref) is true, throw a SyntaxError exception.
    2.  Else, return true.

4.  If IsPropertyReference(ref) is true, then
    1.  Return the result of calling the [[Delete]] internal method on ToObject(GetBase(ref))
        providing GetReferencedName(ref) and IsStrictReference(ref) as the arguments.

5.  Else, ref is a Reference to an Environment Record binding, so
    1.  If IsStrictReference(ref) is true, throw a SyntaxError exception.
    2.  Let bindings be GetBase(ref).
    3.  Return the result of calling the DeleteBinding concrete method of bindings, providing
        GetReferencedName(ref) as the argument.

NOTE When a delete operator occurs within strict mode code, a SyntaxError exception is thrown if its
UnaryExpression is a direct reference to a variable, function argument, or function name. In
addition, if a delete operator occurs within strict mode code and the property to be deleted has the
attribute { [[Configurable]]: false }, a TypeError exception is thrown.

11.4.2 The void Operator
========================

The production UnaryExpression : void UnaryExpression is evaluated as follows:

1.  Let expr be the result of evaluating UnaryExpression.
2.  Call GetValue(expr).
3.  Return undefined.

NOTE GetValue must be called even though its value is not used because it may have observable
side-effects.

11.4.3 The typeof Operator
==========================

The production UnaryExpression : typeof UnaryExpression is evaluated as follows:

1.  Let val be the result of evaluating UnaryExpression.
2.  If Type(val) is Reference, then
    1.  If IsUnresolvableReference(val) is true, return "undefined".
    2.  Let val be GetValue(val).

3.  Return a String determined by Type(val) according to Table 20.

Table 20 — typeof Operator Results

  Type of val                                           Result
  ----------------------------------------------------- -----------------------------------------------------------------------------------------
  Undefined                                             "undefined"
  Null                                                  "object"
  Boolean                                               "boolean"
  Number                                                "number"
  String                                                "string"
  Object (native and does not implement [[Call]])       "object"
  Object (native or host and does implement [[Call]])   "function"
  Object (host and does not implement [[Call]])         Implementation-defined except may not be "undefined", "boolean", "number", or "string".

11.4.4 Prefix Increment Operator
================================

The production UnaryExpression : ++ UnaryExpression is evaluated as follows:

1.  Let expr be the result of evaluating UnaryExpression.
2.  Throw a SyntaxError exception if the following conditions are all true:
    -   Type(expr) is Reference is true
    -   IsStrictReference(expr) is true
    -   Type(GetBase(expr)) is Environment Record
    -   GetReferencedName(expr) is either "eval" or "arguments"

3.  Let oldValue be ToNumber(GetValue(expr)).
4.  Let newValue be the result of adding the value 1 to oldValue, using the same rules as for the +
    operator (see 11.6.3).
5.  Call PutValue(expr, newValue).
6.  Return newValue.

11.4.5 Prefix Decrement Operator
================================

The production UnaryExpression : -- UnaryExpression is evaluated as follows:

1.  Let expr be the result of evaluating UnaryExpression.
2.  Throw a SyntaxError exception if the following conditions are all true:
    -   Type(expr) is Reference is true
    -   IsStrictReference(expr) is true
    -   Type(GetBase(expr)) is Environment Record
    -   GetReferencedName(expr) is either "eval" or "arguments"

3.  Let oldValue be ToNumber(GetValue(expr)).
4.  Let newValue be the result of subtracting the value 1 from oldValue, using the same rules as for
    the - operator (see 11.6.3).
5.  Call PutValue(expr, newValue).
6.  Return newValue.

11.4.6 Unary + Operator
=======================

The unary + operator converts its operand to Number type.

The production UnaryExpression : + UnaryExpression is evaluated as follows:

1.  Let expr be the result of evaluating UnaryExpression.
2.  Return ToNumber(GetValue(expr)).

11.4.7 Unary - Operator
=======================

The unary - operator converts its operand to Number type and then negates it. Note that negating +0
produces −0, and negating −0 produces +0.

The production UnaryExpression : - UnaryExpression is evaluated as follows:

1.  Let expr be the result of evaluating UnaryExpression.
2.  Let oldValue be ToNumber(GetValue(expr)).
3.  If oldValue is NaN, return NaN.
4.  Return the result of negating oldValue; that is, compute a Number with the same magnitude but
    opposite sign.

11.4.8 Bitwise NOT Operator ( ~ )
=================================

The production UnaryExpression : ~ UnaryExpression is evaluated as follows:

1.  Let expr be the result of evaluating UnaryExpression.
2.  Let oldValue be ToInt32(GetValue(expr)).
3.  Return the result of applying bitwise complement to oldValue. The result is a signed 32-bit
    integer.

11.4.9 Logical NOT Operator ( ! )
=================================

The production UnaryExpression : ! UnaryExpression is evaluated as follows:

1.  Let expr be the result of evaluating UnaryExpression.
2.  Let oldValue be ToBoolean(GetValue(expr)).
3.  If oldValue is true, return false.
4.  Return true.

11.5 Multiplicative Operators
=============================

Syntax
------

MultiplicativeExpression :

UnaryExpression

MultiplicativeExpression * UnaryExpression

MultiplicativeExpression / UnaryExpression

MultiplicativeExpression % UnaryExpression

Semantics
---------

The production MultiplicativeExpression : MultiplicativeExpression @ UnaryExpression , where @
stands for one of the operators in the above definitions, is evaluated as follows:

1.  Let left be the result of evaluating MultiplicativeExpression.
2.  Let leftValue be GetValue(left).
3.  Let right be the result of evaluating UnaryExpression.
4.  Let rightValue be GetValue(right).
5.  Let leftNum be ToNumber(leftValue).
6.  Let rightNum be ToNumber(rightValue).
7.  Return the result of applying the specified operation (*, /, or %) to leftNum and rightNum. See
    the Notes below 11.5.1, 11.5.2, 11.5.3.

11.5.1 Applying the * Operator
==============================

The * operator performs multiplication, producing the product of its operands. Multiplication is
commutative. Multiplication is not always associative in ECMAScript, because of finite precision.

The result of a floating-point multiplication is governed by the rules of IEEE 754 binary
double-precision arithmetic:

-   If either operand is NaN, the result is NaN.

-   The sign of the result is positive if both operands have the same sign, negative if the operands
    have different signs.

-   Multiplication of an infinity by a zero results in NaN.

-   Multiplication of an infinity by an infinity results in an infinity. The sign is determined by
    the rule already stated above.

-   Multiplication of an infinity by a finite nonzero value results in a signed infinity. The sign
    is determined by the rule already stated above.

-   In the remaining cases, where neither an infinity or NaN is involved, the product is computed
    and rounded to the nearest representable value using IEEE 754 round-to-nearest mode. If the
    magnitude is too large to represent, the result is then an infinity of appropriate sign. If the
    magnitude is too small to represent, the result is then a zero of appropriate sign. The
    ECMAScript language requires support of gradual underflow as defined by IEEE 754.

11.5.2 Applying the / Operator
==============================

The / operator performs division, producing the quotient of its operands. The left operand is the
dividend and the right operand is the divisor. ECMAScript does not perform integer division. The
operands and result of all division operations are double-precision floating-point numbers. The
result of division is determined by the specification of IEEE 754 arithmetic:

-   If either operand is NaN, the result is NaN.

-   The sign of the result is positive if both operands have the same sign, negative if the operands
    have different signs.

-   Division of an infinity by an infinity results in NaN.

-   Division of an infinity by a zero results in an infinity. The sign is determined by the rule
    already stated above.

-   Division of an infinity by a nonzero finite value results in a signed infinity. The sign is
    determined by the rule already stated above.

-   Division of a finite value by an infinity results in zero. The sign is determined by the rule
    already stated above.

-   Division of a zero by a zero results in NaN; division of zero by any other finite value results
    in zero, with the sign determined by the rule already stated above.

-   Division of a nonzero finite value by a zero results in a signed infinity. The sign is
    determined by the rule already stated above.

-   In the remaining cases, where neither an infinity, nor a zero, nor NaN is involved, the quotient
    is computed and rounded to the nearest representable value using IEEE 754 round-to-nearest mode.
    If the magnitude is too large to represent, the operation overflows; the result is then an
    infinity of appropriate sign. If the magnitude is too small to represent, the operation
    underflows and the result is a zero of the appropriate sign. The ECMAScript language requires
    support of gradual underflow as defined by IEEE 754.

11.5.3 Applying the % Operator
==============================

The % operator yields the remainder of its operands from an implied division; the left operand is
the dividend and the right operand is the divisor.

NOTE In C and C++, the remainder operator accepts only integral operands; in ECMAScript, it also
accepts floating-point operands.

The result of a floating-point remainder operation as computed by the % operator is not the same as
the “remainder” operation defined by IEEE 754. The IEEE 754 “remainder” operation computes the
remainder from a rounding division, not a truncating division, and so its behaviour is not analogous
to that of the usual integer remainder operator. Instead the ECMAScript language defines % on
floating-point operations to behave in a manner analogous to that of the Java integer remainder
operator; this may be compared with the C library function fmod.

The result of an ECMAScript floating-point remainder operation is determined by the rules of IEEE
arithmetic:

-   If either operand is NaN, the result is NaN.

    -   The sign of the result equals the sign of the dividend.

    -   If the dividend is an infinity, or the divisor is a zero, or both, the result is NaN.

    -   If the dividend is finite and the divisor is an infinity, the result equals the dividend.

    -   If the dividend is a zero and the divisor is nonzero and finite, the result is the same as
        the dividend.

    -   In the remaining cases, where neither an infinity, nor a zero, nor NaN is involved, the
        floating-point remainder r from a dividend n and a divisor d is defined by the mathematical
        relation r = n − (d × q) where q is an integer that is negative only if n/d is negative and
        positive only if n/d is positive, and whose magnitude is as large as possible without
        exceeding the magnitude of the true mathematical quotient of n and d. r is computed and
        rounded to the nearest representable value using IEEE 754 round-to-nearest mode.

11.6 Additive Operators
=======================

Syntax
------

AdditiveExpression :

MultiplicativeExpression

AdditiveExpression + MultiplicativeExpression

AdditiveExpression - MultiplicativeExpression

11.6.1 The Addition operator ( + )
==================================

The addition operator either performs string concatenation or numeric addition.

The production AdditiveExpression : AdditiveExpression + MultiplicativeExpression is evaluated as
follows:

1.  Let lref be the result of evaluating AdditiveExpression.
2.  Let lval be GetValue(lref).
3.  Let rref be the result of evaluating MultiplicativeExpression.
4.  Let rval be GetValue(rref).
5.  Let lprim be ToPrimitive(lval).
6.  Let rprim be ToPrimitive(rval).
7.  If Type(lprim) is String or Type(rprim) is String, then
    1.  Return the String that is the result of concatenating ToString(lprim) followed by
        ToString(rprim)

8.  Return the result of applying the addition operation to ToNumber(lprim) and ToNumber(rprim). See
    the Note below 11.6.3.

NOTE 1 No hint is provided in the calls to ToPrimitive in steps 5 and 6. All native ECMAScript
objects except Date objects handle the absence of a hint as if the hint Number were given; Date
objects handle the absence of a hint as if the hint String were given. Host objects may handle the
absence of a hint in some other manner.

NOTE 2 Step 7 differs from step 3 of the comparison algorithm for the relational operators (11.8.5),
by using the logical-or operation instead of the logical-and operation.

11.6.2 The Subtraction Operator ( - )
=====================================

The production AdditiveExpression : AdditiveExpression - MultiplicativeExpression is evaluated as
follows:

1.  Let lref be the result of evaluating AdditiveExpression.
2.  Let lval be GetValue(lref).
3.  Let rref be the result of evaluating MultiplicativeExpression.
4.  Let rval be GetValue(rref).
5.  Let lnum be ToNumber(lval).
6.  Let rnum be ToNumber(rval).
7.  Return the result of applying the subtraction operation to lnum and rnum. See the note below
    11.6.3.

11.6.3 Applying the Additive Operators to Numbers
=================================================

The + operator performs addition when applied to two operands of numeric type, producing the sum of
the operands. The - operator performs subtraction, producing the difference of two numeric operands.

Addition is a commutative operation, but not always associative.

The result of an addition is determined using the rules of IEEE 754 binary double-precision
arithmetic:

-   If either operand is NaN, the result is NaN.

-   The sum of two infinities of opposite sign is NaN.

-   The sum of two infinities of the same sign is the infinity of that sign.

-   The sum of an infinity and a finite value is equal to the infinite operand.

-   The sum of two negative zeroes is −0. The sum of two positive zeroes, or of two zeroes of
    opposite sign, is +0.

-   The sum of a zero and a nonzero finite value is equal to the nonzero operand.

-   The sum of two nonzero finite values of the same magnitude and opposite sign is +0.

-   In the remaining cases, where neither an infinity, nor a zero, nor NaN is involved, and the
    operands have the same sign or have different magnitudes, the sum is computed and rounded to the
    nearest representable value using IEEE 754 round-to-nearest mode. If the magnitude is too large
    to represent, the operation overflows and the result is then an infinity of appropriate sign.
    The ECMAScript language requires support of gradual underflow as defined by IEEE 754.

The - operator performs subtraction when applied to two operands of numeric type, producing the
difference of its operands; the left operand is the minuend and the right operand is the subtrahend.
Given numeric operands a and b, it is always the case that a–b produces the same result as a +(–b).

11.7 Bitwise Shift Operators
============================

Syntax
------

ShiftExpression :

AdditiveExpression

ShiftExpression << AdditiveExpression

ShiftExpression >> AdditiveExpression

ShiftExpression >>> AdditiveExpression

11.7.1 The Left Shift Operator ( << )
=====================================

Performs a bitwise left shift operation on the left operand by the amount specified by the right
operand.

The production ShiftExpression : ShiftExpression << AdditiveExpression is evaluated as follows:

1.  Let lref be the result of evaluating ShiftExpression.
2.  Let lval be GetValue(lref).
3.  Let rref be the result of evaluating AdditiveExpression.
4.  Let rval be GetValue(rref).
5.  Let lnum be ToInt32(lval).
6.  Let rnum be ToUint32(rval).
7.  Let shiftCount be the result of masking out all but the least significant 5 bits of rnum, that
    is, compute rnum & 0x1F.
8.  Return the result of left shifting lnum by shiftCount bits. The result is a signed 32-bit
    integer.

11.7.2 The Signed Right Shift Operator ( >> )
=============================================

Performs a sign-filling bitwise right shift operation on the left operand by the amount specified by
the right operand.

The production ShiftExpression : ShiftExpression >> AdditiveExpression is evaluated as follows:

1.  Let lref be the result of evaluating ShiftExpression.
2.  Let lval be GetValue(lref).
3.  Let rref be the result of evaluating AdditiveExpression.
4.  Let rval be GetValue(rref).
5.  Let lnum be ToInt32(lval).
6.  Let rnum be ToUint32(rval).
7.  Let shiftCount be the result of masking out all but the least significant 5 bits of rnum, that
    is, compute rnum & 0x1F.
8.  Return the result of performing a sign-extending right shift of lnum by shiftCount bits. The
    most significant bit is propagated. The result is a signed 32-bit integer.

11.7.3 The Unsigned Right Shift Operator ( >>> )
================================================

Performs a zero-filling bitwise right shift operation on the left operand by the amount specified by
the right operand.

The production ShiftExpression : ShiftExpression >>> AdditiveExpression is evaluated as follows:

1.  Let lref be the result of evaluating ShiftExpression.
2.  Let lval be GetValue(lref).
3.  Let rref be the result of evaluating AdditiveExpression.
4.  Let rval be GetValue(rref).
5.  Let lnum be ToUint32(lval).
6.  Let rnum be ToUint32(rval).
7.  Let shiftCount be the result of masking out all but the least significant 5 bits of rnum, that
    is, compute rnum & 0x1F.
8.  Return the result of performing a zero-filling right shift of lnum by shiftCount bits. Vacated
    bits are filled with zero. The result is an unsigned 32-bit integer.

11.8 Relational Operators
=========================

Syntax
------

RelationalExpression :

ShiftExpression

RelationalExpression < ShiftExpression

RelationalExpression > ShiftExpression

RelationalExpression <= ShiftExpression

RelationalExpression >= ShiftExpression

RelationalExpression instanceof ShiftExpression

RelationalExpression in ShiftExpression

RelationalExpressionNoIn :

ShiftExpression

RelationalExpressionNoIn < ShiftExpression

RelationalExpressionNoIn > ShiftExpression

RelationalExpressionNoIn <= ShiftExpression

RelationalExpressionNoIn >= ShiftExpression

RelationalExpressionNoIn instanceof ShiftExpression

NOTE The “NoIn” variants are needed to avoid confusing the in operator in a relational expression
with the in operator in a for statement.

Semantics
---------

The result of evaluating a relational operator is always of type Boolean, reflecting whether the
relationship named by the operator holds between its two operands.

The RelationalExpressionNoIn productions are evaluated in the same manner as the
RelationalExpression productions except that the contained RelationalExpressionNoIn is evaluated
instead of the contained RelationalExpression.

11.8.1 The Less-than Operator ( < )
===================================

The production RelationalExpression : RelationalExpression < ShiftExpression is evaluated as
follows:

1.  Let lref be the result of evaluating RelationalExpression.
2.  Let lval be GetValue(lref).
3.  Let rref be the result of evaluating ShiftExpression.
4.  Let rval be GetValue(rref).
5.  Let r be the result of performing abstract relational comparison lval < rval. (see 11.8.5)
6.  If r is undefined, return false. Otherwise, return r.

11.8.2 The Greater-than Operator ( > )
======================================

The production RelationalExpression : RelationalExpression > ShiftExpression is evaluated as
follows:

1.  Let lref be the result of evaluating RelationalExpression.
2.  Let lval be GetValue(lref).
3.  Let rref be the result of evaluating ShiftExpression.
4.  Let rval be GetValue(rref).
5.  Let r be the result of performing abstract relational comparison rval < lval with LeftFirst
    equal to false. (see 11.8.5).
6.  If r is undefined, return false. Otherwise, return r.

11.8.3 The Less-than-or-equal Operator ( <= )
=============================================

The production RelationalExpression : RelationalExpression <= ShiftExpression is evaluated as
follows:

1.  Let lref be the result of evaluating RelationalExpression.
2.  Let lval be GetValue(lref).
3.  Let rref be the result of evaluating ShiftExpression.
4.  Let rval be GetValue(rref).
5.  Let r be the result of performing abstract relational comparison rval < lval with LeftFirst
    equal to false. (see 11.8.5).
6.  If r is true or undefined, return false. Otherwise, return true.

11.8.4 The Greater-than-or-equal Operator ( >= )
================================================

The production RelationalExpression : RelationalExpression >= ShiftExpression is evaluated as
follows:

1.  Let lref be the result of evaluating RelationalExpression.
2.  Let lval be GetValue(lref).
3.  Let rref be the result of evaluating ShiftExpression.
4.  Let rval be GetValue(rref).
5.  Let r be the result of performing abstract relational comparison lval < rval. (see 11.8.5)
6.  If r is true or undefined, return false. Otherwise, return true.

11.8.5 The Abstract Relational Comparison Algorithm
===================================================

The comparison x < y, where x and y are values, produces true, false, or undefined (which indicates
that at least one operand is NaN). In addition to x and y the algorithm takes a Boolean flag named
LeftFirst as a parameter. The flag is used to control the order in which operations with potentially
visible side-effects are performed upon x and y. It is necessary because ECMAScript specifies left
to right evaluation of expressions. The default value of LeftFirst is true and indicates that the x
parameter corresponds to an expression that occurs to the left of the y parameter’s corresponding
expression. If LeftFirst is false, the reverse is the case and operations must be performed upon y
before x. Such a comparison is performed as follows:

1.  If the LeftFirst flag is true, then
    1.  Let px be the result of calling ToPrimitive(x, hint Number).
    2.  Let py be the result of calling ToPrimitive(y, hint Number).

2.  Else the order of evaluation needs to be reversed to preserve left to right evaluation
    1.  Let py be the result of calling ToPrimitive(y, hint Number).
    2.  Let px be the result of calling ToPrimitive(x, hint Number).

3.  If it is not the case that both Type(px) is String and Type(py) is String, then
    1.  Let nx be the result of calling ToNumber(px). Because px and py are primitive values
        evaluation order is not important.
    2.  Let ny be the result of calling ToNumber(py).
    3.  If nx is NaN, return undefined.
    4.  If ny is NaN, return undefined.
    5.  If nx and ny are the same Number value, return false.
    6.  If nx is +0 and ny is −0, return false.
    7.  If nx is −0 and ny is +0, return false.
    8.  If nx is +∞, return false.
    9.  If ny is +∞, return true.
    10. If ny is −∞, return false.
    11. If nx is −∞, return true.
    12. If the mathematical value of nx is less than the mathematical value of ny —note that these
        mathematical values are both finite and not both zero—return true. Otherwise, return false.

4.  Else, both px and py are Strings
    1.  If py is a prefix of px, return false. (A String value p is a prefix of String value q if q
        can be the result of concatenating p and some other String r. Note that any String is a
        prefix of itself, because r may be the empty String.)
    2.  If px is a prefix of py, return true.
    3.  Let k be the smallest nonnegative integer such that the character at position k within px is
        different from the character at position k within py. (There must be such a k, for neither
        String is a prefix of the other.)
    4.  Let m be the integer that is the code unit value for the character at position k within px.
    5.  Let n be the integer that is the code unit value for the character at position k within py.
    6.  If m < n, return true. Otherwise, return false.

NOTE 1 Step 3 differs from step 7 in the algorithm for the addition operator + (11.6.1) in using and
instead of or.

NOTE 2 The comparison of Strings uses a simple lexicographic ordering on sequences of code unit
values. There is no attempt to use the more complex, semantically oriented definitions of character
or string equality and collating order defined in the Unicode specification. Therefore String values
that are canonically equal according to the Unicode standard could test as unequal. In effect this
algorithm assumes that both Strings are already in normalised form. Also, note that for strings
containing supplementary characters, lexicographic ordering on sequences of UTF-16 code unit values
differs from that on sequences of code point values.

11.8.6 The instanceof operator
==============================

The production RelationalExpression : RelationalExpression instanceof ShiftExpression is evaluated
as follows:

1.  Let lref be the result of evaluating RelationalExpression.
2.  Let lval be GetValue(lref).
3.  Let rref be the result of evaluating ShiftExpression.
4.  Let rval be GetValue(rref).
5.  If Type(rval) is not Object, throw a TypeError exception.
6.  If rval does not have a [[HasInstance]] internal method, throw a TypeError exception.
7.  Return the result of calling the [[HasInstance]] internal method of rval with argument lval.

11.8.7 The in operator
======================

The production RelationalExpression : RelationalExpression in ShiftExpression is evaluated as
follows:

1.  Let lref be the result of evaluating RelationalExpression.
2.  Let lval be GetValue(lref).
3.  Let rref be the result of evaluating ShiftExpression.
4.  Let rval be GetValue(rref).
5.  If Type(rval) is not Object, throw a TypeError exception.
6.  Return the result of calling the [[HasProperty]] internal method of rval with argument
    ToString(lval).

11.9 Equality Operators
=======================

Syntax
------

EqualityExpression :

RelationalExpression

EqualityExpression == RelationalExpression

EqualityExpression != RelationalExpression

EqualityExpression === RelationalExpression

EqualityExpression !== RelationalExpression

EqualityExpressionNoIn :

RelationalExpressionNoIn

EqualityExpressionNoIn == RelationalExpressionNoIn

EqualityExpressionNoIn != RelationalExpressionNoIn

EqualityExpressionNoIn === RelationalExpressionNoIn

EqualityExpressionNoIn !== RelationalExpressionNoIn

Semantics
---------

The result of evaluating an equality operator is always of type Boolean, reflecting whether the
relationship named by the operator holds between its two operands.

The EqualityExpressionNoIn productions are evaluated in the same manner as the EqualityExpression
productions except that the contained EqualityExpressionNoIn and RelationalExpressionNoIn are
evaluated instead of the contained EqualityExpression and RelationalExpression, respectively.

11.9.1 The Equals Operator ( == )
=================================

The production EqualityExpression : EqualityExpression == RelationalExpression is evaluated as
follows:

1.  Let lref be the result of evaluating EqualityExpression.
2.  Let lval be GetValue(lref).
3.  Let rref be the result of evaluating RelationalExpression.
4.  Let rval be GetValue(rref).
5.  Return the result of performing abstract equality comparison rval == lval. (see 11.9.3).

11.9.2 The Does-not-equals Operator ( != )
==========================================

The production EqualityExpression : EqualityExpression != RelationalExpression is evaluated as
follows:

1.  Let lref be the result of evaluating EqualityExpression.
2.  Let lval be GetValue(lref).
3.  Let rref be the result of evaluating RelationalExpression.
4.  Let rval be GetValue(rref).
5.  Let r be the result of performing abstract equality comparison rval == lval. (see 11.9.3).
6.  If r is true, return false. Otherwise, return true.

11.9.3 The Abstract Equality Comparison Algorithm
=================================================

The comparison x == y, where x and y are values, produces true or false. Such a comparison is
performed as follows:

1.  If Type(x) is the same as Type(y), then
    1.  If Type(x) is Undefined, return true.
    2.  If Type(x) is Null, return true.
    3.  If Type(x) is Number, then
        1.  If x is NaN, return false.
        2.  If y is NaN, return false.
        3.  If x is the same Number value as y, return true.
        4.  If x is +0 and y is −0, return true.
        5.  If x is −0 and y is +0, return true.
        6.  Return false.

    4.  If Type(x) is String, then return true if x and y are exactly the same sequence of
        characters (same length and same characters in corresponding positions). Otherwise, return
        false.
    5.  If Type(x) is Boolean, return true if x and y are both true or both false. Otherwise, return
        false.
    6.  Return true if x and y refer to the same object. Otherwise, return false.

2.  If x is null and y is undefined, return true.
3.  If x is undefined and y is null, return true.
4.  If Type(x) is Number and Type(y) is String,  
    return the result of the comparison x == ToNumber(y).
5.  If Type(x) is String and Type(y) is Number,  
    return the result of the comparison ToNumber(x) == y.
6.  If Type(x) is Boolean, return the result of the comparison ToNumber(x) == y.
7.  If Type(y) is Boolean, return the result of the comparison x == ToNumber(y).
8.  If Type(x) is either String or Number and Type(y) is Object,  
    return the result of the comparison x == ToPrimitive(y).
9.  If Type(x) is Object and Type(y) is either String or Number,  
    return the result of the comparison ToPrimitive(x) == y.
10. Return false.

NOTE 1 Given the above definition of equality:

-   String comparison can be forced by: "" + a == "" + b.
-   Numeric comparison can be forced by: +a == +b.
-   Boolean comparison can be forced by: !a == !b.

NOTE 2 The equality operators maintain the following invariants:

-   A != B is equivalent to !(A == B).
-   A == B is equivalent to B == A, except in the order of evaluation of A and B.

NOTE 3 The equality operator is not always transitive. For example, there might be two distinct
String objects, each representing the same String value; each String object would be considered
equal to the String value by the == operator, but the two String objects would not be equal to each
other. For Example:

-   new String("a") == "a" and "a" == new             String("a")are both true.
-   new String("a") == new String("a") is false.

NOTE 4 Comparison of Strings uses a simple equality test on sequences of code unit values. There is
no attempt to use the more complex, semantically oriented definitions of character or string
equality and collating order defined in the Unicode specification. Therefore Strings values that are
canonically equal according to the Unicode standard could test as unequal. In effect this algorithm
assumes that both Strings are already in normalised form.

11.9.4 The Strict Equals Operator ( === )
=========================================

The production EqualityExpression : EqualityExpression === RelationalExpression is evaluated as
follows:

1.  Let lref be the result of evaluating EqualityExpression.
2.  Let lval be GetValue(lref).
3.  Let rref be the result of evaluating RelationalExpression.
4.  Let rval be GetValue(rref).
5.  Return the result of performing the strict equality comparison rval === lval. (See 11.9.6)

11.9.5 The Strict Does-not-equal Operator ( !== )
=================================================

The production EqualityExpression : EqualityExpression !== RelationalExpression is evaluated as
follows:

1.  Let lref be the result of evaluating EqualityExpression.
2.  Let lval be GetValue(lref).
3.  Let rref be the result of evaluating RelationalExpression.
4.  Let rval be GetValue(rref).
5.  Let r be the result of performing strict equality comparison rval === lval. (See 11.9.6)
6.  If r is true, return false. Otherwise, return true.

11.9.6 The Strict Equality Comparison Algorithm
===============================================

The comparison x === y, where x and y are values, produces true or false. Such a comparison is
performed as follows:

1.  If Type(x) is different from Type(y), return false.
2.  If Type(x) is Undefined, return true.
3.  If Type(x) is Null, return true.
4.  If Type(x) is Number, then
    1.  If x is NaN, return false.
    2.  If y is NaN, return false.
    3.  If x is the same Number value as y, return true.
    4.  If x is +0 and y is −0, return true.
    5.  If x is −0 and y is +0, return true.
    6.  Return false.

5.  If Type(x) is String, then return true if x and y are exactly the same sequence of characters
    (same length and same characters in corresponding positions); otherwise, return false.
6.  If Type(x) is Boolean, return true if x and y are both true or both false; otherwise, return
    false.
7.  Return true if x and y refer to the same object. Otherwise, return false.

NOTE This algorithm differs from the SameValue Algorithm (9.12) in its treatment of signed zeroes
and NaNs.

11.10 Binary Bitwise Operators
==============================

Syntax
------

BitwiseANDExpression :

EqualityExpression

BitwiseANDExpression & EqualityExpression

BitwiseANDExpressionNoIn :

EqualityExpressionNoIn

BitwiseANDExpressionNoIn & EqualityExpressionNoIn

BitwiseXORExpression :

BitwiseANDExpression

BitwiseXORExpression ^ BitwiseANDExpression

BitwiseXORExpressionNoIn :

BitwiseANDExpressionNoIn

BitwiseXORExpressionNoIn ^ BitwiseANDExpressionNoIn

BitwiseORExpression :

BitwiseXORExpression

BitwiseORExpression | BitwiseXORExpression

BitwiseORExpressionNoIn :

BitwiseXORExpressionNoIn

BitwiseORExpressionNoIn | BitwiseXORExpressionNoIn

Semantics
---------

The production A : A @ B, where @ is one of the bitwise operators in the productions above, is
evaluated as follows:

1.  Let lref be the result of evaluating A.
2.  Let lval be GetValue(lref).
3.  Let rref be the result of evaluating B.
4.  Let rval be GetValue(rref).
5.  Let lnum be ToInt32(lval).
6.  Let rnum be ToInt32(rval).
7.  Return the result of applying the bitwise operator @ to lnum and rnum. The result is a signed 32
    bit integer.

11.11 Binary Logical Operators
==============================

Syntax
------

LogicalANDExpression :

BitwiseORExpression

LogicalANDExpression && BitwiseORExpression

LogicalANDExpressionNoIn :

BitwiseORExpressionNoIn

LogicalANDExpressionNoIn && BitwiseORExpressionNoIn

LogicalORExpression :

LogicalANDExpression

LogicalORExpression || LogicalANDExpression

LogicalORExpressionNoIn :

LogicalANDExpressionNoIn

LogicalORExpressionNoIn || LogicalANDExpressionNoIn

Semantics
---------

The production LogicalANDExpression : LogicalANDExpression && BitwiseORExpression is evaluated as
follows:

1.  Let lref be the result of evaluating LogicalANDExpression.
2.  Let lval be GetValue(lref).
3.  If ToBoolean(lval) is false, return lval.
4.  Let rref be the result of evaluating BitwiseORExpression.
5.  Return GetValue(rref).

The production LogicalORExpression : LogicalORExpression || LogicalANDExpression is evaluated as
follows:

1.  Let lref be the result of evaluating LogicalORExpression.
2.  Let lval be GetValue(lref).
3.  If ToBoolean(lval) is true, return lval.
4.  Let rref be the result of evaluating LogicalANDExpression.
5.  Return GetValue(rref).

The LogicalANDExpressionNoIn and LogicalORExpressionNoIn productions are evaluated in the same
manner as the LogicalANDExpression and LogicalORExpression productions except that the contained
LogicalANDExpressionNoIn, BitwiseORExpressionNoIn and LogicalORExpressionNoIn are evaluated instead
of the contained LogicalANDExpression, BitwiseORExpression and LogicalORExpression, respectively.

NOTE The value produced by a && or || operator is not necessarily of type Boolean. The value
produced will always be the value of one of the two operand expressions.

11.12 Conditional Operator ( ? : )
==================================

Syntax
------

ConditionalExpression :

LogicalORExpression

LogicalORExpression ? AssignmentExpression : AssignmentExpression

ConditionalExpressionNoIn :

LogicalORExpressionNoIn

LogicalORExpressionNoIn ? AssignmentExpression : AssignmentExpressionNoIn

Semantics
---------

The production ConditionalExpression : LogicalORExpression ? AssignmentExpression :
AssignmentExpression is evaluated as follows:

1.  Let lref be the result of evaluating LogicalORExpression.
2.  If ToBoolean(GetValue(lref)) is true, then
    1.  Let trueRef be the result of evaluating the first AssignmentExpression.
    2.  Return GetValue(trueRef).

3.  Else
    1.  Let falseRef be the result of evaluating the second AssignmentExpression.
    2.  Return GetValue(falseRef).

The ConditionalExpressionNoIn production is evaluated in the same manner as the
ConditionalExpression production except that the contained LogicalORExpressionNoIn,
AssignmentExpression and AssignmentExpressionNoIn are evaluated instead of the contained
LogicalORExpression, first AssignmentExpression and second AssignmentExpression, respectively.

NOTE The grammar for a ConditionalExpression in ECMAScript is a little bit different from that in C
and Java, which each allow the second subexpression to be an Expression but restrict the third
expression to be a ConditionalExpression. The motivation for this difference in ECMAScript is to
allow an assignment expression to be governed by either arm of a conditional and to eliminate the
confusing and fairly useless case of a comma expression as the centre expression.

11.13 Assignment Operators
==========================

Syntax
------

AssignmentExpression :

ConditionalExpression

LeftHandSideExpression = AssignmentExpression

LeftHandSideExpression AssignmentOperator AssignmentExpression

AssignmentExpressionNoIn :

ConditionalExpressionNoIn

LeftHandSideExpression = AssignmentExpressionNoIn

LeftHandSideExpression AssignmentOperator AssignmentExpressionNoIn

AssignmentOperator : one of

  ---- ---- ---- ---- ---- ----- ----- ------ ---- ---- ----
  *=   /=   %=   +=   -=   <<=   >>=   >>>=   &=   ^=   |=
  ---- ---- ---- ---- ---- ----- ----- ------ ---- ---- ----

Semantics
---------

The AssignmentExpressionNoIn productions are evaluated in the same manner as the
AssignmentExpression productions except that the contained ConditionalExpressionNoIn and
AssignmentExpressionNoIn are evaluated instead of the contained ConditionalExpression and
AssignmentExpression, respectively.

11.13.1 Simple Assignment ( = )
===============================

The production AssignmentExpression : LeftHandSideExpression = AssignmentExpression is evaluated as
follows:

1.  Let lref be the result of evaluating LeftHandSideExpression.
2.  Let rref be the result of evaluating AssignmentExpression.
3.  Let rval be GetValue(rref).
4.  Throw a SyntaxError exception if the following conditions are all true:
    -   Type(lref) is Reference is true
    -   IsStrictReference(lref) is true
    -   Type(GetBase(lref)) is Environment Record
    -   GetReferencedName(lref) is either "eval" or "arguments"

5.  Call PutValue(lref, rval).
6.  Return rval.

NOTE When an assignment occurs within strict mode code, its LeftHandSide must not evaluate to an
unresolvable reference. If it does a ReferenceError exception is thrown upon assignment. The
LeftHandSide also may not be a reference to a data property with the attribute value
{[[Writable]]:false}, to an accessor property with the attribute value {[[Set]]:undefined}, nor to a
non-existent property of an object whose [[Extensible]] internal property has the value false. In
these cases a TypeError exception is thrown.

11.13.2 Compound Assignment ( op= )
===================================

The production AssignmentExpression : LeftHandSideExpression AssignmentOperator AssignmentExpression
, where AssignmentOperator is @= and @ represents one of the operators indicated above, is evaluated
as follows:

1.  Let lref be the result of evaluating LeftHandSideExpression.
2.  Let lval be GetValue(lref).
3.  Let rref be the result of evaluating AssignmentExpression.
4.  Let rval be GetValue(rref).
5.  Let r be the result of applying operator @ to lval and rval.
6.  Throw a SyntaxError exception if the following conditions are all true:
    -   Type(lref) is Reference is true
    -   IsStrictReference(lref) is true
    -   Type(GetBase(lref)) is Environment Record
    -   GetReferencedName(lref) is either "eval" or "arguments"

7.  Call PutValue(lref, r).
8.  Return r.

NOTE See NOTE 11.13.1.

11.14 Comma Operator ( , )
==========================

Syntax
------

Expression :

AssignmentExpression

Expression , AssignmentExpression

ExpressionNoIn :

AssignmentExpressionNoIn

ExpressionNoIn , AssignmentExpressionNoIn

Semantics
---------

The production Expression : Expression , AssignmentExpression is evaluated as follows:

1.  Let lref be the result of evaluating Expression.
2.  Call GetValue(lref).
3.  Let rref be the result of evaluating AssignmentExpression.
4.  Return GetValue(rref).

The ExpressionNoIn production is evaluated in the same manner as the Expression production except
that the contained ExpressionNoIn and AssignmentExpressionNoIn are evaluated instead of the
contained Expression and AssignmentExpression, respectively.

NOTE GetValue must be called even though its value is not used because it may have observable
side-effects.

12 Statements
=============

Syntax
------

Statement :

Block

VariableStatement

EmptyStatement

ExpressionStatement

IfStatement

IterationStatement

ContinueStatement

BreakStatement

ReturnStatement

WithStatement

LabelledStatement

SwitchStatement

ThrowStatement

TryStatement

DebuggerStatement

Semantics
---------

A Statement can be part of a LabelledStatement, which itself can be part of a LabelledStatement, and
so on. The labels introduced this way are collectively referred to as the “current label set” when
describing the semantics of individual statements. A LabelledStatement has no semantic meaning other
than the introduction of a label to a label set. The label set of an IterationStatement or a
SwitchStatement initially contains the single element empty. The label set of any other statement is
initially empty.

The result of evaluating a Statement is always a Completion value.

NOTE Several widely used implementations of ECMAScript are known to support the use of
FunctionDeclaration as a Statement. However there are significant and irreconcilable variations
among the implementations in the semantics applied to such FunctionDeclarations. Because of these
irreconcilable differences, the use of a FunctionDeclaration as a Statement results in code that is
not reliably portable among implementations. It is recommended that ECMAScript implementations
either disallow this usage of FunctionDeclaration or issue a warning when such a usage is
encountered. Future editions of ECMAScript may define alternative portable means for declaring
functions in a Statement context.

12.1 Block
==========

Syntax
------

Block :

{ StatementListopt }

StatementList :

Statement

StatementList Statement

Semantics
---------

The production Block : { } is evaluated as follows:

1.  Return (normal, empty, empty).

The production Block : { StatementList } is evaluated as follows:

1.  Return the result of evaluating StatementList.

The production StatementList : Statement is evaluated as follows:

1.  Let s be the result of evaluating Statement.
2.  If an exception was thrown, return (throw, V, empty) where V is the exception. (Execution now
    proceeds as if no exception were thrown.)
3.  Return s.

The production StatementList : StatementList Statement is evaluated as follows:

1.  Let sl be the result of evaluating StatementList.
2.  If sl is an abrupt completion, return sl.
3.  Let s be the result of evaluating Statement.
4.  If an exception was thrown, return (throw, V, empty) where V is the exception. (Execution now
    proceeds as if no exception were thrown.)
5.  If s.value is empty, let V = sl.value, otherwise let V = s.value.
6.  Return (s.type, V, s.target).

NOTE Steps 5 and 6 of the above algoritm ensure that the value of a StatementList is the value of
the last value producing Statement in the StatementList. For example, the following calls to the
eval function all return the value 1:

    eval("1;;;;;")

    eval("1;{}")

    eval("1;var a;")

12.2 Variable Statement
=======================

Syntax
------

VariableStatement :

var VariableDeclarationList ;

VariableDeclarationList :

VariableDeclaration

VariableDeclarationList , VariableDeclaration

VariableDeclarationListNoIn :

VariableDeclarationNoIn

VariableDeclarationListNoIn , VariableDeclarationNoIn

VariableDeclaration :

Identifier Initialiseropt

VariableDeclarationNoIn :

Identifier InitialiserNoInopt

Initialiser :

= AssignmentExpression

InitialiserNoIn :

= AssignmentExpressionNoIn

A variable statement declares variables that are created as defined in 10.5. Variables are
initialised to undefined when created. A variable with an Initialiser is assigned the value of its
AssignmentExpression when the VariableStatement is executed, not when the variable is created.

Semantics
---------

The production VariableStatement : var VariableDeclarationList ; is evaluated as follows:

1.  Evaluate VariableDeclarationList.
2.  Return (normal, empty, empty).

The production VariableDeclarationList :VariableDeclaration is evaluated as follows:

1.  Evaluate VariableDeclaration.

The production VariableDeclarationList : VariableDeclarationList , VariableDeclaration is evaluated
as follows:

1.  Evaluate VariableDeclarationList.
2.  Evaluate VariableDeclaration.

The production VariableDeclaration : Identifier is evaluated as follows:

1.  Return a String value containing the same sequence of characters as in the Identifier.

The production VariableDeclaration : Identifier Initialiser is evaluated as follows:

1.  Let lhs be the result of evaluating Identifier as described in 11.1.2.
2.  Let rhs be the result of evaluating Initialiser.
3.  Let value be GetValue(rhs).
4.  Call PutValue(lhs, value).
5.  Return a String value containing the same sequence of characters as in the Identifier.

NOTE The String value of a VariableDeclaration is used in the evaluation of for-in statements
(12.6.4).

If a VariableDeclaration is nested within a with statement and the Identifier in the
VariableDeclaration is the same as a property name of the binding object of the with statement’s
object environment record, then step 4 will assign value to the property instead of to the
VariableEnvironment binding of the Identifier.

The production Initialiser : = AssignmentExpression is evaluated as follows:

1.  Return the result of evaluating AssignmentExpression.

The VariableDeclarationListNoIn, VariableDeclarationNoIn and InitialiserNoIn productions are
evaluated in the same manner as the VariableDeclarationList, VariableDeclaration and Initialiser
productions except that the contained VariableDeclarationListNoIn, VariableDeclarationNoIn,
InitialiserNoIn and AssignmentExpressionNoIn are evaluated instead of the contained
VariableDeclarationList, VariableDeclaration, Initialiser and AssignmentExpression, respectively.

12.2.1 Strict Mode Restrictions
===============================

It is a SyntaxError if a VariableDeclaration or VariableDeclarationNoIn occurs within strict code
and its Identifier is either "eval" or “arguments”.

12.3 Empty Statement
====================

Syntax
------

EmptyStatement :

;

Semantics
---------

The production EmptyStatement : ; is evaluated as follows:

1.  Return (normal, empty, empty).

12.4 Expression Statement
=========================

Syntax
------

ExpressionStatement :

[lookahead ∉ {{, function}] Expression ;

NOTE An ExpressionStatement cannot start with an opening curly brace because that might make it
ambiguous with a Block. Also, an ExpressionStatement cannot start with the function keyword because
that might make it ambiguous with a FunctionDeclaration.

Semantics
---------

The production ExpressionStatement : [lookahead ∉ {{, function}] Expression ; is evaluated as
follows:

1.  Let exprRef be the result of evaluating Expression.
2.  Return (normal, GetValue(exprRef), empty).

12.5 The if Statement
=====================

Syntax
------

IfStatement :

if ( Expression ) Statement else Statement

if ( Expression ) Statement

Each else for which the choice of associated if is ambiguous shall be associated with the nearest
possible if that would otherwise have no corresponding else.

Semantics
---------

The production IfStatement : if ( Expression ) Statement else Statement is evaluated as follows:

1.  Let exprRef be the result of evaluating Expression.
2.  If ToBoolean(GetValue(exprRef)) is true, then
    1.  Return the result of evaluating the first Statement.

3.  Else,
    1.  Return the result of evaluating the second Statement.

The production IfStatement : if ( Expression ) Statement is evaluated as follows:

1.  Let exprRef be the result of evaluating Expression.
2.  If ToBoolean(GetValue(exprRef)) is false, return (normal, empty, empty).
3.  Return the result of evaluating Statement.

12.6 Iteration Statements
=========================

Syntax
------

IterationStatement :

do Statement while ( Expression );

while ( Expression ) Statement

for ( ExpressionNoInopt ; Expressionopt ; Expressionopt ) Statement

for ( var VariableDeclarationListNoIn ; Expressionopt ; Expressionopt ) Statement

for ( LeftHandSideExpression in Expression ) Statement

for ( var VariableDeclarationNoIn in Expression ) Statement

12.6.1 The do-while Statement
=============================

The production do Statement while ( Expression ); is evaluated as follows:

1.  Let V = empty.
2.  Let iterating be true.
3.  Repeat, while iterating is true
    1.  Let stmt be the result of evaluating Statement.
    2.  If stmt.value is not empty, let V = stmt.value.
    3.  If stmt.type is not continue || stmt.target is not in the current label set, then
        1.  If stmt.type is break and stmt.target is in the current label set, return (normal, V,
            empty).
        2.  If stmt is an abrupt completion, return stmt.

    4.  Let exprRef be the result of evaluating Expression.
    5.  If ToBoolean(GetValue(exprRef)) is false, set iterating to false.

4.  Return (normal, V, empty);

12.6.2 The while Statement
==========================

The production IterationStatement : while ( Expression ) Statement is evaluated as follows:

1.  Let V = empty.
2.  Repeat
    1.  Let exprRef be the result of evaluating Expression.
    2.  If ToBoolean(GetValue(exprRef)) is false, return (normal, V, empty).
    3.  Let stmt be the result of evaluating Statement.
    4.  If stmt.value is not empty, let V = stmt.value.
    5.  If stmt.type is not continue || stmt.target is not in the current label set, then
        1.  If stmt.type is break and stmt.target is in the current label set, then
            1.  Return (normal, V, empty).

        2.  If stmt is an abrupt completion, return stmt.

12.6.3 The for Statement
========================

The production  
 IterationStatement : for ( ExpressionNoInopt ; Expressionopt ; Expressionopt ) Statement  
is evaluated as follows:

1.  If ExpressionNoIn is present, then.
    1.  Let exprRef be the result of evaluating ExpressionNoIn.
    2.  Call GetValue(exprRef). (This value is not used but the call may have side-effects.)

2.  Let V = empty.
3.  Repeat
    1.  If the first Expression is present, then
        1.  Let testExprRef be the result of evaluating the first Expression.
        2.  If ToBoolean(GetValue(testExprRef)) is false, return (normal, V, empty).

    2.  Let stmt be the result of evaluating Statement.
    3.  If stmt.value is not empty, let V = stmt.value
    4.  If stmt.type is break and stmt.target is in the current label set, return (normal, V,
        empty).
    5.  If stmt.type is not continue || stmt.target is not in the current label set, then
        1.  If stmt is an abrupt completion, return stmt.

    6.  If the second Expression is present, then
        1.  Let incExprRef be the result of evaluating the second Expression.
        2.  Call GetValue(incExprRef). (This value is not used.)

The production  
 IterationStatement : for ( var VariableDeclarationListNoIn ; Expressionopt ; Expressionopt )
Statement  
is evaluated as follows:

1.  Evaluate VariableDeclarationListNoIn.
2.  Let V = empty.
3.  Repeat
    1.  If the first Expression is present, then
        1.  Let testExprRef be the result of evaluating the first Expression.
        2.  If ToBoolean(GetValue(testExprRef)) is false, then return (normal, V, empty).

    2.  Let stmt be the result of evaluating Statement.
    3.  If stmt.value is not empty, let V = stmt.value.
    4.  If stmt.type is break and stmt.target is in the current label set, return (normal, V,
        empty).
    5.  If stmt.type is not continue || stmt.target is not in the current label set, then
        1.  If stmt is an abrupt completion, return stmt.

    6.  If the second Expression is present, then.
        1.  Let incExprRef be the result of evaluating the second Expression.
        2.  Call GetValue(incExprRef). (This value is not used.)

12.6.4 The for-in Statement
===========================

The production IterationStatement : for ( LeftHandSideExpression in Expression ) Statement is
evaluated as follows:

1.  Let exprRef be the result of evaluating the Expression.
2.  Let experValue be GetValue(exprRef).
3.  If experValue is null or undefined, return (normal, empty, empty).
4.  Let obj be ToObject(experValue).
5.  Let V = empty.
6.  Repeat
    1.  Let P be the name of the next property of obj whose [[Enumerable]] attribute is true. If
        there is no such property, return (normal, V, empty).
    2.  Let lhsRef be the result of evaluating the LeftHandSideExpression ( it may be evaluated
        repeatedly).
    3.  Call PutValue(lhsRef, P).
    4.  Let stmt be the result of evaluating Statement.
    5.  If stmt.value is not empty, let V = stmt.value.
    6.  If stmt.type is break and stmt.target is in the current label set, return (normal, V,
        empty).
    7.  If stmt.type is not continue || stmt.target is not in the current label set, then
        1.  If stmt is an abrupt completion, return stmt.

The production  
 IterationStatement : for ( var VariableDeclarationNoIn in Expression ) Statement  
is evaluated as follows:

1.  Let varName be the result of evaluating VariableDeclarationNoIn.
2.  Let exprRef be the result of evaluating the Expression.
3.  Let experValue be GetValue(exprRef).
4.  If experValue is null or undefined, return (normal, empty, empty).
5.  Let obj be ToObject(experValue).
6.  Let V = empty.
7.  Repeat
    1.  Let P be the name of the next property of obj whose [[Enumerable]] attribute is true. If
        there is no such property, return (normal, V, empty).
    2.  Let varRef be the result of evaluating varName as if it were an Identifier Reference
        (11.1.2); it may be evaluated repeatedly.
    3.  Call PutValue(varRef, P).
    4.  Let stmt be the result of evaluating Statement.
    5.  If stmt.value is not empty, let V = stmt.value.
    6.  If stmt.type is break and stmt.target is in the current label set, return (normal, V,
        empty).
    7.  If stmt.type is not continue || stmt.target is not in the current label set, then
        1.  If stmt is an abrupt completion, return stmt.

The mechanics and order of enumerating the properties (step 6.a in the first algorithm, step 7.a in
the second) is not specified. Properties of the object being enumerated may be deleted during
enumeration. If a property that has not yet been visited during enumeration is deleted, then it will
not be visited. If new properties are added to the object being enumerated during enumeration, the
newly added properties are not guaranteed to be visited in the active enumeration. A property name
must not be visited more than once in any enumeration.

Enumerating the properties of an object includes enumerating properties of its prototype, and the
prototype of the prototype, and so on, recursively; but a property of a prototype is not enumerated
if it is “shadowed” because some previous object in the prototype chain has a property with the same
name. The values of [[Enumerable]] attributes are not considered when determining if a property of a
prototype object is shadowed by a previous object on the prototype chain.

NOTE See NOTE 11.13.1.

12.7 The continue Statement
===========================

Syntax
------

ContinueStatement :

continue ;

continue [no LineTerminator here] Identifier ;

Semantics
---------

A program is considered syntactically incorrect if either of the following is true:

-   The program contains a continue statement without the optional Identifier, which is not nested,
    directly or indirectly (but not crossing function boundaries), within an IterationStatement.

-   The program contains a continue statement with the optional Identifier, where Identifier does
    not appear in the label set of an enclosing (but not crossing function boundaries)
    IterationStatement.

A ContinueStatement without an Identifier is evaluated as follows:

1.  Return (continue, empty, empty).

A ContinueStatement with the optional Identifier is evaluated as follows:

1.  Return (continue, empty, Identifier).

12.8 The break Statement
========================

Syntax
------

BreakStatement :

break ;

break [no LineTerminator here] Identifier ;

Semantics
---------

A program is considered syntactically incorrect if either of the following is true:

-   The program contains a break statement without the optional Identifier, which is not nested,
    directly or indirectly (but not crossing function boundaries), within an IterationStatement or a
    SwitchStatement.

-   The program contains a break statement with the optional Identifier, where Identifier does not
    appear in the label set of an enclosing (but not crossing function boundaries) Statement.

A BreakStatement without an Identifier is evaluated as follows:

1.  Return (break, empty, empty).

A BreakStatement with an Identifier is evaluated as follows:

1.  Return (break, empty, Identifier).

12.9 The return Statement
=========================

Syntax
------

ReturnStatement :

return ;

return [no LineTerminator here] Expression ;

Semantics
---------

An ECMAScript program is considered syntactically incorrect if it contains a return statement that
is not within a FunctionBody. A return statement causes a function to cease execution and return a
value to the caller. If Expression is omitted, the return value is undefined. Otherwise, the return
value is the value of Expression.

A ReturnStatement is evaluated as follows:

1.  If the Expression is not present, return (return, undefined, empty).
2.  Let exprRef be the result of evaluating Expression.
3.  Return (return, GetValue(exprRef), empty).

12.10 The with Statement
========================

Syntax
------

WithStatement :

with ( Expression ) Statement

The with statement adds an object environment record for a computed object to the lexical
environment of the current execution context. It then executes a statement using this augmented
lexical environment. Finally, it restores the original lexical environment.

Semantics
---------

The production WithStatement : with ( Expression ) Statement is evaluated as follows:

1.  Let val be the result of evaluating Expression.
2.  Let obj be ToObject(GetValue(val)).
3.  Let oldEnv be the running execution context’s LexicalEnvironment.
4.  Let newEnv be the result of calling NewObjectEnvironment passing obj and oldEnv as the
    arguments.
5.  Set the provideThis flag of newEnv to true.
6.  Set the running execution context’s LexicalEnvironment to newEnv.
7.  Let C be the result of evaluating Statement but if an exception is thrown during the evaluation,
    let C be (throw, V, empty), where V is the exception. (Execution now proceeds as if no exception
    were thrown.)
8.  Set the running execution context’s Lexical Environment to oldEnv.
9.  Return C.

NOTE No matter how control leaves the embedded Statement, whether normally or by some form of abrupt
completion or exception, the LexicalEnvironment is always restored to its former state.

12.10.1 Strict Mode Restrictions
================================

Strict mode code may not include a WithStatement. The occurrence of a WithStatement in such a
context is treated as a SyntaxError.

12.11 The switch Statement
==========================

Syntax
------

SwitchStatement :

switch ( Expression ) CaseBlock

CaseBlock :

{ CaseClausesopt }

{ CaseClausesopt DefaultClause CaseClausesopt }

CaseClauses :

CaseClause

CaseClauses CaseClause

CaseClause :

case Expression : StatementListopt

DefaultClause :

default : StatementListopt

Semantics
---------

The production SwitchStatement : switch ( Expression ) CaseBlock is evaluated as follows:

1.  Let exprRef be the result of evaluating Expression.
2.  Let R be the result of evaluating CaseBlock, passing it GetValue(exprRef) as a parameter.
3.  If R.type is break and R.target is in the current label set, return (normal, R.value, empty).
4.  Return R.

The production CaseBlock : { CaseClausesopt } is given an input parameter, input, and is evaluated
as follows:

1.  Let V = empty.
2.  Let A be the list of CaseClause items in source text order.
3.  Let searching be true.
4.  Repeat, while searching is true
    1.  Let C be the next CaseClause in A. If there is no such CaseClause, return (normal, V,
        empty).
    2.  Let clauseSelector be the result of evaluating C.
    3.  If input is equal to clauseSelector as defined by the === operator, then
        1.  Set searching to false.
        2.  If C has a StatementList, then
            1.  Evaluate C’s StatementList and let R be the result.
            2.  If R is an abrupt completion, then return R.
            3.  Let V = R.value.

5.  Repeat
    1.  Let C be the next CaseClause in A. If there is no such CaseClause, return (normal, V,
        empty).
    2.  If C has a StatementList, then
        1.  Evaluate C’s StatementList and let R be the result.
        2.  If R.value is not empty, then let V = R.value.
        3.  If R is an abrupt completion, then return (R.type, V, R.target).

The production CaseBlock : { CaseClausesopt DefaultClause CaseClausesopt } is given an input
parameter, input, and is evaluated as follows:

1.  Let V = empty.
2.  Let A be the list of CaseClause items in the first CaseClauses, in source text order.
3.  Let B be the list of CaseClause items in the second CaseClauses, in source text order.
4.  Let found be false.
5.  Repeat letting C be in order each CaseClause in A
    1.  If found is false, then
        1.  Let clauseSelector be the result of evaluating C.
        2.  If input is equal to clauseSelector as defined by the === operator, then set found to
            true.

    2.  If found is true, then
        1.  If C has a StatementList, then
            1.  Evaluate C’s StatementList and let R be the result.
            2.  If R.value is not empty, then let V = R.value.
            3.  R is an abrupt completion, then return (R.type, V, R.target).

6.  Let foundInB be false.
7.  If found is false, then
    1.  Repeat, while foundInB is false and all elements of B have not been processed
        1.  Let C be the next CaseClause in B.
        2.  Let clauseSelector be the result of evaluating C.
        3.  If input is equal to clauseSelector as defined by the === operator, then
            1.  Set foundInB to true.
            2.  If C has a StatementList, then
                1.  Evaluate C’s StatementList and let R be the result.
                2.  If R.value is not empty, then let V = R.value.
                3.  R is an abrupt completion, then return (R.type, V, R.target).

8.  If foundInB is false and the DefaultClause has a StatementList, then
    1.  Evaluate the DefaultClause’s StatementList and let R be the result.
    2.  If R.value is not empty, then let V = R.value.
    3.  If R is an abrupt completion, then return (R.type, V, R.target).

9.  Repeat (Note that if step 7.a.i has been performed this loop does not start at the beginning of
    B)
    1.  Let C be the next CaseClause in B. If there is no such CaseClause, return (normal, V,
        empty).
    2.  If C has a StatementList, then
        1.  Evaluate C’s StatementList and let R be the result.
        2.  If R.value is not empty, then let V = R.value.
        3.  If R is an abrupt completion, then return (R.type, V, R.target).

The production CaseClause : case Expression : StatementListopt is evaluated as follows:

1.  Let exprRef be the result of evaluating Expression.
2.  Return GetValue(exprRef).

NOTE Evaluating CaseClause does not execute the associated StatementList. It simply evaluates the
Expression and returns the value, which the CaseBlock algorithm uses to determine which
StatementList to start executing.

12.12 Labelled Statements
=========================

Syntax
------

LabelledStatement :

Identifier : Statement

Semantics
---------

A Statement may be prefixed by a label. Labelled statements are only used in conjunction with
labelled break and continue statements. ECMAScript has no goto statement.

An ECMAScript program is considered syntactically incorrect if it contains a LabelledStatement that
is enclosed by a LabelledStatement with the same Identifier as label. This does not apply to labels
appearing within the body of a FunctionDeclaration that is nested, directly or indirectly, within a
labelled statement.

The production Identifier : Statement is evaluated by adding Identifier to the label set of
Statement and then evaluating Statement. If the LabelledStatement itself has a non-empty label set,
these labels are also added to the label set of Statement before evaluating it. If the result of
evaluating Statement is (break, V, L) where L is equal to Identifier, the production results in
(normal, V, empty).

Prior to the evaluation of a LabelledStatement, the contained Statement is regarded as possessing an
empty label set, unless it is an IterationStatement or a SwitchStatement, in which case it is
regarded as possessing a label set consisting of the single element, empty.

12.13 The throw Statement
=========================

Syntax
------

ThrowStatement :

throw [no LineTerminator here] Expression ;

Semantics
---------

The production ThrowStatement : throw [no LineTerminator here] Expression ; is evaluated as follows:

1.  Let exprRef be the result of evaluating Expression.
2.  Return (throw, GetValue(exprRef), empty).

12.14 The try Statement
=======================

Syntax
------

TryStatement :

try Block Catch

try Block Finally

try Block Catch Finally

Catch :

catch ( Identifier ) Block

Finally :

finally Block

The try statement encloses a block of code in which an exceptional condition can occur, such as a
runtime error or a throw statement. The catch clause provides the exception-handling code. When a
catch clause catches an exception, its Identifier is bound to that exception.

Semantics
---------

The production TryStatement : try Block Catch is evaluated as follows:

1.  Let B be the result of evaluating Block.
2.  If B.type is not throw, return B.
3.  Return the result of evaluating Catch with parameter B.value.

The production TryStatement : try Block Finally is evaluated as follows:

1.  Let B be the result of evaluating Block.
2.  Let F be the result of evaluating Finally.
3.  If F.type is normal, return B.
4.  Return F.

The production TryStatement : try Block Catch Finally is evaluated as follows:

1.  Let B be the result of evaluating Block.
2.  If B.type is throw, then
    1.  Let C be the result of evaluating Catch with parameter B.value.

3.  Else, B.type is not throw,
    1.  Let C be B.

4.  Let F be the result of evaluating Finally.
5.  If F.type is normal, return C.
6.  Return F.

The production Catch : catch ( Identifier ) Block is evaluated as follows:

1.  Let C be the parameter that has been passed to this production.
2.  Let oldEnv be the running execution context’s LexicalEnvironment.
3.  Let catchEnv be the result of calling NewDeclarativeEnvironment passing oldEnv as the argument.
4.  Call the CreateMutableBinding concrete method of catchEnv passing the Identifier String value as
    the argument.
5.  Call the SetMutableBinding concrete method of catchEnv passing the Identifier, C, and false as
    arguments. Note that the last argument is immaterial in this situation.
6.  Set the running execution context’s LexicalEnvironment to catchEnv.
7.  Let B be the result of evaluating Block.
8.  Set the running execution context’s LexicalEnvironment to oldEnv.
9.  Return B.

NOTE No matter how control leaves the Block the LexicalEnvironment is always restored to its former
state.

The production Finally : finally Block is evaluated as follows:

1.  Return the result of evaluating Block.

12.14.1 Strict Mode Restrictions
================================

It is a SyntaxError if a TryStatement with a Catch occurs within strict code and the Identifier of
the Catch production is either "eval" or “arguments”.

12.15 The debugger statement
============================

Syntax
------

DebuggerStatement :

debugger ;

Semantics
---------

Evaluating the DebuggerStatement production may allow an implementation to cause a breakpoint when
run under a debugger. If a debugger is not present or active this statement has no observable
effect.

The production DebuggerStatement : debugger ; is evaluated as follows:

1.  If an implementation defined debugging facility is available and enabled, then
    1.  Perform an implementation defined debugging action.
    2.  Let result be an implementation defined Completion value.

2.  Else
    1.  Let result be (normal, empty, empty).

3.  Return result.

13 Function Definition
======================

Syntax
------

FunctionDeclaration :

function Identifier ( FormalParameterListopt ) { FunctionBody }

FunctionExpression :

function Identifieropt ( FormalParameterListopt ) { FunctionBody }

FormalParameterList :

Identifier

FormalParameterList , Identifier

FunctionBody :

SourceElementsopt

Semantics
---------

The production  
 FunctionDeclaration : function Identifier ( FormalParameterListopt ) { FunctionBody }  
is instantiated as follows during Declaration Binding instantiation (10.5):

1.  Return the result of creating a new Function object as specified in 13.2 with parameters
    specified by FormalParameterListopt, and body specified by FunctionBody. Pass in the
    VariableEnvironment of the running execution context as the Scope. Pass in true as the Strict
    flag if the FunctionDeclaration is contained in strict code or if its FunctionBody is strict
    code.

The production  
 FunctionExpression : function ( FormalParameterListopt ) { FunctionBody }  
is evaluated as follows:

1.  Return the result of creating a new Function object as specified in 13.2 with parameters
    specified by FormalParameterListopt and body specified by FunctionBody. Pass in the
    LexicalEnvironment of the running execution context as the Scope. Pass in true as the Strict
    flag if the FunctionExpression is contained in strict code or if its FunctionBody is strict
    code.

The production  
 FunctionExpression : function Identifier ( FormalParameterListopt ) { FunctionBody }  
is evaluated as follows:

1.  Let funcEnv be the result of calling NewDeclarativeEnvironment passing the running execution
    context’s Lexical Environment as the argument
2.  Let envRec be funcEnv’s environment record.
3.  Call the CreateImmutableBinding concrete method of envRec passing the String value of Identifier
    as the argument.
4.  Let closure be the result of creating a new Function object as specified in 13.2 with parameters
    specified by FormalParameterListopt and body specified by FunctionBody. Pass in funcEnv as the
    Scope. Pass in true as the Strict flag if the FunctionExpression is contained in strict code or
    if its FunctionBody is strict code.
5.  Call the InitializeImmutableBinding concrete method of envRec passing the String value of
    Identifier and closure as the arguments.
6.  Return closure.

NOTE The Identifier in a FunctionExpression can be referenced from inside the FunctionExpression’s
FunctionBody to allow the function to call itself recursively. However, unlike in a
FunctionDeclaration, the Identifier in a FunctionExpression cannot be referenced from and does not
affect the scope enclosing the FunctionExpression.

The production FunctionBody : SourceElementsopt is evaluated as follows:

1.  The code of this FunctionBody is strict mode code if it is part of a FunctionDeclaration or
    FunctionExpression that is contained in strict mode code or if the Directive Prologue (14.1) of
    its SourceElements contains a Use Strict Directive or if any of the conditions in 10.1.1 apply.
    If the code of this FunctionBody is strict mode code, SourceElements is evaluated in the
    following steps as strict mode code. Otherwise, SourceElements is evaluated in the following
    steps as non-strict mode code.
2.  If SourceElements is present return the result of evaluating SourceElements.
3.  Else return (normal, undefined, empty).

13.1 Strict Mode Restrictions
=============================

It is a SyntaxError if any Identifier value occurs more than once within a FormalParameterList of a
strict mode FunctionDeclaration or FunctionExpression.

It is a SyntaxError if the Identifier "eval" or the Identifier "arguments" occurs within a
FormalParameterList of a strict mode FunctionDeclaration or FunctionExpression.

It is a SyntaxError if the Identifier "eval" or the Identifier "arguments" occurs as the Identifier
of a strict mode FunctionDeclaration or FunctionExpression.

13.2 Creating Function Objects
==============================

Given an optional parameter list specified by FormalParameterList, a body specified by FunctionBody,
a Lexical Environment specified by Scope, and a Boolean flag Strict, a Function object is
constructed as follows:

1.  Create a new native ECMAScript object and let F be that object.
2.  Set all the internal methods, except for [[Get]], of F as described in 8.12.
3.  Set the [[Class]] internal property of F to "Function".
4.  Set the [[Prototype]] internal property of F to the standard built-in Function prototype object
    as specified in 15.3.3.1.
5.  Set the [[Get]] internal property of F as described in 15.3.5.4.
6.  Set the [[Call]] internal property of F as described in 13.2.1.
7.  Set the [[Construct]] internal property of F as described in 13.2.2.
8.  Set the [[HasInstance]] internal property of F as described in 15.3.5.3.
9.  Set the [[Scope]] internal property of F to the value of Scope.
10. Let names be a List containing, in left to right textual order, the Strings corresponding to the
    identifiers of FormalParameterList. If no parameters are specified, let names be the empty list.
11. Set the [[FormalParameters]] internal property of F to names.
12. Set the [[Code]] internal property of F to FunctionBody.
13. Set the [[Extensible]] internal property of F to true.
14. Let len be the number of formal parameters specified in FormalParameterList. If no parameters
    are specified, let len be 0.
15. Call the [[DefineOwnProperty]] internal method of F with arguments "length", Property Descriptor
    {[[Value]]: len, [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false}, and
    false.
16. Let proto be the result of creating a new object as would be constructed by the expression
    new           Object()where Object is the standard built-in constructor with that name.
17. Call the [[DefineOwnProperty]] internal method of proto with arguments "constructor", Property
    Descriptor {[[Value]]: F, { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true},
    and false.
18. Call the [[DefineOwnProperty]] internal method of F with arguments "prototype", Property
    Descriptor {[[Value]]: proto, { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]:
    false}, and false.
19. If Strict is true, then
    1.  Let thrower be the [[ThrowTypeError]] function Object (13.2.3).
    2.  Call the [[DefineOwnProperty]] internal method of F with arguments "caller",
        PropertyDescriptor {[[Get]]: thrower, [[Set]]: thrower, [[Enumerable]]: false,
        [[Configurable]]: false}, and false.
    3.  Call the [[DefineOwnProperty]] internal method of F with arguments "arguments",
        PropertyDescriptor {[[Get]]: thrower, [[Set]]: thrower, [[Enumerable]]: false,
        [[Configurable]]: false}, and false.

20. Return F.

NOTE A prototype property is automatically created for every function, to allow for the possibility
that the function will be used as a constructor.

13.2.1 [[Call]]
===============

When the [[Call]] internal method for a Function object F is called with a this value and a list of
arguments, the following steps are taken:

1.  Let funcCtx be the result of establishing a new execution context for function code using the
    value of F’s [[FormalParameters]] internal property, the passed arguments List args, and the
    this value as described in 10.4.3.
2.  Let result be the result of evaluating the FunctionBody that is the value of F’s [[Code]]
    internal property. If F does not have a [[Code]] internal property or if its value is an empty
    FunctionBody, then result is (normal, undefined, empty).
3.  Exit the execution context funcCtx, restoring the previous execution context.
4.  If result.type is throw then throw result.value.
5.  If result.type is return then return result.value.
6.  Otherwise result.type must be normal. Return undefined.

13.2.2 [[Construct]]
====================

When the [[Construct]] internal method for a Function object F is called with a possibly empty list
of arguments, the following steps are taken:

1.  Let obj be a newly created native ECMAScript object.
2.  Set all the internal methods of obj as specified in 8.12.
3.  Set the [[Class]] internal property of obj to "Object".
4.  Set the [[Extensible]] internal property of obj to true.
5.  Let proto be the value of calling the [[Get]] internal property of F with argument "prototype".
6.  If Type(proto) is Object, set the [[Prototype]] internal property of obj to proto.
7.  If Type(proto) is not Object, set the [[Prototype]] internal property of obj to the standard
    built-in Object prototype object as described in 15.2.4.
8.  Let result be the result of calling the [[Call]] internal property of F, providing obj as the
    this value and providing the argument list passed into [[Construct]] as args.
9.  If Type(result) is Object then return result.
10. Return obj.

13.2.3 The [[ThrowTypeError]] Function Object
=============================================

The [[ThrowTypeError]] object is a unique function object that is defined once as follows:

1.  Create a new native ECMAScript object and let F be that object.
2.  Set all the internal methods of F as described in 8.12.
3.  Set the [[Class]] internal property of F to "Function".
4.  Set the [[Prototype]] internal property of F to the standard built-in Function prototype object
    as specified in 15.3.3.1.
5.  Set the [[Call]] internal property of F as described in 13.2.1.
6.  Set the [[Scope]] internal property of F to the Global Environment.
7.  Set the [[FormalParameters]] internal property of F to an empty List.
8.  Set the [[Code]] internal property of F to be a FunctionBody that unconditionally throws a
    TypeError exception and performs no other action.
9.  Call the [[DefineOwnProperty]] internal method of F with arguments "length", Property Descriptor
    {[[Value]]: 0, [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false}, and false.
10. Set the [[Extensible]] internal property of F to false.
11. Let [[ThrowTypeError]] be F.

14 Program
==========

Syntax
------

Program :

SourceElementsopt

SourceElements :

SourceElement

SourceElements SourceElement

SourceElement :

Statement

FunctionDeclaration

Semantics
---------

The production Program : SourceElementsopt is evaluated as follows:

1.  The code of this Program is strict mode code if the Directive Prologue (14.1) of its
    SourceElements contains a Use Strict Directive or if any of the conditions of 10.1.1 apply. If
    the code of this Program is strict mode code, SourceElements is evaluated in the following steps
    as strict mode code. Otherwise SourceElements is evaluated in the following steps as non-strict
    mode code.
2.  If SourceElements is not present, return (normal, empty, empty).
3.  Let progCxt be a new execution context for global code as described in 10.4.1.
4.  Let result be the result of evaluating SourceElements.
5.  Exit the execution context progCxt.
6.  Return result.

NOTE The processes for initiating the evaluation of a Program and for dealing with the result of
such an evaluation are defined by an ECMAScript implementation and not by this specification.

The production SourceElements : SourceElements SourceElement is evaluated as follows:

1.  Let headResult be the result of evaluating SourceElements.
2.  If headResult is an abrupt completion, return headResult.
3.  Let tailResult be result of evaluating SourceElement.
4.  If tailResult.value is empty, let V = headResult.value, otherwise let V = tailResult.value.
5.  Return (tailResult.type, V, tailResult.target)

The production SourceElement : Statement is evaluated as follows:

1.  Return the result of evaluating Statement.

The production SourceElement : FunctionDeclaration is evaluated as follows:

1.  Return (normal, empty, empty).

14.1 Directive Prologues and the Use Strict Directive
=====================================================

A Directive Prologue is the longest sequence of ExpressionStatement productions occurring as the
initial SourceElement productions of a Program or FunctionBody and where each ExpressionStatement in
the sequence consists entirely of a StringLiteral token followed a semicolon. The semicolon may
appear explicitly or may be inserted by automatic semicolon insertion. A Directive Prologue may be
an empty sequence.

A Use Strict Directive is an ExpressionStatement in a Directive Prologue whose StringLiteral is
either the exact character sequences "use strict" or 'use strict'. A Use Strict Directive may not
contain an EscapeSequence or LineContinuation.

A Directive Prologue may contain more than one Use Strict Directive. However, an implementation may
issue a warning if this occurs.

NOTE The ExpressionStatement productions of a Directive Prologue are evaluated normally during
evaluation of the containing SourceElements production. Implementations may define implementation
specific meanings for ExpressionStatement productions which are not a Use Strict Directive and which
occur in a Directive Prologue. If an appropriate notification mechanism exists, an implementation
should issue a warning if it encounters in a Directive Prologue an ExpressionStatement that is not a
Use Strict Directive or which does not have a meaning defined by the implementation.

15 Standard Built-in ECMAScript Objects
=======================================

There are certain built-in objects available whenever an ECMAScript program begins execution. One,
the global object, is part of the lexical environment of the executing program. Others are
accessible as initial properties of the global object.

Unless specified otherwise, the [[Class]] internal property of a built-in object is "Function" if
that built-in object has a [[Call]] internal property, or "Object" if that built-in object does not
have a [[Call]] internal property. Unless specified otherwise, the [[Extensible]] internal property
of a built-in object initially has the value true.

Many built-in objects are functions: they can be invoked with arguments. Some of them furthermore
are constructors: they are functions intended for use with the new operator. For each built-in
function, this specification describes the arguments required by that function and properties of the
Function object. For each built-in constructor, this specification furthermore describes properties
of the prototype object of that constructor and properties of specific object instances returned by
a new expression that invokes that constructor.

Unless otherwise specified in the description of a particular function, if a function or constructor
described in this clause is given fewer arguments than the function is specified to require, the
function or constructor shall behave exactly as if it had been given sufficient additional
arguments, each such argument being the undefined value.

Unless otherwise specified in the description of a particular function, if a function or constructor
described in this clause is given more arguments than the function is specified to allow, the extra
arguments are evaluated by the call and then ignored by the function. However, an implementation may
define implementation specific behaviour relating to such arguments as long as the behaviour is not
the throwing of a TypeError exception that is predicated simply on the presence of an extra
argument.

NOTE Implementations that add additional capabilities to the set of built-in functions are
encouraged to do so by adding new functions rather than adding new parameters to existing functions.

Every built-in function and every built-in constructor has the Function prototype object, which is
the initial value of the expression Function.prototype (15.3.4), as the value of its [[Prototype]]
internal property.

Unless otherwise specified every built-in prototype object has the Object prototype object, which is
the initial value of the expression Object.prototype (15.2.4), as the value of its [[Prototype]]
internal property, except the Object prototype object itself.

None of the built-in functions described in this clause that are not constructors shall implement
the [[Construct]] internal method unless otherwise specified in the description of a particular
function. None of the built-in functions described in this clause shall have a prototype property
unless otherwise specified in the description of a particular function.

This clause generally describes distinct behaviours for when a constructor is “called as a function”
and for when it is “called as part of a new expression”. The “called as a function” behaviour
corresponds to the invocation of the constructor’s [[Call]] internal method and the “called as part
of a new expression” behaviour corresponds to the invocation of the constructor’s [[Construct]]
internal method.

Every built-in Function object described in this clause—whether as a constructor, an ordinary
function, or both—has a length property whose value is an integer. Unless otherwise specified, this
value is equal to the largest number of named arguments shown in the subclause headings for the
function description, including optional parameters.

NOTE For example, the Function object that is the initial value of the slice property of the String
prototype object is described under the subclause heading “String.prototype.slice (start, end)”
which shows the two named arguments start and end; therefore the value of the length property of
that Function object is 2.

In every case, the length property of a built-in Function object described in this clause has the
attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }. Every other
property described in this clause has the attributes { [[Writable]]: true, [[Enumerable]]: false,
[[Configurable]]: true } unless otherwise specified.

15.1 The Global Object
======================

The unique global object is created before control enters any execution context.

Unless otherwise specified, the standard built-in properties of the global object have attributes
{[[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true}.

The global object does not have a [[Construct]] internal property; it is not possible to use the
global object as a constructor with the new operator.

The global object does not have a [[Call]] internal property; it is not possible to invoke the
global object as a function.

The values of the [[Prototype]] and [[Class]] internal properties of the global object are
implementation-dependent.

In addition to the properties defined in this specification the global object may have additional
host defined properties. This may include a property whose value is the global object itself; for
example, in the HTML document object model the window property of the global object is the global
object itself.

15.1.1 Value Properties of the Global Object
============================================

15.1.1.1 NaN
============

The value of NaN is NaN (see 8.5). This property has the attributes { [[Writable]]: false,
[[Enumerable]]: false, [[Configurable]]: false }.

15.1.1.2 Infinity
=================

The value of Infinity is +∞ (see 8.5). This property has the attributes { [[Writable]]: false,
[[Enumerable]]: false, [[Configurable]]: false }.

15.1.1.3 undefined
==================

The value of undefined is undefined (see 8.1). This property has the attributes { [[Writable]]:
false, [[Enumerable]]: false, [[Configurable]]: false }.

15.1.2 Function Properties of the Global Object
===============================================

15.1.2.1 eval (x)
=================

When the eval function is called with one argument x, the following steps are taken:

1.  If Type(x) is not String, return x.
2.  Let prog be the ECMAScript code that is the result of parsing x as a Program. If the parse
    fails, throw a SyntaxError exception (but see also clause 16).
3.  Let evalCtx be the result of establishing a new execution context (10.4.2) for the eval code
    prog.
4.  Let result be the result of evaluating the program prog.
5.  Exit the running execution context evalCtx, restoring the previous execution context.
6.  If result.type is normal and its completion value is a value V, then return the value V.
7.  If result.type is normal and its completion value is empty, then return the value undefined.
8.  Otherwise, result.type must be throw. Throw result.value as an exception.

15.1.2.1.1 Direct Call to Eval
==============================

A direct call to the eval function is one that is expressed as a CallExpression that meets the
following two conditions:

The Reference that is the result of evaluating the MemberExpression in the CallExpression has an
environment record as its base value and its reference name is "eval".

The result of calling the abstract operation GetValue with that Reference as the argument is the
standard built-in function defined in 15.1.2.1.

15.1.2.2 parseInt (string , radix)
==================================

The parseInt function produces an integer value dictated by interpretation of the contents of the
string argument according to the specified radix. Leading white space in string is ignored. If radix
is undefined or 0, it is assumed to be 10 except when the number begins with the character pairs 0x
or 0X, in which case a radix of 16 is assumed. If radix is 16, the number may also optionally begin
with the character pairs 0x or 0X.

When the parseInt function is called, the following steps are taken:

1.  Let inputString be ToString(string).
2.  Let S be a newly created substring of inputString consisting of the first character that is not
    a StrWhiteSpaceChar and all characters following that character. (In other words, remove leading
    white space.) If inputString does not contain any such characters, let S be the empty string.
3.  Let sign be 1.
4.  If S is not empty and the first character of S is a minus sign -, let sign be −1.
5.  If S is not empty and the first character of S is a plus sign + or a minus sign -, then remove
    the first character from S.
6.  Let R = ToInt32(radix).
7.  Let stripPrefix be true.
8.  If R ≠ 0, then
    1.  If R < 2 or R > 36, then return NaN.
    2.  If R ≠ 16, let stripPrefix be false.

9.  Else, R = 0
    1.  Let R = 10.

10. If stripPrefix is true, then
    1.  If the length of S is at least 2 and the first two characters of S are either “0x” or “0X”,
        then remove the first two characters from S and let R = 16.

11. If S contains any character that is not a radix-R digit, then let Z be the substring of S
    consisting of all characters before the first such character; otherwise, let Z be S.
12. If Z is empty, return NaN.
13. Let mathInt be the mathematical integer value that is represented by Z in radix-R notation,
    using the letters A-Z and a-z for digits with values 10 through 35. (However, if R is 10 and Z
    contains more than 20 significant digits, every significant digit after the 20th may be replaced
    by a 0 digit, at the option of the implementation; and if R is not 2, 4, 8, 10, 16, or 32, then
    mathInt may be an implementation-dependent approximation to the mathematical integer value that
    is represented by Z in radix-R notation.)
14. Let number be the Number value for mathInt.
15. Return sign × number.

NOTE parseInt may interpret only a leading portion of string as an integer value; it ignores any
characters that cannot be interpreted as part of the notation of an integer, and no indication is
given that any such characters were ignored.

15.1.2.3 parseFloat (string)
============================

The parseFloat function produces a Number value dictated by interpretation of the contents of the
string argument as a decimal literal.

When the parseFloat function is called, the following steps are taken:

1.  Let inputString be ToString(string).
2.  Let trimmedString be a substring of inputString consisting of the leftmost character that is not
    a StrWhiteSpaceChar and all characters to the right of that character. (In other words, remove
    leading white space.) If inputString does not contain any such characters, let trimmedString be
    the empty string.
3.  If neither trimmedString nor any prefix of trimmedString satisfies the syntax of a
    StrDecimalLiteral (see 9.3.1), return NaN.
4.  Let numberString be the longest prefix of trimmedString, which might be trimmedString itself,
    that satisfies the syntax of a StrDecimalLiteral.
5.  Return the Number value for the MV of numberString.

NOTE parseFloat may interpret only a leading portion of string as a Number value; it ignores any
characters that cannot be interpreted as part of the notation of an decimal literal, and no
indication is given that any such characters were ignored.

15.1.2.4 isNaN (number)
=======================

Returns true if the argument coerces to NaN, and otherwise returns false.

1.  If ToNumber(number) is NaN, return true.
2.  Otherwise, return false.

NOTE A reliable way for ECMAScript code to test if a value X is a NaN is an expression of the form X
!== X. The result will be true if and only if X is a NaN.

15.1.2.5 isFinite (number)
==========================

Returns false if the argument coerces to NaN, +∞, or −∞, and otherwise returns true.

1.  If ToNumber(number) is NaN, +∞, or −∞, return false.
2.  Otherwise, return true.

15.1.3 URI Handling Function Properties
=======================================

Uniform Resource Identifiers, or URIs, are Strings that identify resources (e.g. web pages or files)
and transport protocols by which to access them (e.g. HTTP or FTP) on the Internet. The ECMAScript
language itself does not provide any support for using URIs except for functions that encode and
decode URIs as described in 15.1.3.1, 15.1.3.2, 15.1.3.3 and 15.1.3.4.

NOTE Many implementations of ECMAScript provide additional functions and methods that manipulate web
pages; these functions are beyond the scope of this standard.

A URI is composed of a sequence of components separated by component separators. The general form
is:

Scheme : First / Second ; Third ? Fourth

where the italicised names represent components and “:”, “/”, “;” and “?” are reserved characters
used as separators. The encodeURI and decodeURI functions are intended to work with complete URIs;
they assume that any reserved characters in the URI are intended to have special meaning and so are
not encoded. The encodeURIComponent and decodeURIComponent functions are intended to work with the
individual component parts of a URI; they assume that any reserved characters represent text and so
must be encoded so that they are not interpreted as reserved characters when the component is part
of a complete URI.

The following lexical grammar specifies the form of encoded URIs.

Syntax
------

uri :::

uriCharactersopt

uriCharacters :::

uriCharacter uriCharactersopt

uriCharacter :::

uriReserved

uriUnescaped

uriEscaped

uriReserved ::: one of

; / ? : @ & = + $ ,

uriUnescaped :::

uriAlpha

DecimalDigit

uriMark

uriEscaped :::

% HexDigit HexDigit

uriAlpha ::: one of

a b c d e f g h i j k l m n o p q r s t u v w x y z

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

uriMark ::: one of

- _ . ! ~ * ' ( )

NOTE The above syntax is based upon RFC 2396 and does not reflect changes introduced by the more
recent RFC 3986.

When a character to be included in a URI is not listed above or is not intended to have the special
meaning sometimes given to the reserved characters, that character must be encoded. The character is
transformed into its UTF-8 encoding, with surrogate pairs first converted from UTF-16 to the
corresponding code point value. (Note that for code units in the range [0,127] this results in a
single octet with the same value.) The resulting sequence of octets is then transformed into a
String with each octet represented by an escape sequence of the form “%xx”.

The encoding and escaping process is described by the abstract operation Encode taking two String
arguments string and unescapedSet.

1.  Let strLen be the number of characters in string.
2.  Let R be the empty String.
3.  Let k be 0.
4.  Repeat
    1.  If k equals strLen, return R.
    2.  Let C be the character at position k within string.
    3.  If C is in unescapedSet, then
        1.  Let S be a String containing only the character C.
        2.  Let R be a new String value computed by concatenating the previous value of R and S.

    4.  Else, C is not in unescapedSet
        1.  If the code unit value of C is not less than 0xDC00 and not greater than 0xDFFF, throw a
            URIError exception.
        2.  If the code unit value of C is less than 0xD800 or greater than 0xDBFF, then
            1.  Let V be the code unit value of C.

        3.  Else,
            1.  Increase k by 1.
            2.  If k equals strLen, throw a URIError exception.
            3.  Let kChar be the code unit value of the character at position k within string.
            4.  If kChar is less than 0xDC00 or greater than 0xDFFF, throw a URIError exception.
            5.  Let V be (((the code unit value of C) – 0xD800) × 0x400 + (kChar – 0xDC00) +
                0x10000).

        4.  Let Octets be the array of octets resulting by applying the UTF-8 transformation to V,
            and let L be the array size.
        5.  Let j be 0.
        6.  Repeat, while j < L
            1.  Let jOctet be the value at position j within Octets.
            2.  Let S be a String containing three characters “%XY” where XY are two uppercase
                hexadecimal digits encoding the value of jOctet.
            3.  Let R be a new String value computed by concatenating the previous value of R and S.
            4.  Increase j by 1.

    5.  Increase k by 1.

The unescaping and decoding process is described by the abstract operation Decode taking two String
arguments string and reservedSet.

1.  Let strLen be the number of characters in string.
2.  Let R be the empty String.
3.  Let k be 0.
4.  Repeat
    1.  If k equals strLen, return R.
    2.  Let C be the character at position k within string.
    3.  If C is not ‘%’, then
        1.  Let S be the String containing only the character C.

    4.  Else, C is ‘%’
        1.  Let start be k.
        2.  If k + 2 is greater than or equal to strLen, throw a URIError exception.
        3.  If the characters at position (k+1) and (k + 2) within string do not represent
            hexadecimal digits, throw a URIError exception.
        4.  Let B be the 8-bit value represented by the two hexadecimal digits at position (k + 1)
            and (k + 2).
        5.  Increment k by 2.
        6.  If the most significant bit in B is 0, then
            1.  Let C be the character with code unit value B.
            2.  If C is not in reservedSet, then
                1.  Let S be the String containing only the character C.

            3.  Else, C is in reservedSet
                1.  Let S be the substring of string from position start to position k included.

        7.  Else, the most significant bit in B is 1
            1.  Let n be the smallest non-negative number such that (B << n) & 0x80 is equal to 0.
            2.  If n equals 1 or n is greater than 4, throw a URIError exception.
            3.  Let Octets be an array of 8-bit integers of size n.
            4.  Put B into Octets at position 0.
            5.  If k + (3 × (n – 1)) is greater than or equal to strLen, throw a URIError exception.
            6.  Let j be 1.
            7.  Repeat, while j < n
                1.  Increment k by 1.
                2.  If the character at position k is not ‘%’, throw a URIError exception.
                3.  If the characters at position (k +1) and (k + 2) within string do not represent
                    hexadecimal digits, throw a URIError exception.
                4.  Let B be the 8-bit value represented by the two hexadecimal digits at position
                    (k + 1) and (k + 2).
                5.  If the two most significant bits in B are not 10, throw a URIError exception.
                6.  Increment k by 2.
                7.  Put B into Octets at position j.
                8.  Increment j by 1.

            8.  Let V be the value obtained by applying the UTF-8 transformation to Octets, that is,
                from an array of octets into a 21-bit value. If Octets does not contain a valid
                UTF-8 encoding of a Unicode code point throw an URIError exception.
            9.  If V is less than 0x10000, then
                1.  Let C be the character with code unit value V.
                2.  If C is not in reservedSet, then
                    1.  Let S be the String containing only the character C.

                3.  Else, C is in reservedSet
                    1.  Let S be the substring of string from position start to position k included.

            10. Else, V is ≥ 0x10000
                1.  Let L be (((V – 0x10000) & 0x3FF) + 0xDC00).
                2.  Let H be ((((V – 0x10000) >> 10) & 0x3FF) + 0xD800).
                3.  Let S be the String containing the two characters with code unit values H and L.

    5.  Let R be a new String value computed by concatenating the previous value of R and S.
    6.  Increase k by 1.

NOTE This syntax of Uniform Resource Identifiers is based upon RFC 2396 and does not reflect the
more recent RFC 3986 which replaces RFC 2396. A formal description and implementation of UTF-8 is
given in RFC 3629.

In UTF-8, characters are encoded using sequences of 1 to 6 octets. The only octet of a “sequence” of
one has the higher-order bit set to 0, the remaining 7 bits being used to encode the character
value. In a sequence of n octets, n>1, the initial octet has the n higher-order bits set to 1,
followed by a bit set to 0. The remaining bits of that octet contain bits from the value of the
character to be encoded. The following octets all have the higher-order bit set to 1 and the
following bit set to 0, leaving 6 bits in each to contain bits from the character to be encoded. The
possible UTF-8 encodings of ECMAScript characters are specified in Table 21.

Table 21 — UTF-8 Encodings

+------------------+------------------+------------------+------------------+------------------+------------------+
| Code Unit Value  | Representation   | 1st Octet        | 2nd Octet        | 3rd Octet        | 4th Octet        |
+==================+==================+==================+==================+==================+==================+
| 0x0000 - 0x007F  | 00000000         | 0zzzzzzz         |                  |                  |                  |
|                  | 0zzzzzzz         |                  |                  |                  |                  |
+------------------+------------------+------------------+------------------+------------------+------------------+
| 0x0080 - 0x07FF  | 00000yyy         | 110yyyyy         | 10zzzzzz         |                  |                  |
|                  | yyzzzzzz         |                  |                  |                  |                  |
+------------------+------------------+------------------+------------------+------------------+------------------+
| 0x0800 - 0xD7FF  | xxxxyyyy         | 1110xxxx         | 10yyyyyy         | 10zzzzzz         |                  |
|                  | yyzzzzzz         |                  |                  |                  |                  |
+------------------+------------------+------------------+------------------+------------------+------------------+
| 0xD800 - 0xDBFF  | 110110vv         | 11110uuu         | 10uuwwww         | 10xxyyyy         | 10zzzzzz         |
|                  | vvwwwwxx         |                  |                  |                  |                  |
| followed by      |                  |                  |                  |                  |                  |
|                  | followed by      |                  |                  |                  |                  |
| 0xDC00 – 0xDFFF  |                  |                  |                  |                  |                  |
|                  | 110111yy         |                  |                  |                  |                  |
|                  | yyzzzzzz         |                  |                  |                  |                  |
+------------------+------------------+------------------+------------------+------------------+------------------+
| 0xD800 - 0xDBFF  | causes URIError  |                  |                  |                  |                  |
|                  |                  |                  |                  |                  |                  |
| not followed by  |                  |                  |                  |                  |                  |
|                  |                  |                  |                  |                  |                  |
| 0xDC00 – 0xDFFF  |                  |                  |                  |                  |                  |
+------------------+------------------+------------------+------------------+------------------+------------------+
| 0xDC00 – 0xDFFF  | causes URIError  |                  |                  |                  |                  |
+------------------+------------------+------------------+------------------+------------------+------------------+
| 0xE000 - 0xFFFF  | xxxxyyyy         | 1110xxxx         | 10yyyyyy         | 10zzzzzz         |                  |
|                  | yyzzzzzz         |                  |                  |                  |                  |
+------------------+------------------+------------------+------------------+------------------+------------------+

Where

uuuuu = vvvv + 1

to account for the addition of 0x10000 as in Surrogates, section 3.7, of the Unicode Standard.

The range of code unit values 0xD800-0xDFFF is used to encode surrogate pairs; the above
transformation combines a UTF-16 surrogate pair into a UTF-32 representation and encodes the
resulting 21-bit value in UTF-8. Decoding reconstructs the surrogate pair.

RFC 3629 prohibits the decoding of invalid UTF-8 octet sequences. For example, the invalid sequence
C0 80 must not decode into the character U+0000. Implementations of the Decode algorithm are
required to throw a URIError when encountering such invalid sequences.

15.1.3.1 decodeURI (encodedURI)
===============================

The decodeURI function computes a new version of a URI in which each escape sequence and UTF-8
encoding of the sort that might be introduced by the encodeURI function is replaced with the
character that it represents. Escape sequences that could not have been introduced by encodeURI are
not replaced.

When the decodeURI function is called with one argument encodedURI, the following steps are taken:

1.  Let uriString be ToString(encodedURI).
2.  Let reservedURISet be a String containing one instance of each character valid in uriReserved
    plus “#”.
3.  Return the result of calling Decode(uriString, reservedURISet)

NOTE The character “#” is not decoded from escape sequences even though it is not a reserved URI
character.

15.1.3.2 decodeURIComponent (encodedURIComponent)
=================================================

The decodeURIComponent function computes a new version of a URI in which each escape sequence and
UTF-8 encoding of the sort that might be introduced by the encodeURIComponent function is replaced
with the character that it represents.

When the decodeURIComponent function is called with one argument encodedURIComponent, the following
steps are taken:

1.  Let componentString be ToString(encodedURIComponent).
2.  Let reservedURIComponentSet be the empty String.
3.  Return the result of calling Decode(componentString, reservedURIComponentSet)

15.1.3.3 encodeURI (uri)
========================

The encodeURI function computes a new version of a URI in which each instance of certain characters
is replaced by one, two, three, or four escape sequences representing the UTF-8 encoding of the
character.

When the encodeURI function is called with one argument uri, the following steps are taken:

1.  Let uriString be ToString(uri).
2.  Let unescapedURISet be a String containing one instance of each character valid in uriReserved
    and uriUnescaped plus “#”.
3.  Return the result of calling Encode(uriString, unescapedURISet)

NOTE The character “#” is not encoded to an escape sequence even though it is not a reserved or
unescaped URI character.

15.1.3.4 encodeURIComponent (uriComponent)
==========================================

The encodeURIComponent function computes a new version of a URI in which each instance of certain
characters is replaced by one, two, three, or four escape sequences representing the UTF-8 encoding
of the character.

When the encodeURIComponent function is called with one argument uriComponent, the following steps
are taken:

1.  Let componentString be ToString(uriComponent).
2.  Let unescapedURIComponentSet be a String containing one instance of each character valid in
    uriUnescaped.
3.  Return the result of calling Encode(componentString, unescapedURIComponentSet)

15.1.4 Constructor Properties of the Global Object
==================================================

15.1.4.1 Object ( . . . )
=========================

See 15.2.1 and 15.2.2.

15.1.4.2 Function ( . . . )
===========================

See 15.3.1 and 15.3.2.

15.1.4.3 Array ( . . . )
========================

See 15.4.1 and 15.4.2.

15.1.4.4 String ( . . . )
=========================

See 15.5.1 and 15.5.2.

15.1.4.5 Boolean ( . . . )
==========================

See 15.6.1 and 15.6.2.

15.1.4.6 Number ( . . . )
=========================

See 15.7.1 and 15.7.2.

15.1.4.7 Date ( . . . )
=======================

See 15.9.2.

15.1.4.8 RegExp ( . . . )
=========================

See 15.10.3 and 15.10.4.

15.1.4.9 Error ( . . . )
========================

See 15.11.1 and 15.11.2.

15.1.4.10 EvalError ( . . . )
=============================

See 15.11.6.1.

15.1.4.11 RangeError ( . . . )
==============================

See 15.11.6.2.

15.1.4.12 ReferenceError ( . . . )
==================================

See 15.11.6.3.

15.1.4.13 SyntaxError ( . . . )
===============================

See 15.11.6.4.

15.1.4.14 TypeError ( . . . )
=============================

See 15.11.6.5.

15.1.4.15 URIError ( . . . )
============================

See 15.11.6.6.

15.1.5 Other Properties of the Global Object
============================================

15.1.5.1 Math
=============

See 15.8.

15.1.5.2 JSON
=============

See 15.12.

15.2 Object Objects
===================

15.2.1 The Object Constructor Called as a Function
==================================================

When Object is called as a function rather than as a constructor, it performs a type conversion.

15.2.1.1 Object ( [ value ] )
=============================

When the Object function is called with no arguments or with one argument value, the following steps
are taken:

1.  If value is null, undefined or not supplied, create and return a new Object object exactly as if
    the standard built-in Object constructor had been called with the same arguments (15.2.2.1).
2.  Return ToObject(value).

15.2.2 The Object Constructor
=============================

When Object is called as part of a new expression, it is a constructor that may create an object.

15.2.2.1 new Object ( [ value ] )
=================================

When the Object constructor is called with no arguments or with one argument value, the following
steps are taken:

1.  If value is supplied, then
    1.  If Type(value) is Object, then
        1.  If the value is a native ECMAScript object, do not create a new object but simply return
            value.
        2.  If the value is a host object, then actions are taken and a result is returned in an
            implementation-dependent manner that may depend on the host object.

    2.  If Type(value) is String, return ToObject(value).
    3.  If Type(value) is Boolean, return ToObject(value).
    4.  If Type(value) is Number, return ToObject(value).

2.  Assert: The argument value was not supplied or its type was Null or Undefined.
3.  Let obj be a newly created native ECMAScript object.
4.  Set the [[Prototype]] internal property of obj to the standard built-in Object prototype object
    (15.2.4).
5.  Set the [[Class]] internal property of obj to "Object".
6.  Set the [[Extensible]] internal property of obj to true.
7.  Set all the internal methods of obj as specified in 8.12.
8.  Return obj.

15.2.3 Properties of the Object Constructor
===========================================

The value of the [[Prototype]] internal property of the Object constructor is the standard built-in
Function prototype object.

Besides the internal properties and the length property (whose value is 1), the Object constructor
has the following properties:

15.2.3.1 Object.prototype
=========================

The initial value of Object.prototype is the standard built-in Object prototype object (15.2.4).

This property has the attributes {[[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.

15.2.3.2 Object.getPrototypeOf ( O )
====================================

When the getPrototypeOf function is called with argument O, the following steps are taken:

1.  If Type(O) is not Object throw a TypeError exception.
2.  Return the value of the [[Prototype]] internal property of O.

15.2.3.3 Object.getOwnPropertyDescriptor ( O, P )
=================================================

When the getOwnPropertyDescriptor function is called, the following steps are taken:

1.  If Type(O) is not Object throw a TypeError exception.
2.  Let name be ToString(P).
3.  Let desc be the result of calling the [[GetOwnProperty]] internal method of O with argument
    name.
4.  Return the result of calling FromPropertyDescriptor(desc) (8.10.4).

15.2.3.4 Object.getOwnPropertyNames ( O )
=========================================

When the getOwnPropertyNames function is called, the following steps are taken:

1.  If Type(O) is not Object throw a TypeError exception.
2.  Let array be the result of creating a new object as if by the expression new Array () where
    Array is the standard built-in constructor with that name.
3.  Let n be 0.
4.  For each named own property P of O
    1.  Let name be the String value that is the name of P.
    2.  Call the [[DefineOwnProperty]] internal method of array with arguments ToString(n), the
        PropertyDescriptor {[[Value]]: name, [[Writable]]: true, [[Enumerable]]: true,
        [[Configurable]]: true}, and false.
    3.  Increment n by 1.

5.  Return array.

NOTE If O is a String instance, the set of own properties processed in step 4 includes the implicit
properties defined in 15.5.5.2 that correspond to character positions within the object’s
[[PrimitiveValue]] String.

15.2.3.5 Object.create ( O [, Properties] )
===========================================

The create function creates a new object with a specified prototype. When the create function is
called, the following steps are taken:

1.  If Type(O) is not Object or Null throw a TypeError exception.
2.  Let obj be the result of creating a new object as if by the expression new Object() where Object
    is the standard built-in constructor with that name
3.  Set the [[Prototype]] internal property of obj to O.
4.  If the argument Properties is present and not undefined, add own properties to obj as if by
    calling the standard built-in function Object.defineProperties with arguments obj and
    Properties.
5.  Return obj.

15.2.3.6 Object.defineProperty ( O, P, Attributes )
===================================================

The defineProperty function is used to add an own property and/or update the attributes of an
existing own property of an object. When the defineProperty function is called, the following steps
are taken:

1.  If Type(O) is not Object throw a TypeError exception.
2.  Let name be ToString(P).
3.  Let desc be the result of calling ToPropertyDescriptor with Attributes as the argument.
4.  Call the [[DefineOwnProperty]] internal method of O with arguments name, desc, and true.
5.  Return O.

15.2.3.7 Object.defineProperties ( O, Properties )
==================================================

The defineProperties function is used to add own properties and/or update the attributes of existing
own properties of an object. When the defineProperties function is called, the following steps are
taken:

1.  If Type(O) is not Object throw a TypeError exception.
2.  Let props be ToObject(Properties).
3.  Let names be an internal list containing the names of each enumerable own property of props.
4.  Let descriptors be an empty internal List.
5.  For each element P of names in list order,
    1.  Let descObj be the result of calling the [[Get]] internal method of props with P as the
        argument.
    2.  Let desc be the result of calling ToPropertyDescriptor with descObj as the argument.
    3.  Append the pair (a two element List) consisting of P and desc to the end of descriptors.

6.  For each pair from descriptors in list order,
    1.  Let P be the first element of pair.
    2.  Let desc be the second element of pair.
    3.  Call the [[DefineOwnProperty]] internal method of O with arguments P, desc, and true.

7.  Return O.

If an implementation defines a specific order of enumeration for the for-in statement, that same
enumeration order must be used to order the list elements in step 3 of this algorithm.

15.2.3.8 Object.seal ( O )
==========================

When the seal function is called, the following steps are taken:

1.  If Type(O) is not Object throw a TypeError exception.
2.  For each named own property name P of O,
    1.  Let desc be the result of calling the [[GetOwnProperty]] internal method of O with P.
    2.  If desc.[[Configurable]] is true, set desc.[[Configurable]] to false.
    3.  Call the [[DefineOwnProperty]] internal method of O with P, desc, and true as arguments.

3.  Set the [[Extensible]] internal property of O to false.
4.  Return O.

15.2.3.9 Object.freeze ( O )
============================

When the freeze function is called, the following steps are taken:

1.  If Type(O) is not Object throw a TypeError exception.
2.  For each named own property name P of O,
    1.  Let desc be the result of calling the [[GetOwnProperty]] internal method of O with P.
    2.  If IsDataDescriptor(desc) is true, then
        1.  If desc.[[Writable]] is true, set desc.[[Writable]] to false.

    3.  If desc.[[Configurable]] is true, set desc.[[Configurable]] to false.
    4.  Call the [[DefineOwnProperty]] internal method of O with P, desc, and true as arguments.

3.  Set the [[Extensible]] internal property of O to false.
4.  Return O.

15.2.3.10 Object.preventExtensions ( O )
========================================

When the preventExtensions function is called, the following steps are taken:

1.  If Type(O) is not Object throw a TypeError exception.
2.  Set the [[Extensible]] internal property of O to false.
3.  Return O.

15.2.3.11 Object.isSealed ( O )
===============================

When the isSealed function is called with argument O, the following steps are taken:

1.  If Type(O) is not Object throw a TypeError exception.
2.  For each named own property name P of O,
    1.  Let desc be the result of calling the [[GetOwnProperty]] internal method of O with P.
    2.  If desc.[[Configurable]] is true, then return false.

3.  If the [[Extensible]] internal property of O is false, then return true.
4.  Otherwise, return false.

15.2.3.12 Object.isFrozen ( O )
===============================

When the isFrozen function is called with argument O, the following steps are taken:

1.  If Type(O) is not Object throw a TypeError exception.
2.  For each named own property name P of O,
    1.  Let desc be the result of calling the [[GetOwnProperty]] internal method of O with P.
    2.  If IsDataDescriptor(desc) is true then
        1.  If desc.[[Writable]] is true, return false.

    3.  If desc.[[Configurable]] is true, then return false.

3.  If the [[Extensible]] internal property of O is false, then return true.
4.  Otherwise, return false.

15.2.3.13 Object.isExtensible ( O )
===================================

When the isExtensible function is called with argument O, the following steps are taken:

1.  If Type(O) is not Object throw a TypeError exception.
2.  Return the Boolean value of the [[Extensible]] internal property of O.

15.2.3.14 Object.keys ( O )
===========================

When the keys function is called with argument O, the following steps are taken:

1.  If the Type(O) is not Object, throw a TypeError exception.
2.  Let n be the number of own enumerable properties of O
3.  Let array be the result of creating a new Object as if by the expression new Array(n) where
    Array is the standard built-in constructor with that name.
4.  Let index be 0.
5.  For each own enumerable property of O whose name String is P
    1.  Call the [[DefineOwnProperty]] internal method of array with arguments ToString(index), the
        PropertyDescriptor {[[Value]]: P, [[Writable]]: true, [[Enumerable]]: true,
        [[Configurable]]: true}, and false.
    2.  Increment index by 1.

6.  Return array.

If an implementation defines a specific order of enumeration for the for-in statement, that same
enumeration order must be used in step 5 of this algorithm.

15.2.4 Properties of the Object Prototype Object
================================================

The value of the [[Prototype]] internal property of the Object prototype object is null, the value
of the [[Class]] internal property is "Object", and the initial value of the [[Extensible]] internal
property is true.

15.2.4.1 Object.prototype.constructor
=====================================

The initial value of Object.prototype.constructor is the standard built-in Object constructor.

15.2.4.2 Object.prototype.toString ( )
======================================

When the toString method is called, the following steps are taken:

1.  If the this value is undefined, return "[object Undefined]".
2.  If the this value is null, return "[object Null]".
3.  Let O be the result of calling ToObject passing the this value as the argument.
4.  Let class be the value of the [[Class]] internal property of O.
5.  Return the String value that is the result of concatenating the three Strings "[object ", class,
    and "]".

15.2.4.3 Object.prototype.toLocaleString ( )
============================================

When the toLocaleString method is called, the following steps are taken:

1.  Let O be the result of calling ToObject passing the this value as the argument.
2.  Let toString be the result of calling the [[Get]] internal method of O passing “toString” as the
    argument.
3.  If IsCallable(toString) is false, throw a TypeError exception.
4.  Return the result of calling the [[Call]] internal method of toString passing O as the this
    value and no arguments.

NOTE 1 This function is provided to give all Objects a generic toLocaleString interface, even though
not all may use it. Currently, Array, Number, and Date provide their own locale-sensitive
toLocaleString methods.

NOTE 2 The first parameter to this function is likely to be used in a future version of this
standard; it is recommended that implementations do not use this parameter position for anything
else.

15.2.4.4 Object.prototype.valueOf ( )
=====================================

When the valueOf method is called, the following steps are taken:

1.  Let O be the result of calling ToObject passing the this value as the argument.
2.  If O is the result of calling the Object constructor with a host object (15.2.2.1), then
    1.  Return either O or another value such as the host object originally passed to the
        constructor. The specific result that is returned is implementation-defined.

3.  Return O.

15.2.4.5 Object.prototype.hasOwnProperty (V)
============================================

When the hasOwnProperty method is called with argument V, the following steps are taken:

1.  Let P be ToString(V).
2.  Let O be the result of calling ToObject passing the this value as the argument.
3.  Let desc be the result of calling the [[GetOwnProperty]] internal method of O passing P as the
    argument.
4.  If desc is undefined, return false.
5.  Return true.

NOTE 1 Unlike [[HasProperty]] (8.12.6), this method does not consider objects in the prototype
chain.

NOTE 2 The ordering of steps 1 and 2 is chosen to ensure that any exception that would have been
thrown by step 1 in previous editions of this specification will continue to be thrown even if the
this value is undefined or null.

15.2.4.6 Object.prototype.isPrototypeOf (V)
===========================================

When the isPrototypeOf method is called with argument V, the following steps are taken:

1.  If V is not an object, return false.
2.  Let O be the result of calling ToObject passing the this value as the argument.
3.  Repeat
    1.  Let V be the value of the [[Prototype]] internal property of V.
    2.  if V is null, return false
    3.  If O and V refer to the same object, return true.

NOTE The ordering of steps 1 and 2 is chosen to preserve the behaviour specified by previous
editions of this specification for the case where V is not an object and the this value is undefined
or null.

15.2.4.7 Object.prototype.propertyIsEnumerable (V)
==================================================

When the propertyIsEnumerable method is called with argument V, the following steps are taken:

1.  Let P be ToString(V).
2.  Let O be the result of calling ToObject passing the this value as the argument.
3.  Let desc be the result of calling the [[GetOwnProperty]] internal method of O passing P as the
    argument.
4.  If desc is undefined, return false.
5.  Return the value of desc.[[Enumerable]].

NOTE 1 This method does not consider objects in the prototype chain.

NOTE 2 The ordering of steps 1 and 2 is chosen to ensure that any exception that would have been
thrown by step 1 in previous editions of this specification will continue to be thrown even if the
this value is undefined or null.

15.2.5 Properties of Object Instances
=====================================

Object instances have no special properties beyond those inherited from the Object prototype object.

15.3 Function Objects
=====================

15.3.1 The Function Constructor Called as a Function
====================================================

When Function is called as a function rather than as a constructor, it creates and initialises a new
Function object. Thus the function call Function(…) is equivalent to the object creation expression
new Function(…) with the same arguments.

15.3.1.1 Function (p1, p2, … , pn, body)
========================================

When the Function function is called with some arguments p1, p2, … , pn, body (where n might be 0,
that is, there are no “p” arguments, and where body might also not be provided), the following steps
are taken:

1.  Create and return a new Function object as if the standard built-in constructor Function was
    used in a new expression with the same arguments (15.3.2.1).

15.3.2 The Function Constructor
===============================

When Function is called as part of a new expression, it is a constructor: it initialises the newly
created object.

15.3.2.1 new Function (p1, p2, … , pn, body)
============================================

The last argument specifies the body (executable code) of a function; any preceding arguments
specify formal parameters.

When the Function constructor is called with some arguments p1, p2, … , pn, body (where n might be
0, that is, there are no “p” arguments, and where body might also not be provided), the following
steps are taken:

1.  Let argCount be the total number of arguments passed to this function invocation.
2.  Let P be the empty String.
3.  If argCount = 0, let body be the empty String.
4.  Else if argCount = 1, let body be that argument.
5.  Else, argCount > 1
    1.  Let firstArg be the first argument.
    2.  Let P be ToString(firstArg).
    3.  Let k be 2.
    4.  Repeat, while k < argCount
        1.  Let nextArg be the k’th argument.
        2.  Let P be the result of concatenating the previous value of P, the String "," (a comma),
            and ToString(nextArg).
        3.  Increase k by 1.

    5.  Let body be the k’th argument.

6.  Let body be ToString(body).
7.  If P is not parsable as a FormalParameterListopt then throw a SyntaxError exception.
8.  If body is not parsable as FunctionBody then throw a SyntaxError exception.
9.  If body is strict mode code (see 10.1.1) then let strict be true, else let strict be false.
10. If strict is true, throw any exceptions specified in 13.1 that apply.
11. Return a new Function object created as specified in 13.2 passing P as the
    FormalParameterListopt and body as the FunctionBody. Pass in the Global Environment as the Scope
    parameter and strict as the Strict flag.

A prototype property is automatically created for every function, to provide for the possibility
that the function will be used as a constructor.

NOTE It is permissible but not necessary to have one argument for each formal parameter to be
specified. For example, all three of the following expressions produce the same result:

    new Function("a", "b", "c", "return a+b+c")

    new Function("a, b, c", "return a+b+c")

    new Function("a,b", "c", "return a+b+c")

15.3.3 Properties of the Function Constructor
=============================================

The Function constructor is itself a Function object and its [[Class]] is "Function". The value of
the [[Prototype]] internal property of the Function constructor is the standard built-in Function
prototype object (15.3.4).

The value of the [[Extensible]] internal property of the Function constructor is true.

The Function constructor has the following properties:

15.3.3.1 Function.prototype
===========================

The initial value of Function.prototype is the standard built-in Function prototype object (15.3.4).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.

15.3.3.2 Function.length
========================

This is a data property with a value of 1. This property has the attributes { [[Writable]]: false,
[[Enumerable]]: false, [[Configurable]]: false }.

15.3.4 Properties of the Function Prototype Object
==================================================

The Function prototype object is itself a Function object (its [[Class]] is "Function") that, when
invoked, accepts any arguments and returns undefined.

The value of the [[Prototype]] internal property of the Function prototype object is the standard
built-in Object prototype object (15.2.4). The initial value of the [[Extensible]] internal property
of the Function prototype object is true.

The Function prototype object does not have a valueOf property of its own; however, it inherits the
valueOf property from the Object prototype Object.

The length property of the Function prototype object is 0.

15.3.4.1 Function.prototype.constructor
=======================================

The initial value of Function.prototype.constructor is the built-in Function constructor.

15.3.4.2 Function.prototype.toString ( )
========================================

An implementation-dependent representation of the function is returned. This representation has the
syntax of a FunctionDeclaration. Note in particular that the use and placement of white space, line
terminators, and semicolons within the representation String is implementation-dependent.

The toString function is not generic; it throws a TypeError exception if its this value is not a
Function object. Therefore, it cannot be transferred to other kinds of objects for use as a method.

15.3.4.3 Function.prototype.apply (thisArg, argArray)
=====================================================

When the apply method is called on an object func with arguments thisArg and argArray, the following
steps are taken:

1.  If IsCallable(func) is false, then throw a TypeError exception.
2.  If argArray is null or undefined, then
    1.  Return the result of calling the [[Call]] internal method of func, providing thisArg as the
        this value and an empty list of arguments.

3.  If Type(argArray) is not Object, then throw a TypeError exception.
4.  Let len be the result of calling the [[Get]] internal method of argArray with argument "length".
5.  Let n be ToUint32(len).
6.  Let argList be an empty List.
7.  Let index be 0.
8.  Repeat while index < n
    1.  Let indexName be ToString(index).
    2.  Let nextArg be the result of calling the [[Get]] internal method of argArray with indexName
        as the argument.
    3.  Append nextArg as the last element of argList.
    4.  Set index to index + 1.

9.  Return the result of calling the [[Call]] internal method of func, providing thisArg as the this
    value and argList as the list of arguments.

The length property of the apply method is 2.

NOTE The thisArg value is passed without modification as the this value. This is a change from
Edition 3, where a undefined or null thisArg is replaced with the global object and ToObject is
applied to all other values and that result is passed as the this value.

15.3.4.4 Function.prototype.call (thisArg [ , arg1 [ , arg2, … ] ] )
====================================================================

When the call method is called on an object func with argument thisArg and optional arguments arg1,
arg2 etc, the following steps are taken:

1.  If IsCallable(func) is false, then throw a TypeError exception.
2.  Let argList be an empty List.
3.  If this method was called with more than one argument then in left to right order starting with
    arg1 append each argument as the last element of argList
4.  Return the result of calling the [[Call]] internal method of func, providing thisArg as the this
    value and argList as the list of arguments.

The length property of the call method is 1.

NOTE The thisArg value is passed without modification as the this value. This is a change from
Edition 3, where a undefined or null thisArg is replaced with the global object and ToObject is
applied to all other values and that result is passed as the this value.

15.3.4.5 Function.prototype.bind (thisArg [, arg1 [, arg2, …]])
===============================================================

The bind method takes one or more arguments, thisArg and (optionally) arg1, arg2, etc, and returns a
new function object by performing the following steps:

1.  Let Target be the this value.
2.  If IsCallable(Target) is false, throw a TypeError exception.
3.  Let A be a new (possibly empty) internal list of all of the argument values provided after
    thisArg (arg1, arg2 etc), in order.
4.  Let F be a new native ECMAScript object .
5.  Set all the internal methods, except for [[Get]], of F as specified in 8.12.
6.  Set the [[Get]] internal property of F as specified in 15.3.5.4.
7.  Set the [[TargetFunction]] internal property of F to Target.
8.  Set the [[BoundThis]] internal property of F to the value of thisArg.
9.  Set the [[BoundArgs]] internal property of F to A.
10. Set the [[Class]] internal property of F to “Function”.
11. Set the [[Prototype]] internal property of F to the standard built-in Function prototype object
    as specified in 15.3.3.1.
12. Set the [[Call]] internal property of F as described in 15.3.4.5.1.
13. Set the [[Construct]] internal property of F as described in 15.3.4.5.2.
14. Set the [[HasInstance]] internal property of F as described in 15.3.4.5.3.
15. If the [[Class]] internal property of Target is “Function”, then
    1.  Let L be the length property of Target minus the length of A.
    2.  Set the length own property of F to either 0 or L, whichever is larger.

16. Else set the length own property of F to 0.
17. Set the attributes of the length own property of F to the values specified in 15.3.5.1.
18. Set the [[Extensible]] internal property of F to true.
19. Let thrower be the [[ThrowTypeError]] function Object (13.2.3).
20. Call the [[DefineOwnProperty]] internal method of F with arguments "caller", PropertyDescriptor
    {[[Get]]: thrower, [[Set]]: thrower, [[Enumerable]]: false, [[Configurable]]: false}, and false.
21. Call the [[DefineOwnProperty]] internal method of F with arguments "arguments",
    PropertyDescriptor {[[Get]]: thrower, [[Set]]: thrower, [[Enumerable]]: false, [[Configurable]]:
    false}, and false.
22. Return F.

The length property of the bind method is 1.

NOTE Function objects created using Function.prototype.bind do not have a prototype property or the
[[Code]], [[FormalParameters]], and [[Scope]] internal properties.

15.3.4.5.1 [[Call]]
===================

When the [[Call]] internal method of a function object, F, which was created using the bind function
is called with a this value and a list of arguments ExtraArgs, the following steps are taken:

1.  Let boundArgs be the value of F’s [[BoundArgs]] internal property.
2.  Let boundThis be the value of F’s [[BoundThis]] internal property.
3.  Let target be the value of F’s [[TargetFunction]] internal property.
4.  Let args be a new list containing the same values as the list boundArgs in the same order
    followed by the same values as the list ExtraArgs in the same order.
5.  Return the result of calling the [[Call]] internal method of target providing boundThis as the
    this value and providing args as the arguments.

15.3.4.5.2 [[Construct]]
========================

When the [[Construct]] internal method of a function object, F that was created using the bind
function is called with a list of arguments ExtraArgs, the following steps are taken:

1.  Let target be the value of F’s [[TargetFunction]] internal property.
2.  If target has no [[Construct]] internal method, a TypeError exception is thrown.
3.  Let boundArgs be the value of F’s [[BoundArgs]] internal property.
4.  Let args be a new list containing the same values as the list boundArgs in the same order
    followed by the same values as the list ExtraArgs in the same order.
5.  Return the result of calling the [[Construct]] internal method of target providing args as the
    arguments.

15.3.4.5.3 [[HasInstance]] (V)
==============================

When the [[HasInstance]] internal method of a function object F, that was created using the bind
function is called with argument V, the following steps are taken:

1.  Let target be the value of F’s [[TargetFunction]] internal property.
2.  If target has no [[HasInstance]] internal method, a TypeError exception is thrown.
3.  Return the result of calling the [[HasInstance]] internal method of target providing V as the
    argument.

15.3.5 Properties of Function Instances
=======================================

In addition to the required internal properties, every function instance has a [[Call]] internal
property and in most cases uses a different version of the [[Get]] internal property. Depending on
how they are created (see 8.6.2, 13.2, 15, and 15.3.4.5), function instances may have a
[[HasInstance]] internal property, a [[Scope]] internal property, a [[Construct]] internal property,
a [[FormalParameters]] internal property, a [[Code]] internal property, a [[TargetFunction]]
internal property, a [[BoundThis]] internal property, and a [[BoundArgs]] internal property.

The value of the [[Class]] internal property is “Function”.

Function instances that correspond to strict mode functions (13.2) and function instances created
using the Function.prototype.bind method (15.3.4.5) have properties named “caller” and “arguments”
that throw a TypeError exception. An ECMAScript implementation must not associate any implementation
specific behaviour with accesses of these properties from strict mode function code.

15.3.5.1 length
===============

The value of the length property is an integer that indicates the “typical” number of arguments
expected by the function. However, the language permits the function to be invoked with some other
number of arguments. The behaviour of a function when invoked on a number of arguments other than
the number specified by its length property depends on the function. This property has the
attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

15.3.5.2 prototype
==================

The value of the prototype property is used to initialise the [[Prototype]] internal property of a
newly created object before the Function object is invoked as a constructor for that newly created
object. This property has the attribute { [[Writable]]: true, [[Enumerable]]: false,
[[Configurable]]: false }.

NOTE Function objects created using Function.prototype.bind do not have a prototype property.

15.3.5.3 [[HasInstance]] (V)
============================

Assume F is a Function object.

When the [[HasInstance]] internal method of F is called with value V, the following steps are taken:

1.  If V is not an object, return false.
2.  Let O be the result of calling the [[Get]] internal method of F with property name "prototype".
3.  If Type(O) is not Object, throw a TypeError exception.
4.  Repeat
    1.  Let V be the value of the [[Prototype]] internal property of V.
    2.  If V is null, return false.
    3.  If O and V refer to the same object, return true.

NOTE Function objects created using Function.prototype.bind have a different implementation of
[[HasInstance]] defined in 15.3.4.5.3.

15.3.5.4 [[Get]] (P)
====================

Function objects use a variation of the [[Get]] internal method used for other native ECMAScript
objects (8.12.3).

Assume F is a Function object. When the [[Get]] internal method of F is called with property name P,
the following steps are taken:

1.  Let v be the result of calling the default [[Get]] internal method (8.12.3) on F passing P as
    the property name argument.
2.  If P is "caller" and v is a strict mode Function object, throw a TypeError exception.
3.  Return v.

NOTE Function objects created using Function.prototype.bind use the default [[Get]] internal method.

15.4 Array Objects
==================

Array objects give special treatment to a certain class of property names. A property name P (in the
form of a String value) is an array index if and only if ToString(ToUint32(P)) is equal to P and
ToUint32(P) is not equal to 2^32−1. A property whose property name is an array index is also called
an element. Every Array object has a length property whose value is always a nonnegative integer
less than 2^32. The value of the length property is numerically greater than the name of every
property whose name is an array index; whenever a property of an Array object is created or changed,
other properties are adjusted as necessary to maintain this invariant. Specifically, whenever a
property is added whose name is an array index, the length property is changed, if necessary, to be
one more than the numeric value of that array index; and whenever the length property is changed,
every property whose name is an array index whose value is not smaller than the new length is
automatically deleted. This constraint applies only to own properties of an Array object and is
unaffected by length or array index properties that may be inherited from its prototypes.

An object, O, is said to be sparse if the following algorithm returns true:

1.  Let len be the result of calling the [[Get]] internal method of O with argument “length”.
2.  For each integer i in the range 0≤i<ToUint32(len)
    1.  Let elem be the result of calling the [[GetOwnProperty]] internal method of O with argument
        ToString(i).
    2.  If elem is undefined, return true.

3.  Return false.

15.4.1 The Array Constructor Called as a Function
=================================================

When Array is called as a function rather than as a constructor, it creates and initialises a new
Array object. Thus the function call Array(…) is equivalent to the object creation expression
new Array(…) with the same arguments.

15.4.1.1 Array ( [ item1 [ , item2 [ , … ] ] ] )
================================================

When the Array function is called the following steps are taken:

1.  Create and return a new Array object exactly as if the standard built-in constructor Array was
    used in a new expression with the same arguments (15.4.2).

15.4.2 The Array Constructor
============================

When Array is called as part of a new expression, it is a constructor: it initialises the newly
created object.

15.4.2.1 new Array ( [ item0 [ , item1 [ , … ] ] ] )
====================================================

This description applies if and only if the Array constructor is given no arguments or at least two
arguments.

The [[Prototype]] internal property of the newly constructed object is set to the original Array
prototype object, the one that is the initial value of Array.prototype (15.4.3.1).

The [[Class]] internal property of the newly constructed object is set to "Array".

The [[Extensible]] internal property of the newly constructed object is set to true.

The length property of the newly constructed object is set to the number of arguments.

The 0 property of the newly constructed object is set to item0 (if supplied); the 1 property of the
newly constructed object is set to item1 (if supplied); and, in general, for as many arguments as
there are, the k property of the newly constructed object is set to argument k, where the first
argument is considered to be argument number 0. These properties all have the attributes
{[[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}.

15.4.2.2 new Array (len)
========================

The [[Prototype]] internal property of the newly constructed object is set to the original Array
prototype object, the one that is the initial value of Array.prototype (15.4.3.1). The [[Class]]
internal property of the newly constructed object is set to "Array". The [[Extensible]] internal
property of the newly constructed object is set to true.

If the argument len is a Number and ToUint32(len) is equal to len, then the length property of the
newly constructed object is set to ToUint32(len). If the argument len is a Number and ToUint32(len)
is not equal to len, a RangeError exception is thrown.

If the argument len is not a Number, then the length property of the newly constructed object is set
to 1 and the 0 property of the newly constructed object is set to len with attributes {[[Writable]]:
true, [[Enumerable]]: true, [[Configurable]]: true}.

15.4.3 Properties of the Array Constructor
==========================================

The value of the [[Prototype]] internal property of the Array constructor is the Function prototype
object (15.3.4).

Besides the internal properties and the length property (whose value is 1), the Array constructor
has the following properties:

15.4.3.1 Array.prototype
========================

The initial value of Array.prototype is the Array prototype object (15.4.4).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.

15.4.3.2 Array.isArray ( arg )
==============================

The isArray function takes one argument arg, and returns the Boolean value true if the argument is
an object whose class internal property is "Array"; otherwise it returns false. The following steps
are taken:

1.  If Type(arg) is not Object, return false.
2.  If the value of the [[Class]] internal property of arg is "Array", then return true.
3.  Return false.

15.4.4 Properties of the Array Prototype Object
===============================================

The value of the [[Prototype]] internal property of the Array prototype object is the standard
built-in Object prototype object (15.2.4).

The Array prototype object is itself an array; its [[Class]] is "Array", and it has a length
property (whose initial value is +0) and the special [[DefineOwnProperty]] internal method described
in 15.4.5.1.

In following descriptions of functions that are properties of the Array prototype object, the phrase
“this object” refers to the object that is the this value for the invocation of the function. It is
permitted for the this to be an object for which the value of the [[Class]] internal property is not
"Array".

NOTE The Array prototype object does not have a valueOf property of its own; however, it inherits
the valueOf property from the standard built-in Object prototype Object.

15.4.4.1 Array.prototype.constructor
====================================

The initial value of Array.prototype.constructor is the standard built-in Array constructor.

15.4.4.2 Array.prototype.toString ( )
=====================================

When the toString method is called, the following steps are taken:

1.  Let array be the result of calling ToObject on the this value.
2.  Let func be the result of calling the [[Get]] internal method of array with argument "join".
3.  If IsCallable(func) is false, then let func be the standard built-in method
    Object.prototype.toString (15.2.4.2).
4.  Return the result of calling the [[Call]] internal method of func providing array as the this
    value and an empty arguments list.

NOTE The toString function is intentionally generic; it does not require that its this value be an
Array object. Therefore it can be transferred to other kinds of objects for use as a method. Whether
the toString function can be applied successfully to a host object is implementation-dependent.

15.4.4.3 Array.prototype.toLocaleString ( )
===========================================

The elements of the array are converted to Strings using their toLocaleString methods, and these
Strings are then concatenated, separated by occurrences of a separator String that has been derived
in an implementation-defined locale-specific way. The result of calling this function is intended to
be analogous to the result of toString, except that the result of this function is intended to be
locale-specific.

The result is calculated as follows:

1.  Let array be the result of calling ToObject passing the this value as the argument.
2.  Let arrayLen be the result of calling the [[Get]] internal method of array with argument
    "length".
3.  Let len be ToUint32(arrayLen).
4.  Let separator be the String value for the list-separator String appropriate for the host
    environment’s current locale (this is derived in an implementation-defined way).
5.  If len is zero, return the empty String.
6.  Let firstElement be the result of calling the [[Get]] internal method of array with argument
    "0".
7.  If firstElement is undefined or null, then
    1.  Let R be the empty String.

8.  Else
    1.  Let elementObj be ToObject(firstElement).
    2.  Let func be the result of calling the [[Get]] internal method of elementObj with argument
        "toLocaleString".
    3.  If IsCallable(func) is false, throw a TypeError exception.
    4.  Let R be the result of calling the [[Call]] internal method of func providing elementObj as
        the this value and an empty arguments list.

9.  Let k be 1.
10. Repeat, while k < len
    1.  Let S be a String value produced by concatenating R and separator.
    2.  Let nextElement be the result of calling the [[Get]] internal method of array with argument
        ToString(k).
    3.  If nextElement is undefined or null, then
        1.  Let R be the empty String.

    4.  Else
        1.  Let elementObj be ToObject(nextElement).
        2.  Let func be the result of calling the [[Get]] internal method of elementObj with
            argument "toLocaleString".
        3.  If IsCallable(func) is false, throw a TypeError exception.
        4.  Let R be the result of calling the [[Call]] internal method of func providing elementObj
            as the this value and an empty arguments list.

    5.  Let R be a String value produced by concatenating S and R.
    6.  Increase k by 1.

11. Return R.

NOTE 1 The first parameter to this function is likely to be used in a future version of this
standard; it is recommended that implementations do not use this parameter position for anything
else.

NOTE 2 The toLocaleString function is intentionally generic; it does not require that its this value
be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
Whether the toLocaleString function can be applied successfully to a host object is
implementation-dependent.

15.4.4.4 Array.prototype.concat ( [ item1 [ , item2 [ , … ] ] ] )
=================================================================

When the concat method is called with zero or more arguments item1, item2, etc., it returns an array
containing the array elements of the object followed by the array elements of each argument in
order.

The following steps are taken:

1.  Let O be the result of calling ToObject passing the this value as the argument.
2.  Let A be a new array created as if by the expression new Array() where Array is the standard
    built-in constructor with that name.
3.  Let n be 0.
4.  Let items be an internal List whose first element is O and whose subsequent elements are, in
    left to right order, the arguments that were passed to this function invocation.
5.  Repeat, while items is not empty
    1.  Remove the first element from items and let E be the value of the element.
    2.  If the value of the [[Class]] internal property of E is "Array", then
        1.  Let k be 0.
        2.  Let len be the result of calling the [[Get]] internal method of E with argument
            "length".
        3.  Repeat, while k < len
            1.  Let P be ToString(k).
            2.  Let exists be the result of calling the [[HasProperty]] internal method of E with P.
            3.  If exists is true, then
                1.  Let subElement be the result of calling the [[Get]] internal method of E with
                    argument P.
                2.  Call the [[DefineOwnProperty]] internal method of A with arguments ToString(n),
                    Property Descriptor {[[Value]]: subElement, [[Writable]]: true, [[Enumerable]]:
                    true, [[Configurable]]: true}, and false.

            4.  Increase n by 1.
            5.  Increase k by 1.

    3.  Else, E is not an Array
        1.  Call the [[DefineOwnProperty]] internal method of A with arguments ToString(n), Property
            Descriptor {[[Value]]: E, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]:
            true}, and false.
        2.  Increase n by 1.

6.  Return A.

The length property of the concat method is 1.

NOTE The concat function is intentionally generic; it does not require that its this value be an
Array object. Therefore it can be transferred to other kinds of objects for use as a method. Whether
the concat function can be applied successfully to a host object is implementation-dependent.

15.4.4.5 Array.prototype.join (separator)
=========================================

The elements of the array are converted to Strings, and these Strings are then concatenated,
separated by occurrences of the separator. If no separator is provided, a single comma is used as
the separator.

The join method takes one argument, separator, and performs the following steps:

1.  Let O be the result of calling ToObject passing the this value as the argument.
2.  Let lenVal be the result of calling the [[Get]] internal method of O with argument "length".
3.  Let len be ToUint32(lenVal).
4.  If separator is undefined, let separator be the single-character String ",".
5.  Let sep be ToString(separator).
6.  If len is zero, return the empty String.
7.  Let element0 be the result of calling the [[Get]] internal method of O with argument "0".
8.  If element0 is undefined or null, let R be the empty String; otherwise, Let R be
    ToString(element0).
9.  Let k be 1.
10. Repeat, while k < len
    1.  Let S be the String value produced by concatenating R and sep.
    2.  Let element be the result of calling the [[Get]] internal method of O with argument
        ToString(k).
    3.  If element is undefined or null, Let next be the empty String; otherwise, let next be
        ToString(element).
    4.  Let R be a String value produced by concatenating S and next.
    5.  Increase k by 1.

11. Return R.

The length property of the join method is 1.

NOTE The join function is intentionally generic; it does not require that its this value be an Array
object. Therefore, it can be transferred to other kinds of objects for use as a method. Whether the
join function can be applied successfully to a host object is implementation-dependent.

15.4.4.6 Array.prototype.pop ( )
================================

The last element of the array is removed from the array and returned.

1.  Let O be the result of calling ToObject passing the this value as the argument.
2.  Let lenVal be the result of calling the [[Get]] internal method of O with argument "length".
3.  Let len be ToUint32(lenVal).
4.  If len is zero,
    1.  Call the [[Put]] internal method of O with arguments "length", 0, and true.
    2.  Return undefined.

5.  Else, len > 0
    1.  Let indx be ToString(len–1).
    2.  Let element be the result of calling the [[Get]] internal method of O with argument indx.
    3.  Call the [[Delete]] internal method of O with arguments indx and true.
    4.  Call the [[Put]] internal method of O with arguments "length", indx, and true.
    5.  Return element.

NOTE The pop function is intentionally generic; it does not require that its this value be an Array
object. Therefore it can be transferred to other kinds of objects for use as a method. Whether the
pop function can be applied successfully to a host object is implementation-dependent.

15.4.4.7 Array.prototype.push ( [ item1 [ , item2 [ , … ] ] ] )
===============================================================

The arguments are appended to the end of the array, in the order in which they appear. The new
length of the array is returned as the result of the call.

When the push method is called with zero or more arguments item1, item2, etc., the following steps
are taken:

1.  Let O be the result of calling ToObject passing the this value as the argument.
2.  Let lenVal be the result of calling the [[Get]] internal method of O with argument "length".
3.  Let n be ToUint32(lenVal).
4.  Let items be an internal List whose elements are, in left to right order, the arguments that
    were passed to this function invocation.
5.  Repeat, while items is not empty
    1.  Remove the first element from items and let E be the value of the element.
    2.  Call the [[Put]] internal method of O with arguments ToString(n), E, and true.
    3.  Increase n by 1.

6.  Call the [[Put]] internal method of O with arguments "length", n, and true.
7.  Return n.

The length property of the push method is 1.

NOTE The push function is intentionally generic; it does not require that its this value be an Array
object. Therefore it can be transferred to other kinds of objects for use as a method. Whether the
push function can be applied successfully to a host object is implementation-dependent.

15.4.4.8 Array.prototype.reverse ( )
====================================

The elements of the array are rearranged so as to reverse their order. The object is returned as the
result of the call.

1.  Let O be the result of calling ToObject passing the this value as the argument.
2.  Let lenVal be the result of calling the [[Get]] internal method of O with argument "length".
3.  Let len be ToUint32(lenVal).
4.  Let middle be floor(len/2).
5.  Let lower be 0.
6.  Repeat, while lower ≠ middle
    1.  Let upper be len− lower −1.
    2.  Let upperP be ToString(upper).
    3.  Let lowerP be ToString(lower).
    4.  Let lowerValue be the result of calling the [[Get]] internal method of O with argument
        lowerP.
    5.  Let upperValue be the result of calling the [[Get]] internal method of O with argument
        upperP .
    6.  Let lowerExists be the result of calling the [[HasProperty]] internal method of O with
        argument lowerP.
    7.  Let upperExists be the result of calling the [[HasProperty]] internal method of O with
        argument upperP.
    8.  If lowerExists is true and upperExists is true, then
        1.  Call the [[Put]] internal method of O with arguments lowerP, upperValue, and true .
        2.  Call the [[Put]] internal method of O with arguments upperP, lowerValue, and true .

    9.  Else if lowerExists is false and upperExists is true, then
        1.  Call the [[Put]] internal method of O with arguments lowerP, upperValue, and true .
        2.  Call the [[Delete]] internal method of O, with arguments upperP and true.

    10. Else if lowerExists is true and upperExists is false, then
        1.  Call the [[Delete]] internal method of O, with arguments lowerP and true .
        2.  Call the [[Put]] internal method of O with arguments upperP, lowerValue, and true .

    11. Else, both lowerExists and upperExists are false
        1.  No action is required.

    12. Increase lower by 1.

7.  Return O .

NOTE The reverse function is intentionally generic; it does not require that its this value be an
Array object. Therefore, it can be transferred to other kinds of objects for use as a method.
Whether the reverse function can be applied successfully to a host object is
implementation-dependent.

15.4.4.9 Array.prototype.shift ( )
==================================

The first element of the array is removed from the array and returned.

1.  Let O be the result of calling ToObject passing the this value as the argument.
2.  Let lenVal be the result of calling the [[Get]] internal method of O with argument "length".
3.  Let len be ToUint32(lenVal).
4.  If len is zero, then
    1.  Call the [[Put]] internal method of O with arguments "length", 0, and true.
    2.  Return undefined.

5.  Let first be the result of calling the [[Get]] internal method of O with argument "0".
6.  Let k be 1.
7.  Repeat, while k < len
    1.  Let from be ToString(k).
    2.  Let to be ToString(k–1).
    3.  Let fromPresent be the result of calling the [[HasProperty]] internal method of O with
        argument from.
    4.  If fromPresent is true, then
        1.  Let fromVal be the result of calling the [[Get]] internal method of O with argument
            from.
        2.  Call the [[Put]] internal method of O with arguments to, fromVal, and true.

    5.  Else, fromPresent is false
        1.  Call the [[Delete]] internal method of O with arguments to and true.

    6.  Increase k by 1.

8.  Call the [[Delete]] internal method of O with arguments ToString(len–1) and true.
9.  Call the [[Put]] internal method of O with arguments "length", (len–1) , and true.
10. Return first.

NOTE The shift function is intentionally generic; it does not require that its this value be an
Array object. Therefore it can be transferred to other kinds of objects for use as a method. Whether
the shift function can be applied successfully to a host object is implementation-dependent.

15.4.4.10 Array.prototype.slice (start, end)
============================================

The slice method takes two arguments, start and end, and returns an array containing the elements of
the array from element start up to, but not including, element end (or through the end of the array
if end is undefined). If start is negative, it is treated as length+start where length is the length
of the array. If end is negative, it is treated as length+end where length is the length of the
array. The following steps are taken:

1.  Let O be the result of calling ToObject passing the this value as the argument.
2.  Let A be a new array created as if by the expression new Array() where Array is the standard
    built-in constructor with that name.
3.  Let lenVal be the result of calling the [[Get]] internal method of O with argument "length".
4.  Let len be ToUint32(lenVal).
5.  Let relativeStart be ToInteger(start).
6.  If relativeStart is negative, let k be max((len + relativeStart),0); else let k be
    min(relativeStart, len).
7.  If end is undefined, let relativeEnd be len; else let relativeEnd be ToInteger(end).
8.  If relativeEnd is negative, let final be max((len + relativeEnd),0); else let final be
    min(relativeEnd, len).
9.  Let n be 0.
10. Repeat, while k < final
    1.  Let Pk be ToString(k).
    2.  Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument
        Pk.
    3.  If kPresent is true, then
        1.  Let kValue be the result of calling the [[Get]] internal method of O with argument Pk.
        2.  Call the [[DefineOwnProperty]] internal method of A with arguments ToString(n), Property
            Descriptor {[[Value]]: kValue, [[Writable]]: true, [[Enumerable]]: true,
            [[Configurable]]: true}, and false.

    4.  Increase k by 1.
    5.  Increase n by 1.

11. Return A.

The length property of the slice method is 2.

NOTE The slice function is intentionally generic; it does not require that its this value be an
Array object. Therefore it can be transferred to other kinds of objects for use as a method. Whether
the slice function can be applied successfully to a host object is implementation-dependent.

15.4.4.11 Array.prototype.sort (comparefn)
==========================================

The elements of this array are sorted. The sort is not necessarily stable (that is, elements that
compare equal do not necessarily remain in their original order). If comparefn is not undefined, it
should be a function that accepts two arguments x and y and returns a negative value if x < y, zero
if x = y, or a positive value if x > y.

Let obj be the result of calling ToObject passing the this value as the argument.

Let len be the result of applying Uint32 to the result of calling the [[Get]] internal method of obj
with argument "length".

If comparefn is not undefined and is not a consistent comparison function for the elements of this
array (see below), the behaviour of sort is implementation-defined.

Let proto be the value of the [[Prototype]] internal property of obj. If proto is not null and there
exists an integer j such that all of the conditions below are satisfied then the behaviour of sort
is implementation-defined:

-   obj is sparse (15.4)
-   0 ≤ j < len
-   The result of calling the [[HasProperty]] internal method of proto with argument ToString(j) is
    true.

The behaviour of sort is also implementation defined if obj is sparse and any of the following
conditions are true:

-   The [[Extensible]] internal property of obj is false.

-   Any array index property of obj whose name is a nonnegative integer less than len is a data
    property whose [[Configurable]] attribute is false.

The behaviour of sort is also implementation defined if any array index property of obj whose name
is a nonnegative integer less than len is an accessor property or is a data property whose
[[Writable]] attribute is false.

Otherwise, the following steps are taken.

1.  Perform an implementation-dependent sequence of calls to the [[Get]] , [[Put]], and [[Delete]]
    internal methods of obj and to SortCompare (described below), where the first argument for each
    call to [[Get]], [[Put]], or [[Delete]] is a nonnegative integer less than len and where the
    arguments for calls to SortCompare are results of previous calls to the [[Get]] internal method.
    The throw argument to the [[Put]] and [[Delete]] internal methods will be the value true. If obj
    is not sparse then [[Delete]] must not be called.
2.  Return obj.

The returned object must have the following two properties.

-   There must be some mathematical permutation π of the nonnegative integers less than len, such
    that for every nonnegative integer j less than len, if property old[j] existed, then new[π(j)]
    is exactly the same value as old[j],. But if property old[j] did not exist, then new[π(j)] does
    not exist.

-   Then for all nonnegative integers j and k, each less than len, if SortCompare(j,k) < 0 (see
    SortCompare below), then π(j) < π(k).

Here the notation old[j] is used to refer to the hypothetical result of calling the [[Get]] internal
method of obj with argument j before this function is executed, and the notation new[j] to refer to
the hypothetical result of calling the [[Get]] internal method of obj with argument j after this
function has been executed.

A function comparefn is a consistent comparison function for a set of values S if all of the
requirements below are met for all values a, b, and c (possibly the same value) in the set S: The
notation a <CF b means comparefn(a,b) < 0; a =CF b means comparefn(a,b) = 0 (of either sign); and
a >CF b means comparefn(a,b) > 0.

-   Calling comparefn(a,b) always returns the same value v when given a specific pair of values a
    and b as its two arguments. Furthermore, Type(v) is Number, and v is not NaN. Note that this
    implies that exactly one of a <CF b, a =CF b, and a >CF b will be true for a given pair of a and
    b.

-   Calling comparefn(a,b) does not modify the this object.

-   a =CF a (reflexivity)

-   If a =CF b, then b =CF a (symmetry)

-   If a =CF b and b =CF c, then a =CF c (transitivity of =CF)

-   If a <CF b and b <CF c, then a <CF c (transitivity of <CF)

-   If a >CF b and b >CF c, then a >CF c (transitivity of >CF)

NOTE The above conditions are necessary and sufficient to ensure that comparefn divides the set S
into equivalence classes and that these equivalence classes are totally ordered.

When the SortCompare abstract operation is called with two arguments j and k, the following steps
are taken:

1.  Let jString be ToString(j).
2.  Let kString be ToString(k).
3.  Let hasj be the result of calling the [[HasProperty]] internal method of obj with argument
    jString.
4.  Let hask be the result of calling the [[HasProperty]] internal method of obj with argument
    kString.
5.  If hasj and hask are both false, then return +0.
6.  If hasj is false, then return 1.
7.  If hask is false, then return –1.
8.  Let x be the result of calling the [[Get]] internal method of obj with argument jString.
9.  Let y be the result of calling the [[Get]] internal method of obj with argument kString.
10. If x and y are both undefined, return +0.
11. If x is undefined, return 1.
12. If y is undefined, return −1.
13. If the argument comparefn is not undefined, then
    1.  If IsCallable(comparefn) is false, throw a TypeError exception.
    2.  Return the result of calling the [[Call]] internal method of comparefn passing undefined as
        the this value and with arguments x and y.

14. Let xString be ToString(x).
15. Let yString be ToString(y).
16. If xString < yString, return −1.
17. If xString > yString, return 1.
18. Return +0.

NOTE 1 Because non-existent property values always compare greater than undefined property values,
and undefined always compares greater than any other value, undefined property values always sort to
the end of the result, followed by non-existent property values.

NOTE 2 The sort function is intentionally generic; it does not require that its this value be an
Array object. Therefore, it can be transferred to other kinds of objects for use as a method.
Whether the sort function can be applied successfully to a host object is implementation-dependent.

15.4.4.12 Array.prototype.splice (start, deleteCount [ , item1 [ , item2 [ , … ] ] ] )
======================================================================================

When the splice method is called with two or more arguments start, deleteCount and (optionally)
item1, item2, etc., the deleteCount elements of the array starting at array index start are replaced
by the arguments item1, item2, etc. An Array object containing the deleted elements (if any) is
returned. The following steps are taken:

1.  Let O be the result of calling ToObject passing the this value as the argument.
2.  Let A be a new array created as if by the expression new Array()where Array is the standard
    built-in constructor with that name.
3.  Let lenVal be the result of calling the [[Get]] internal method of O with argument "length".
4.  Let len be ToUint32(lenVal).
5.  Let relativeStart be ToInteger(start).
6.  If relativeStart is negative, let actualStart be max((len + relativeStart),0); else let
    actualStart be min(relativeStart, len).
7.  Let actualDeleteCount be min(max(ToInteger(deleteCount),0), len – actualStart).
8.  Let k be 0.
9.  Repeat, while k < actualDeleteCount
    1.  Let from be ToString(actualStart+k).
    2.  Let fromPresent be the result of calling the [[HasProperty]] internal method of O with
        argument from.
    3.  If fromPresent is true, then
        1.  Let fromValue be the result of calling the [[Get]] internal method of O with argument
            from.
        2.  Call the [[DefineOwnProperty]] internal method of A with arguments ToString(k), Property
            Descriptor {[[Value]]: fromValue, [[Writable]]: true, [[Enumerable]]: true,
            [[Configurable]]: true}, and false.

    4.  Increment k by 1.

10. Let items be an internal List whose elements are, in left to right order, the portion of the
    actual argument list starting with item1. The list will be empty if no such items are present.
11. Let itemCount be the number of elements in items.
12. If itemCount < actualDeleteCount, then
    1.  Let k be actualStart.
    2.  Repeat, while k < (len – actualDeleteCount)
        1.  Let from be ToString(k+actualDeleteCount).
        2.  Let to be ToString(k+itemCount).
        3.  Let fromPresent be the result of calling the [[HasProperty]] internal method of O with
            argument from.
        4.  If fromPresent is true, then
            1.  Let fromValue be the result of calling the [[Get]] internal method of O with
                argument from.
            2.  Call the [[Put]] internal method of O with arguments to, fromValue, and true.

        5.  Else, fromPresent is false
            1.  Call the [[Delete]] internal method of O with arguments to and true.

        6.  Increase k by 1.

    3.  Let k be len.
    4.  Repeat, while k > (len – actualDeleteCount + itemCount)
        1.  Call the [[Delete]] internal method of O with arguments ToString(k–1) and true.
        2.  Decrease k by 1.

13. Else if itemCount > actualDeleteCount, then
    1.  Let k be (len – actualDeleteCount).
    2.  Repeat, while k > actualStart
        1.  Let from be ToString(k + actualDeleteCount – 1).
        2.  Let to be ToString(k + itemCount – 1)
        3.  Let fromPresent be the result of calling the [[HasProperty]] internal method of O with
            argument from.
        4.  If fromPresent is true, then
            1.  Let fromValue be the result of calling the [[Get]] internal method of O with
                argument from.
            2.  Call the [[Put]] internal method of O with arguments to, fromValue, and true.

        5.  Else, fromPresent is false
            1.  Call the [[Delete]] internal method of O with argument to and true.

        6.  Decrease k by 1.

14. Let k be actualStart.
15. Repeat, while items is not empty
    1.  Remove the first element from items and let E be the value of that element.
    2.  Call the [[Put]] internal method of O with arguments ToString(k), E, and true.
    3.  Increase k by 1.

16. Call the [[Put]] internal method of O with arguments "length", (len – actualDeleteCount +
    itemCount), and true.
17. Return A.

The length property of the splice method is 2.

NOTE The splice function is intentionally generic; it does not require that its this value be an
Array object. Therefore it can be transferred to other kinds of objects for use as a method. Whether
the splice function can be applied successfully to a host object is implementation-dependent.

15.4.4.13 Array.prototype.unshift ( [ item1 [ , item2 [ , … ] ] ] )
===================================================================

The arguments are prepended to the start of the array, such that their order within the array is the
same as the order in which they appear in the argument list.

When the unshift method is called with zero or more arguments item1, item2, etc., the following
steps are taken:

1.  Let O be the result of calling ToObject passing the this value as the argument.
2.  Let lenVal be the result of calling the [[Get]] internal method of O with argument "length".
3.  Let len be ToUint32(lenVal).
4.  Let argCount be the number of actual arguments.
5.  Let k be len.
6.  Repeat, while k > 0,
    1.  Let from be ToString(k–1).
    2.  Let to be ToString(k+argCount –1).
    3.  Let fromPresent be the result of calling the [[HasProperty]] internal method of O with
        argument from.
    4.  If fromPresent is true, then
        1.  Let fromValue be the result of calling the [[Get]] internal method of O with argument
            from.
        2.  Call the [[Put]] internal method of O with arguments to, fromValue, and true.

    5.  Else, fromPresent is false
        1.  Call the [[Delete]] internal method of O with arguments to, and true.

    6.  Decrease k by 1.

7.  Let j be 0.
8.  Let items be an internal List whose elements are, in left to right order, the arguments that
    were passed to this function invocation.
9.  Repeat, while items is not empty
    1.  Remove the first element from items and let E be the value of that element.
    2.  Call the [[Put]] internal method of O with arguments ToString(j), E, and true.
    3.  Increase j by 1.

10. Call the [[Put]] internal method of O with arguments "length", len+argCount, and true.
11. Return len+argCount.

The length property of the unshift method is 1.

NOTE The unshift function is intentionally generic; it does not require that its this value be an
Array object. Therefore it can be transferred to other kinds of objects for use as a method. Whether
the unshift function can be applied successfully to a host object is implementation-dependent.

15.4.4.14 Array.prototype.indexOf ( searchElement [ , fromIndex ] )
===================================================================

indexOf compares searchElement to the elements of the array, in ascending order, using the internal
Strict Equality Comparison Algorithm (11.9.6), and if found at one or more positions, returns the
index of the first such position; otherwise, -1 is returned.

The optional second argument fromIndex defaults to 0 (i.e. the whole array is searched). If it is
greater than or equal to the length of the array, -1 is returned, i.e. the array will not be
searched. If it is negative, it is used as the offset from the end of the array to compute
fromIndex. If the computed index is less than 0, the whole array will be searched.

When the indexOf method is called with one or two arguments, the following steps are taken:

1.  Let O be the result of calling ToObject passing the this value as the argument.
2.  Let lenValue be the result of calling the [[Get]] internal method of O with the argument
    "length".
3.  Let len be ToUint32(lenValue).
4.  If len is 0, return -1.
5.  If argument fromIndex was passed let n be ToInteger(fromIndex); else let n be 0.
6.  If n ≥ len, return -1.
7.  If n ≥ 0, then
    1.  Let k be n.

8.  Else, n<0
    1.  Let k be len - abs(n).
    2.  If k is less than 0, then let k be 0.

9.  Repeat, while k<len
    1.  Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument
        ToString(k).
    2.  If kPresent is true, then
        1.  Let elementK be the result of calling the [[Get]] internal method of O with the argument
            ToString(k).
        2.  Let same be the result of applying the Strict Equality Comparison Algorithm to
            searchElement and elementK.
        3.  If same is true, return k.

    3.  Increase k by 1.

10. Return -1.

The length property of the indexOf method is 1.

NOTE The indexOf function is intentionally generic; it does not require that its this value be an
Array object. Therefore it can be transferred to other kinds of objects for use as a method. Whether
the indexOf function can be applied successfully to a host object is implementation-dependent.

15.4.4.15 Array.prototype.lastIndexOf ( searchElement [ , fromIndex ] )
=======================================================================

lastIndexOf compares searchElement to the elements of the array in descending order using the
internal Strict Equality Comparison Algorithm (11.9.6), and if found at one or more positions,
returns the index of the last such position; otherwise, -1 is returned.

The optional second argument fromIndex defaults to the array’s length minus one (i.e. the whole
array is searched). If it is greater than or equal to the length of the array, the whole array will
be searched. If it is negative, it is used as the offset from the end of the array to compute
fromIndex. If the computed index is less than 0, -1 is returned.

When the lastIndexOf method is called with one or two arguments, the following steps are taken:

1.  Let O be the result of calling ToObject passing the this value as the argument.
2.  Let lenValue be the result of calling the [[Get]] internal method of O with the argument
    "length".
3.  Let len be ToUint32(lenValue).
4.  If len is 0, return -1.
5.  If argument fromIndex was passed let n be ToInteger(fromIndex); else let n be len-1.
6.  If n ≥ 0, then let k be min(n, len – 1).
7.  Else, n < 0
    1.  Let k be len - abs(n).

8.  Repeat, while k≥ 0
    1.  Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument
        ToString(k).
    2.  If kPresent is true, then
        1.  Let elementK be the result of calling the [[Get]] internal method of O with the argument
            ToString(k).
        2.  Let same be the result of applying the Strict Equality Comparison Algorithm to
            searchElement and elementK.
        3.  If same is true, return k.

    3.  Decrease k by 1.

9.  Return -1.

The length property of the lastIndexOf method is 1.

NOTE The lastIndexOf function is intentionally generic; it does not require that its this value be
an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
Whether the lastIndexOf function can be applied successfully to a host object is
implementation-dependent.

15.4.4.16 Array.prototype.every ( callbackfn [ , thisArg ] )
============================================================

callbackfn should be a function that accepts three arguments and returns a value that is coercible
to the Boolean value true or false. every calls callbackfn once for each element present in the
array, in ascending order, until it finds one where callbackfn returns false. If such an element is
found, every immediately returns false. Otherwise, if callbackfn returned true for all elements,
every will return true. callbackfn is called only for elements of the array which actually exist; it
is not called for missing elements of the array.

If a thisArg parameter is provided, it will be used as the this value for each invocation of
callbackfn. If it is not provided, undefined is used instead.

callbackfn is called with three arguments: the value of the element, the index of the element, and
the object being traversed.

every does not directly mutate the object on which it is called but the object may be mutated by the
calls to callbackfn.

The range of elements processed by every is set before the first call to callbackfn. Elements which
are appended to the array after the call to every begins will not be visited by callbackfn. If
existing elements of the array are changed, their value as passed to callbackfn will be the value at
the time every visits them; elements that are deleted after the call to every begins and before
being visited are not visited. every acts like the “for all” quantifier in mathematics. In
particular, for an empty array, it returns true.

When the every method is called with one or two arguments, the following steps are taken:

1.  Let O be the result of calling ToObject passing the this value as the argument.
2.  Let lenValue be the result of calling the [[Get]] internal method of O with the argument
    "length".
3.  Let len be ToUint32(lenValue).
4.  If IsCallable(callbackfn) is false, throw a TypeError exception.
5.  If thisArg was supplied, let T be thisArg; else let T be undefined.
6.  Let k be 0.
7.  Repeat, while k < len
    1.  Let Pk be ToString(k).
    2.  Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument
        Pk.
    3.  If kPresent is true, then
        1.  Let kValue be the result of calling the [[Get]] internal method of O with argument Pk.
        2.  Let testResult be the result of calling the [[Call]] internal method of callbackfn with
            T as the this value and argument list containing kValue, k, and O.
        3.  If ToBoolean(testResult) is false, return false.

    4.  Increase k by 1.

8.  Return true.

The length property of the every method is 1.

NOTE The every function is intentionally generic; it does not require that its this value be an
Array object. Therefore it can be transferred to other kinds of objects for use as a method. Whether
the every function can be applied successfully to a host object is implementation-dependent.

15.4.4.17 Array.prototype.some ( callbackfn [ , thisArg ] )
===========================================================

callbackfn should be a function that accepts three arguments and returns a value that is coercible
to the Boolean value true or false. some calls callbackfn once for each element present in the
array, in ascending order, until it finds one where callbackfn returns true. If such an element is
found, some immediately returns true. Otherwise, some returns false. callbackfn is called only for
elements of the array which actually exist; it is not called for missing elements of the array.

If a thisArg parameter is provided, it will be used as the this value for each invocation of
callbackfn. If it is not provided, undefined is used instead.

callbackfn is called with three arguments: the value of the element, the index of the element, and
the object being traversed.

some does not directly mutate the object on which it is called but the object may be mutated by the
calls to callbackfn.

The range of elements processed by some is set before the first call to callbackfn. Elements that
are appended to the array after the call to some begins will not be visited by callbackfn. If
existing elements of the array are changed, their value as passed to callbackfn will be the value at
the time that some visits them; elements that are deleted after the call to some begins and before
being visited are not visited. some acts like the “exists” quantifier in mathematics. In particular,
for an empty array, it returns false.

When the some method is called with one or two arguments, the following steps are taken:

1.  Let O be the result of calling ToObject passing the this value as the argument.
2.  Let lenValue be the result of calling the [[Get]] internal method of O with the argument
    "length".
3.  Let len be ToUint32(lenValue).
4.  If IsCallable(callbackfn) is false, throw a TypeError exception.
5.  If thisArg was supplied, let T be thisArg; else let T be undefined.
6.  Let k be 0.
7.  Repeat, while k < len
    1.  Let Pk be ToString(k).
    2.  Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument
        Pk.
    3.  If kPresent is true, then
        1.  Let kValue be the result of calling the [[Get]] internal method of O with argument Pk.
        2.  Let testResult be the result of calling the [[Call]] internal method of callbackfn with
            T as the this value and argument list containing kValue, k, and O.
        3.  If ToBoolean(testResult) is true, return true.

    4.  Increase k by 1.

8.  Return false.

The length property of the some method is 1.

NOTE The some function is intentionally generic; it does not require that its this value be an Array
object. Therefore it can be transferred to other kinds of objects for use as a method. Whether the
some function can be applied successfully to a host object is implementation-dependent.

15.4.4.18 Array.prototype.forEach ( callbackfn [ , thisArg ] )
==============================================================

callbackfn should be a function that accepts three arguments. forEach calls callbackfn once for each
element present in the array, in ascending order. callbackfn is called only for elements of the
array which actually exist; it is not called for missing elements of the array.

If a thisArg parameter is provided, it will be used as the this value for each invocation of
callbackfn. If it is not provided, undefined is used instead.

callbackfn is called with three arguments: the value of the element, the index of the element, and
the object being traversed.

forEach does not directly mutate the object on which it is called but the object may be mutated by
the calls to callbackfn.

The range of elements processed by forEach is set before the first call to callbackfn. Elements
which are appended to the array after the call to forEach begins will not be visited by callbackfn.
If existing elements of the array are changed, their value as passed to callback will be the value
at the time forEach visits them; elements that are deleted after the call to forEach begins and
before being visited are not visited.

When the forEach method is called with one or two arguments, the following steps are taken:

1.  Let O be the result of calling ToObject passing the this value as the argument.
2.  Let lenValue be the result of calling the [[Get]] internal method of O with the argument
    "length".
3.  Let len be ToUint32(lenValue).
4.  If IsCallable(callbackfn) is false, throw a TypeError exception.
5.  If thisArg was supplied, let T be thisArg; else let T be undefined.
6.  Let k be 0.
7.  Repeat, while k < len
    1.  Let Pk be ToString(k).
    2.  Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument
        Pk.
    3.  If kPresent is true, then
        1.  Let kValue be the result of calling the [[Get]] internal method of O with argument Pk.
        2.  Call the [[Call]] internal method of callbackfn with T as the this value and argument
            list containing kValue, k, and O.

    4.  Increase k by 1.

8.  Return undefined.

The length property of the forEach method is 1.

NOTE The forEach function is intentionally generic; it does not require that its this value be an
Array object. Therefore it can be transferred to other kinds of objects for use as a method. Whether
the forEach function can be applied successfully to a host object is implementation-dependent.

15.4.4.19 Array.prototype.map ( callbackfn [ , thisArg ] )
==========================================================

callbackfn should be a function that accepts three arguments. map calls callbackfn once for each
element in the array, in ascending order, and constructs a new Array from the results. callbackfn is
called only for elements of the array which actually exist; it is not called for missing elements of
the array.

If a thisArg parameter is provided, it will be used as the this value for each invocation of
callbackfn. If it is not provided, undefined is used instead.

callbackfn is called with three arguments: the value of the element, the index of the element, and
the object being traversed.

map does not directly mutate the object on which it is called but the object may be mutated by the
calls to callbackfn.

The range of elements processed by map is set before the first call to callbackfn. Elements which
are appended to the array after the call to map begins will not be visited by callbackfn. If
existing elements of the array are changed, their value as passed to callbackfn will be the value at
the time map visits them; elements that are deleted after the call to map begins and before being
visited are not visited.

When the map method is called with one or two arguments, the following steps are taken:

1.  Let O be the result of calling ToObject passing the this value as the argument.
2.  Let lenValue be the result of calling the [[Get]] internal method of O with the argument
    "length".
3.  Let len be ToUint32(lenValue).
4.  If IsCallable(callbackfn) is false, throw a TypeError exception.
5.  If thisArg was supplied, let T be thisArg; else let T be undefined.
6.  Let A be a new array created as if by the expression new Array(len) where Array is the standard
    built-in constructor with that name and len is the value of len.
7.  Let k be 0.
8.  Repeat, while k < len
    1.  Let Pk be ToString(k).
    2.  Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument
        Pk.
    3.  If kPresent is true, then
        1.  Let kValue be the result of calling the [[Get]] internal method of O with argument Pk.
        2.  Let mappedValue be the result of calling the [[Call]] internal method of callbackfn with
            T as the this value and argument list containing kValue, k, and O.
        3.  Call the [[DefineOwnProperty]] internal method of A with arguments Pk, Property
            Descriptor {[[Value]]: mappedValue, [[Writable]]: true, [[Enumerable]]: true,
            [[Configurable]]: true}, and false.

    4.  Increase k by 1.

9.  Return A.

The length property of the map method is 1.

NOTE The map function is intentionally generic; it does not require that its this value be an Array
object. Therefore it can be transferred to other kinds of objects for use as a method. Whether the
map function can be applied successfully to a host object is implementation-dependent.

15.4.4.20 Array.prototype.filter ( callbackfn [ , thisArg ] )
=============================================================

callbackfn should be a function that accepts three arguments and returns a value that is coercible
to the Boolean value true or false. filter calls callbackfn once for each element in the array, in
ascending order, and constructs a new array of all the values for which callbackfn returns true.
callbackfn is called only for elements of the array which actually exist; it is not called for
missing elements of the array.

If a thisArg parameter is provided, it will be used as the this value for each invocation of
callbackfn. If it is not provided, undefined is used instead.

callbackfn is called with three arguments: the value of the element, the index of the element, and
the object being traversed.

filter does not directly mutate the object on which it is called but the object may be mutated by
the calls to callbackfn.

The range of elements processed by filter is set before the first call to callbackfn. Elements which
are appended to the array after the call to filter begins will not be visited by callbackfn. If
existing elements of the array are changed their value as passed to callbackfn will be the value at
the time filter visits them; elements that are deleted after the call to filter begins and before
being visited are not visited.

When the filter method is called with one or two arguments, the following steps are taken:

1.  Let O be the result of calling ToObject passing the this value as the argument.
2.  Let lenValue be the result of calling the [[Get]] internal method of O with the argument
    "length".
3.  Let len be ToUint32(lenValue).
4.  If IsCallable(callbackfn) is false, throw a TypeError exception.
5.  If thisArg was supplied, let T be thisArg; else let T be undefined.
6.  Let A be a new array created as if by the expression new Array() where Array is the standard
    built-in constructor with that name.
7.  Let k be 0.
8.  Let to be 0.
9.  Repeat, while k < len
    1.  Let Pk be ToString(k).
    2.  Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument
        Pk.
    3.  If kPresent is true, then
        1.  Let kValue be the result of calling the [[Get]] internal method of O with argument Pk.
        2.  Let selected be the result of calling the [[Call]] internal method of callbackfn with T
            as the this value and argument list containing kValue, k, and O.
        3.  If ToBoolean(selected) is true, then
            1.  Call the [[DefineOwnProperty]] internal method of A with arguments ToString(to),
                Property Descriptor {[[Value]]: kValue, [[Writable]]: true, [[Enumerable]]: true,
                [[Configurable]]: true}, and false.
            2.  Increase to by 1.

    4.  Increase k by 1.

10. Return A.

The length property of the filter method is 1.

NOTE The filter function is intentionally generic; it does not require that its this value be an
Array object. Therefore it can be transferred to other kinds of objects for use as a method. Whether
the filter function can be applied successfully to a host object is implementation-dependent.

15.4.4.21 Array.prototype.reduce ( callbackfn [ , initialValue ] )
==================================================================

callbackfn should be a function that takes four arguments. reduce calls the callback, as a function,
once for each element present in the array, in ascending order.

callbackfn is called with four arguments: the previousValue (or value from the previous call to
callbackfn), the currentValue (value of the current element), the currentIndex, and the object being
traversed. The first time that callback is called, the previousValue and currentValue can be one of
two values. If an initialValue was provided in the call to reduce, then previousValue will be equal
to initialValue and currentValue will be equal to the first value in the array. If no initialValue
was provided, then previousValue will be equal to the first value in the array and currentValue will
be equal to the second. It is a TypeError if the array contains no elements and initialValue is not
provided.

reduce does not directly mutate the object on which it is called but the object may be mutated by
the calls to callbackfn.

The range of elements processed by reduce is set before the first call to callbackfn. Elements that
are appended to the array after the call to reduce begins will not be visited by callbackfn. If
existing elements of the array are changed, their value as passed to callbackfn will be the value at
the time reduce visits them; elements that are deleted after the call to reduce begins and before
being visited are not visited.

When the reduce method is called with one or two arguments, the following steps are taken:

1.  Let O be the result of calling ToObject passing the this value as the argument.
2.  Let lenValue be the result of calling the [[Get]] internal method of O with the argument
    "length".
3.  Let len be ToUint32(lenValue).
4.  If IsCallable(callbackfn) is false, throw a TypeError exception.
5.  If len is 0 and initialValue is not present, throw a TypeError exception.
6.  Let k be 0.
7.  If initialValue is present, then
    1.  Set accumulator to initialValue.

8.  Else, initialValue is not present
    1.  Let kPresent be false.
    2.  Repeat, while kPresent is false and k < len
        1.  Let Pk be ToString(k).
        2.  Let kPresent be the result of calling the [[HasProperty]] internal method of O with
            argument Pk.
        3.  If kPresent is true, then
            1.  Let accumulator be the result of calling the [[Get]] internal method of O with
                argument Pk.

        4.  Increase k by 1.

    3.  If kPresent is false, throw a TypeError exception.

9.  Repeat, while k < len
    1.  Let Pk be ToString(k).
    2.  Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument
        Pk.
    3.  If kPresent is true, then
        1.  Let kValue be the result of calling the [[Get]] internal method of O with argument Pk.
        2.  Let accumulator be the result of calling the [[Call]] internal method of callbackfn with
            undefined as the this value and argument list containing accumulator, kValue, k, and O.

    4.  Increase k by 1.

10. Return accumulator.

The length property of the reduce method is 1.

NOTE The reduce function is intentionally generic; it does not require that its this value be an
Array object. Therefore it can be transferred to other kinds of objects for use as a method. Whether
the reduce function can be applied successfully to a host object is implementation-dependent.

15.4.4.22 Array.prototype.reduceRight ( callbackfn [ , initialValue ] )
=======================================================================

callbackfn should be a function that takes four arguments. reduceRight calls the callback, as a
function, once for each element present in the array, in descending order.

callbackfn is called with four arguments: the previousValue (or value from the previous call to
callbackfn), the currentValue (value of the current element), the currentIndex, and the object being
traversed. The first time the function is called, the previousValue and currentValue can be one of
two values. If an initialValue was provided in the call to reduceRight, then previousValue will be
equal to initialValue and currentValue will be equal to the last value in the array. If no
initialValue was provided, then previousValue will be equal to the last value in the array and
currentValue will be equal to the second-to-last value. It is a TypeError if the array contains no
elements and initialValue is not provided.

reduceRight does not directly mutate the object on which it is called but the object may be mutated
by the calls to callbackfn.

The range of elements processed by reduceRight is set before the first call to callbackfn. Elements
that are appended to the array after the call to reduceRight begins will not be visited by
callbackfn. If existing elements of the array are changed by callbackfn, their value as passed to
callbackfn will be the value at the time reduceRight visits them; elements that are deleted after
the call to reduceRight begins and before being visited are not visited.

When the reduceRight method is called with one or two arguments, the following steps are taken:

1.  Let O be the result of calling ToObject passing the this value as the argument.
2.  Let lenValue be the result of calling the [[Get]] internal method of O with the argument
    "length".
3.  Let len be ToUint32(lenValue).
4.  If IsCallable(callbackfn) is false, throw a TypeError exception.
5.  If len is 0 and initialValue is not present, throw a TypeError exception.
6.  Let k be len-1.
7.  If initialValue is present, then
    1.  Set accumulator to initialValue.

8.  Else, initialValue is not present
    1.  Let kPresent be false.
    2.  Repeat, while kPresent is false and k ≥ 0
        1.  Let Pk be ToString(k).
        2.  Let kPresent be the result of calling the [[HasProperty]] internal method of O with
            argument Pk.
        3.  If kPresent is true, then
            1.  Let accumulator be the result of calling the [[Get]] internal method of O with
                argument Pk.

        4.  Decrease k by 1.

    3.  If kPresent is false, throw a TypeError exception.

9.  Repeat, while k ≥ 0
    1.  Let Pk be ToString(k).
    2.  Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument
        Pk.
    3.  If kPresent is true, then
        1.  Let kValue be the result of calling the [[Get]] internal method of O with argument Pk.
        2.  Let accumulator be the result of calling the [[Call]] internal method of callbackfn with
            undefined as the this value and argument list containing accumulator, kValue, k, and O.

    4.  Decrease k by 1.

10. Return accumulator.

The length property of the reduceRight method is 1.

NOTE The reduceRight function is intentionally generic; it does not require that its this value be
an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
Whether the reduceRight function can be applied successfully to a host object is
implementation-dependent.

15.4.5 Properties of Array Instances
====================================

Array instances inherit properties from the Array prototype object and their [[Class]] internal
property value is "Array". Array instances also have the following properties.

15.4.5.1 [[DefineOwnProperty]] ( P, Desc, Throw )
=================================================

Array objects use a variation of the [[DefineOwnProperty]] internal method used for other native
ECMAScript objects (8.12.9).

Assume A is an Array object, Desc is a Property Descriptor, and Throw is a Boolean flag.

In the following algorithm, the term “Reject” means “If Throw is true, then throw a TypeError
exception, otherwise return false.”

When the [[DefineOwnProperty]] internal method of A is called with property P, Property Descriptor
Desc, and Boolean flag Throw, the following steps are taken:

1.  Let oldLenDesc be the result of calling the [[GetOwnProperty]] internal method of A passing
    "length" as the argument. The result will never be undefined or an accessor descriptor because
    Array objects are created with a length data property that cannot be deleted or reconfigured.
2.  Let oldLen be oldLenDesc.[[Value]].
3.  If P is "length", then
    1.  If the [[Value]] field of Desc is absent, then
        1.  Return the result of calling the default [[DefineOwnProperty]] internal method (8.12.9)
            on A passing "length", Desc, and Throw as arguments.

    2.  Let newLenDesc be a copy of Desc.
    3.  Let newLen be ToUint32(Desc.[[Value]]).
    4.  If newLen is not equal to ToNumber( Desc.[[Value]]), throw a RangeError exception.
    5.  Set newLenDesc.[[Value] to newLen.
    6.  If newLen ≥oldLen, then
        1.  Return the result of calling the default [[DefineOwnProperty]] internal method (8.12.9)
            on A passing "length", newLenDesc, and Throw as arguments.

    7.  Reject if oldLenDesc.[[Writable]] is false.
    8.  If newLenDesc.[[Writable]] is absent or has the value true, let newWritable be true.
    9.  Else,
        1.  Need to defer setting the [[Writable]] attribute to false in case any elements cannot be
            deleted.
        2.  Let newWritable be false.
        3.  Set newLenDesc.[[Writable] to true.

    10. Let succeeded be the result of calling the default [[DefineOwnProperty]] internal method
        (8.12.9) on A passing "length", newLenDesc, and Throw as arguments.
    11. If succeeded is false, return false.
    12. While newLen < oldLen repeat,
        1.  Set oldLen to oldLen – 1.
        2.  Let deleteSucceeded be the result of calling the [[Delete]] internal method of A passing
            ToString(oldLen) and false as arguments.
        3.  If deleteSucceeded is false, then
            1.  Set newLenDesc.[[Value] to oldLen+1.
            2.  If newWritable is false, set newLenDesc.[[Writable] to false.
            3.  Call the default [[DefineOwnProperty]] internal method (8.12.9) on A passing
                "length", newLenDesc, and false as arguments.
            4.  Reject.

    13. If newWritable is false, then
        1.  Call the default [[DefineOwnProperty]] internal method (8.12.9) on A passing "length",
            Property Descriptor{[[Writable]]: false}, and false as arguments. This call will always
            return true.

    14. Return true.

4.  Else if P is an array index (15.4), then
    1.  Let index be ToUint32(P).
    2.  Reject if index ≥ oldLen and oldLenDesc.[[Writable]] is false.
    3.  Let succeeded be the result of calling the default [[DefineOwnProperty]] internal method
        (8.12.9) on A passing P, Desc, and false as arguments.
    4.  Reject if succeeded is false.
    5.  If index ≥ oldLen
        1.  Set oldLenDesc.[[Value]] to index + 1.
        2.  Call the default [[DefineOwnProperty]] internal method (8.12.9) on A passing "length",
            oldLenDesc, and false as arguments. This call will always return true.

    6.  Return true.

5.  Return the result of calling the default [[DefineOwnProperty]] internal method (8.12.9) on A
    passing P, Desc, and Throw as arguments.

15.4.5.2 length
===============

The length property of this Array object is a data property whose value is always numerically
greater than the name of every deletable property whose name is an array index.

The length property initially has the attributes { [[Writable]]: true, [[Enumerable]]: false,
[[Configurable]]: false }.

NOTE Attempting to set the length property of an Array object to a value that is numerically less
than or equal to the largest numeric property name of an existing array indexed non-deletable
property of the array will result in the length being set to a numeric value that is one greater
than that largest numeric property name. See 15.4.5.1.

15.5 String Objects
===================

15.5.1 The String Constructor Called as a Function
==================================================

When String is called as a function rather than as a constructor, it performs a type conversion.

15.5.1.1 String ( [ value ] )
=============================

Returns a String value (not a String object) computed by ToString(value). If value is not supplied,
the empty String "" is returned.

15.5.2 The String Constructor
=============================

When String is called as part of a new expression, it is a constructor: it initialises the newly
created object.

15.5.2.1 new String ( [ value ] )
=================================

The [[Prototype]] internal property of the newly constructed object is set to the standard built-in
String prototype object that is the initial value of String.prototype (15.5.3.1).

The [[Class]] internal property of the newly constructed object is set to "String".

The [[Extensible]] internal property of the newly constructed object is set to true.

The [[PrimitiveValue]] internal property of the newly constructed object is set to ToString(value),
or to the empty String if value is not supplied.

15.5.3 Properties of the String Constructor
===========================================

The value of the [[Prototype]] internal property of the String constructor is the standard built-in
Function prototype object (15.3.4).

Besides the internal properties and the length property (whose value is 1), the String constructor
has the following properties:

15.5.3.1 String.prototype
=========================

The initial value of String.prototype is the standard built-in String prototype object (15.5.4).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.

15.5.3.2 String.fromCharCode ( [ char0 [ , char1 [ , … ] ] ] )
==============================================================

Returns a String value containing as many characters as the number of arguments. Each argument
specifies one character of the resulting String, with the first argument specifying the first
character, and so on, from left to right. An argument is converted to a character by applying the
operation ToUint16 (9.7) and regarding the resulting 16-bit integer as the code unit value of a
character. If no arguments are supplied, the result is the empty String.

The length property of the fromCharCode function is 1.

15.5.4 Properties of the String Prototype Object
================================================

The String prototype object is itself a String object (its [[Class]] is "String") whose value is an
empty String.

The value of the [[Prototype]] internal property of the String prototype object is the standard
built-in Object prototype object (15.2.4).

15.5.4.1 String.prototype.constructor
=====================================

The initial value of String.prototype.constructor is the built-in String constructor.

15.5.4.2 String.prototype.toString ( )
======================================

Returns this String value. (Note that, for a String object, the toString method happens to return
the same thing as the valueOf method.)

The toString function is not generic; it throws a TypeError exception if its this value is not a
String or a String object. Therefore, it cannot be transferred to other kinds of objects for use as
a method.

15.5.4.3 String.prototype.valueOf ( )
=====================================

Returns this String value.

The valueOf function is not generic; it throws a TypeError exception if its this value is not a
String or String object. Therefore, it cannot be transferred to other kinds of objects for use as a
method.

15.5.4.4 String.prototype.charAt (pos)
======================================

Returns a String containing the character at position pos in the String resulting from converting
this object to a String. If there is no character at that position, the result is the empty String.
The result is a String value, not a String object.

If pos is a value of Number type that is an integer, then the result of x.charAt(pos) is equal to
the result of x.substring(pos, pos+1).

When the charAt method is called with one argument pos, the following steps are taken:

1.  Call CheckObjectCoercible passing the this value as its argument.
2.  Let S be the result of calling ToString, giving it the this value as its argument.
3.  Let position be ToInteger(pos).
4.  Let size be the number of characters in S.
5.  If position < 0 or position ≥ size, return the empty String.
6.  Return a String of length 1, containing one character from S, namely the character at position
    position, where the first (leftmost) character in S is considered to be at position 0, the next
    one at position 1, and so on.

NOTE The charAt function is intentionally generic; it does not require that its this value be a
String object. Therefore, it can be transferred to other kinds of objects for use as a method.

15.5.4.5 String.prototype.charCodeAt (pos)
==========================================

Returns a Number (a nonnegative integer less than 2^16) representing the code unit value of the
character at position pos in the String resulting from converting this object to a String. If there
is no character at that position, the result is NaN.

When the charCodeAt method is called with one argument pos, the following steps are taken:

1.  Call CheckObjectCoercible passing the this value as its argument.
2.  Let S be the result of calling ToString, giving it the this value as its argument.
3.  Let position be ToInteger(pos).
4.  Let size be the number of characters in S.
5.  If position < 0 or position ≥ size, return NaN.
6.  Return a value of Number type, whose value is the code unit value of the character at position
    position in the String S, where the first (leftmost) character in S is considered to be at
    position 0, the next one at position 1, and so on.

NOTE The charCodeAt function is intentionally generic; it does not require that its this value be a
String object. Therefore it can be transferred to other kinds of objects for use as a method.

15.5.4.6 String.prototype.concat ( [ string1 [ , string2 [ , … ] ] ] )
======================================================================

When the concat method is called with zero or more arguments string1, string2, etc., it returns a
String consisting of the characters of this object (converted to a String) followed by the
characters of each of string1, string2, etc. (where each argument is converted to a String). The
result is a String value, not a String object. The following steps are taken:

1.  Call CheckObjectCoercible passing the this value as its argument.
2.  Let S be the result of calling ToString, giving it the this value as its argument.
3.  Let args be an internal list that is a copy of the argument list passed to this function.
4.  Let R be S.
5.  Repeat, while args is not empty
    1.  Remove the first element from args and let next be the value of that element.
    2.  Let R be the String value consisting of the characters in the previous value of R followed
        by the characters of ToString(next).

6.  Return R.

The length property of the concat method is 1.

NOTE The concat function is intentionally generic; it does not require that its this value be a
String object. Therefore it can be transferred to other kinds of objects for use as a method.

15.5.4.7 String.prototype.indexOf (searchString, position)
==========================================================

If searchString appears as a substring of the result of converting this object to a String, at one
or more positions that are greater than or equal to position, then the index of the smallest such
position is returned; otherwise, ‑1 is returned. If position is undefined, 0 is assumed, so as to
search all of the String.

The indexOf method takes two arguments, searchString and position, and performs the following steps:

1.  Call CheckObjectCoercible passing the this value as its argument.
2.  Let S be the result of calling ToString, giving it the this value as its argument.
3.  Let searchStr be ToString(searchString).
4.  Let pos be ToInteger(position). (If position is undefined, this step produces the value 0).
5.  Let len be the number of characters in S.
6.  Let start be min(max(pos, 0), len).
7.  Let searchLen be the number of characters in searchStr.
8.  Return the smallest possible integer k not smaller than start such that k+ searchLen is not
    greater than len, and for all nonnegative integers j less than searchLen, the character at
    position k+j of S is the same as the character at position j of searchStr; but if there is no
    such integer k, then return the value -1.

The length property of the indexOf method is 1.

NOTE The indexOf function is intentionally generic; it does not require that its this value be a
String object. Therefore, it can be transferred to other kinds of objects for use as a method.

15.5.4.8 String.prototype.lastIndexOf (searchString, position)
==============================================================

If searchString appears as a substring of the result of converting this object to a String at one or
more positions that are smaller than or equal to position, then the index of the greatest such
position is returned; otherwise, ‑1 is returned. If position is undefined, the length of the String
value is assumed, so as to search all of the String.

The lastIndexOf method takes two arguments, searchString and position, and performs the following
steps:

1.  Call CheckObjectCoercible passing the this value as its argument.
2.  Let S be the result of calling ToString, giving it the this value as its argument.
3.  Let searchStr be ToString(searchString).
4.  Let numPos be ToNumber(position). (If position is undefined, this step produces the value NaN).
5.  If numPos is NaN, let pos be +∞; otherwise, let pos be ToInteger(numPos).
6.  Let len be the number of characters in S.
7.  Let start min(max(pos, 0), len).
8.  Let searchLen be the number of characters in searchStr.
9.  Return the largest possible nonnegative integer k not larger than start such that k+ searchLen
    is not greater than len, and for all nonnegative integers j less than searchLen, the character
    at position k+j of S is the same as the character at position j of searchStr; but if there is no
    such integer k, then return the value -1.

The length property of the lastIndexOf method is 1.

NOTE The lastIndexOf function is intentionally generic; it does not require that its this value be a
String object. Therefore, it can be transferred to other kinds of objects for use as a method.

15.5.4.9 String.prototype.localeCompare (that)
==============================================

When the localeCompare method is called with one argument that, it returns a Number other than NaN
that represents the result of a locale-sensitive String comparison of the this value (converted to a
String) with that (converted to a String). The two Strings are S and That. The two Strings are
compared in an implementation-defined fashion. The result is intended to order String values in the
sort order specified by the system default locale, and will be negative, zero, or positive,
depending on whether S comes before That in the sort order, the Strings are equal, or S comes after
That in the sort order, respectively.

Before perform the comparisons the following steps are performed to prepare the Strings:

1.  Call CheckObjectCoercible passing the this value as its argument.
2.  Let S be the result of calling ToString, giving it the this value as its argument.
3.  Let That be ToString(that).

The localeCompare method, if considered as a function of two arguments this and that, is a
consistent comparison function (as defined in 15.4.4.11) on the set of all Strings.

The actual return values are implementation-defined to permit implementers to encode additional
information in the value, but the function is required to define a total ordering on all Strings and
to return 0 when comparing Strings that are considered canonically equivalent by the Unicode
standard.

If no language-sensitive comparison at all is available from the host environment, this function may
perform a bitwise comparison.

NOTE 1 The localeCompare method itself is not directly suitable as an argument to
Array.prototype.sort because the latter requires a function of two arguments.

NOTE 2 This function is intended to rely on whatever language-sensitive comparison functionality is
available to the ECMAScript environment from the host environment, and to compare according to the
rules of the host environment’s current locale. It is strongly recommended that this function treat
Strings that are canonically equivalent according to the Unicode standard as identical (in other
words, compare the Strings as if they had both been converted to Normalised Form C or D first). It
is also recommended that this function not honour Unicode compatibility equivalences or
decompositions.

NOTE 3 The second parameter to this function is likely to be used in a future version of this
standard; it is recommended that implementations do not use this parameter position for anything
else.

NOTE 4 The localeCompare function is intentionally generic; it does not require that its this value
be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

15.5.4.10 String.prototype.match (regexp)
=========================================

When the match method is called with argument regexp, the following steps are taken:

1.  Call CheckObjectCoercible passing the this value as its argument.
2.  Let S be the result of calling ToString, giving it the this value as its argument.
3.  If Type(regexp) is Object and the value of the [[Class]] internal property of regexp is
    "RegExp", then let rx be regexp;
4.  Else, let rx be a new RegExp object created as if by the expression
    new               RegExp(regexp) where RegExp is the standard built-in constructor with that
    name.
5.  Let global be the result of calling the [[Get]] internal method of rx with argument "global".
6.  Let exec be the standard built-in function RegExp.prototype.exec (see 15.10.6.2)
7.  If global is not true, then
    1.  Return the result of calling the [[Call]] internal method of exec with rx as the this value
        and argument list containing S.

8.  Else, global is true
    1.  Call the [[Put]] internal method of rx with arguments "lastIndex" and 0.
    2.  Let A be a new array created as if by the expression new Array() where Array is the standard
        built-in constructor with that name.
    3.  Let previousLastIndex be 0.
    4.  Let n be 0.
    5.  Let lastMatch be true.
    6.  Repeat, while lastMatch is true
        1.  Let result be the result of calling the [[Call]] internal method of exec with rx as the
            this value and argument list containing S.
        2.  If result is null, then set lastMatch to false.
        3.  Else, result is not null
            1.  Let thisIndex be the result of calling the [[Get]] internal method of rx with
                argument "lastIndex".
            2.  If thisIndex = previousLastIndex then
                1.  Call the [[Put]] internal method of rx with arguments "lastIndex" and
                    thisIndex+1.
                2.  Set previousLastIndex to thisIndex+1.

            3.  Else, set previousLastIndex to thisIndex.
            4.  Let matchStr be the result of calling the [[Get]] internal method of result with
                argument "0".
            5.  Call the [[DefineOwnProperty]] internal method of A with arguments ToString(n), the
                Property Descriptor {[[Value]]: matchStr, [[Writable]]: true, [[Enumerable]]: true,
                [[configurable]]: true}, and false.
            6.  Increment n.

    7.  If n = 0, then return null.
    8.  Return A.

NOTE The match function is intentionally generic; it does not require that its this value be a
String object. Therefore, it can be transferred to other kinds of objects for use as a method.

15.5.4.11 String.prototype.replace (searchValue, replaceValue)
==============================================================

First set string according to the following steps:

1.  Call CheckObjectCoercible passing the this value as its argument.
2.  Let string be the result of calling ToString, giving it the this value as its argument.

If searchValue is a regular expression (an object whose [[Class]] internal property is "RegExp"), do
the following: If searchValue.global is false, then search string for the first match of the regular
expression searchValue. If searchValue.global is true, then search string for all matches of the
regular expression searchValue. Do the search in the same manner as in String.prototype.match,
including the update of searchValue.lastIndex. Let m be the number of left capturing parentheses in
searchValue (using NcapturingParens as specified in 15.10.2.1).

If searchValue is not a regular expression, let searchString be ToString(searchValue) and search
string for the first occurrence of searchString. Let m be 0.

If replaceValue is a function, then for each matched substring, call the function with the following
m + 3 arguments. Argument 1 is the substring that matched. If searchValue is a regular expression,
the next m arguments are all of the captures in the MatchResult (see 15.10.2.1). Argument m + 2 is
the offset within string where the match occurred, and argument m + 3 is string. The result is a
String value derived from the original input by replacing each matched substring with the
corresponding return value of the function call, converted to a String if need be.

Otherwise, let newstring denote the result of converting replaceValue to a String. The result is a
String value derived from the original input String by replacing each matched substring with a
String derived from newstring by replacing characters in newstring by replacement text as specified
in Table 22. These $ replacements are done left-to-right, and, once such a replacement is performed,
the new replacement text is not subject to further replacements. For example,
"$1,$2".replace(/(\$(\d))/g, "$$1-$1$2") returns "$1-$11,$1-$22". A $ in newstring that does not
match any of the forms below is left as is.

Table 22 — Replacement Text Symbol Substitutions

  Characters   Replacement text
  ------------ --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
  $$           $
  $&           The matched substring.
  $‘           The portion of string that precedes the matched substring.
  $’           The portion of string that follows the matched substring.
  $n           The nth capture, where n is a single digit in the range 1 to 9 and $n is not followed by a decimal digit. If n≤m and the nth capture is undefined, use the empty String instead. If n>m, the result is implementation-defined.
  $nn          The nnth capture, where nn is a two-digit decimal number in the range 01 to 99. If nn≤m and the nnth capture is undefined, use the empty String instead. If nn>m, the result is implementation-defined.

NOTE The replace function is intentionally generic; it does not require that its this value be a
String object. Therefore, it can be transferred to other kinds of objects for use as a method.

15.5.4.12 String.prototype.search (regexp)
==========================================

When the search method is called with argument regexp, the following steps are taken:

1.  Call CheckObjectCoercible passing the this value as its argument.
2.  Let string be the result of calling ToString, giving it the this value as its argument.
3.  If Type(regexp) is Object and the value of the [[Class]] internal property of regexp is
    "RegExp", then let rx be regexp;
4.  Else, let rx be a new RegExp object created as if by the expression
    new               RegExp(regexp) where RegExp is the standard built-in constructor with that
    name.
5.  Search the value string from its beginning for an occurrence of the regular expression pattern
    rx. Let result be a Number indicating the offset within string where the pattern matched, or –1
    if there was no match. The lastIndex and global properties of regexp are ignored when performing
    the search. The lastIndex property of regexp is left unchanged.
6.  Return result.

NOTE The search function is intentionally generic; it does not require that its this value be a
String object. Therefore, it can be transferred to other kinds of objects for use as a method.

15.5.4.13 String.prototype.slice (start, end)
=============================================

The slice method takes two arguments, start and end, and returns a substring of the result of
converting this object to a String, starting from character position start and running to, but not
including, character position end (or through the end of the String if end is undefined). If start
is negative, it is treated as sourceLength+start where sourceLength is the length of the String. If
end is negative, it is treated as sourceLength+end where sourceLength is the length of the String.
The result is a String value, not a String object. The following steps are taken:

1.  Call CheckObjectCoercible passing the this value as its argument.
2.  Let S be the result of calling ToString, giving it the this value as its argument.
3.  Let len be the number of characters in S.
4.  Let intStart be ToInteger(start).
5.  If end is undefined, let intEnd be len; else let intEnd be ToInteger(end).
6.  If intStart is negative, let from be max(len + intStart,0); else let from be min(intStart, len).
7.  If intEnd is negative, let to be max(len + intEnd,0); else let to be min(intEnd, len).
8.  Let span be max(to – from,0).
9.  Return a String containing span consecutive characters from S beginning with the character at
    position from.

The length property of the slice method is 2.

NOTE The slice function is intentionally generic; it does not require that its this value be a
String object. Therefore it can be transferred to other kinds of objects for use as a method.

15.5.4.14 String.prototype.split (separator, limit)
===================================================

Returns an Array object into which substrings of the result of converting this object to a String
have been stored. The substrings are determined by searching from left to right for occurrences of
separator; these occurrences are not part of any substring in the returned array, but serve to
divide up the String value. The value of separator may be a String of any length or it may be a
RegExp object (i.e., an object whose [[Class]] internal property is "RegExp"; see 15.10).

The value of separator may be an empty String, an empty regular expression, or a regular expression
that can match an empty String. In this case, separator does not match the empty substring at the
beginning or end of the input String, nor does it match the empty substring at the end of the
previous separator match. (For example, if separator is the empty String, the String is split up
into individual characters; the length of the result array equals the length of the String, and each
substring contains one character.) If separator is a regular expression, only the first match at a
given position of the this String is considered, even if backtracking could yield a
non-empty-substring match at that position. (For example, "ab".split(/a*?/) evaluates to the array
["a","b"], while "ab".split(/a*/) evaluates to the array["","b"].)

If the this object is (or converts to) the empty String, the result depends on whether separator can
match the empty String. If it can, the result array contains no elements. Otherwise, the result
array contains one element, which is the empty String.

If separator is a regular expression that contains capturing parentheses, then each time separator
is matched the results (including any undefined results) of the capturing parentheses are spliced
into the output array. For example,

    "A<B>bold</B>and<CODE>coded</CODE>".split(/<(\/)?([^<>]+)>/)

evaluates to the array

    ["A", undefined, "B", "bold", "/", "B", "and", undefined, "CODE", "coded", "/", "CODE", ""]

If separator is undefined, then the result array contains just one String, which is the this value
(converted to a String). If limit is not undefined, then the output array is truncated so that it
contains no more than limit elements.

When the split method is called, the following steps are taken:

1.  Call CheckObjectCoercible passing the this value as its argument.
2.  Let S be the result of calling ToString, giving it the this value as its argument.
3.  Let A be a new array created as if by the expression new Array()where Array is the standard
    built-in constructor with that name.
4.  Let lengthA be 0.
5.  If limit is undefined, let lim = 2^32–1; else let lim = ToUint32(limit).
6.  Let s be the number of characters in S.
7.  Let p = 0.
8.  If separator is a RegExp object (its [[Class]] is "RegExp"), let R = separator; otherwise let R
    = ToString(separator).
9.  If lim = 0, return A.
10. If separator is undefined, then
    1.  Call the [[DefineOwnProperty]] internal method of A with arguments "0", Property Descriptor
        {[[Value]]: S, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
    2.  Return A.

11. If s = 0, then
    1.  Call SplitMatch(S, 0, R) and let z be its MatchResult result.
    2.  If z is not failure, return A.
    3.  Call the [[DefineOwnProperty]] internal method of A with arguments "0", Property Descriptor
        {[[Value]]: S, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
    4.  Return A.

12. Let q = p.
13. Repeat, while q ≠ s
    1.  Call SplitMatch(S, q, R) and let z be its MatchResult result.
    2.  If z is failure, then let q = q+1.
    3.  Else, z is not failure
        1.  z must be a State. Let e be z’s endIndex and let cap be z’s captures array.
        2.  If e = p, then let q = q+1.
        3.  Else, e ≠ p
            1.  Let T be a String value equal to the substring of S consisting of the characters at
                positions p (inclusive) through q (exclusive).
            2.  Call the [[DefineOwnProperty]] internal method of A with arguments
                ToString(lengthA), Property Descriptor {[[Value]]: T, [[Writable]]: true,
                [[Enumerable]]: true, [[Configurable]]: true}, and false.
            3.  Increment lengthA by 1.
            4.  If lengthA = lim, return A.
            5.  Let p = e.
            6.  Let i = 0.
            7.  Repeat, while i is not equal to the number of elements in cap.
                1.  Let i = i+1.
                2.  Call the [[DefineOwnProperty]] internal method of A with arguments
                    ToString(lengthA), Property Descriptor {[[Value]]: cap[i], [[Writable]]: true,
                    [[Enumerable]]: true, [[Configurable]]: true}, and false.
                3.  Increment lengthA by 1.
                4.  If lengthA = lim, return A.

            8.  Let q = p.

14. Let T be a String value equal to the substring of S consisting of the characters at positions p
    (inclusive) through s (exclusive).
15. Call the [[DefineOwnProperty]] internal method of A with arguments ToString(lengthA), Property
    Descriptor {[[Value]]: T, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and
    false.
16. Return A.

The abstract operation SplitMatch takes three parameters, a String S, an integer q, and a String or
RegExp R, and performs the following in order to return a MatchResult (see 15.10.2.1):

1.  If R is a RegExp object (its [[Class]] is "RegExp"), then
    1.  Call the [[Match]] internal method of R giving it the arguments S and q, and return the
        MatchResult result.

2.  Type(R) must be String. Let r be the number of characters in R.
3.  Let s be the number of characters in S.
4.  If q+r > s then return the MatchResult failure.
5.  If there exists an integer i between 0 (inclusive) and r (exclusive) such that the character at
    position q+i of S is different from the character at position i of R, then return failure.
6.  Let cap be an empty array of captures (see 15.10.2.1).
7.  Return the State (q+r, cap). (see 15.10.2.1)

The length property of the split method is 2.

NOTE 1 The split method ignores the value of separator.global for separators that are RegExp
objects.

NOTE 2 The split function is intentionally generic; it does not require that its this value be a
String object. Therefore, it can be transferred to other kinds of objects for use as a method.

15.5.4.15 String.prototype.substring (start, end)
=================================================

The substring method takes two arguments, start and end, and returns a substring of the result of
converting this object to a String, starting from character position start and running to, but not
including, character position end of the String (or through the end of the String is end is
undefined). The result is a String value, not a String object.

If either argument is NaN or negative, it is replaced with zero; if either argument is larger than
the length of the String, it is replaced with the length of the String.

If start is larger than end, they are swapped.

The following steps are taken:

1.  Call CheckObjectCoercible passing the this value as its argument.
2.  Let S be the result of calling ToString, giving it the this value as its argument.
3.  Let len be the number of characters in S.
4.  Let intStart be ToInteger(start).
5.  If end is undefined, let intEnd be len; else let intEnd be ToInteger(end).
6.  Let finalStart be min(max(intStart, 0), len).
7.  Let finalEnd be min(max(intEnd, 0), len).
8.  Let from be min(finalStart, finalEnd).
9.  Let to be max(finalStart, finalEnd).
10. Return a String whose length is to - from, containing characters from S, namely the characters
    with indices from through to −1, in ascending order.

The length property of the substring method is 2.

NOTE The substring function is intentionally generic; it does not require that its this value be a
String object. Therefore, it can be transferred to other kinds of objects for use as a method.

15.5.4.16 String.prototype.toLowerCase ( )
==========================================

The following steps are taken:

1.  Call CheckObjectCoercible passing the this value as its argument.
2.  Let S be the result of calling ToString, giving it the this value as its argument.
3.  Let L be a String where each character of L is either the Unicode lowercase equivalent of the
    corresponding character of S or the actual corresponding character of S if no Unicode lowercase
    equivalent exists.
4.  Return L.

For the purposes of this operation, the 16-bit code units of the Strings are treated as code points
in the Unicode Basic Multilingual Plane. Surrogate code points are directly transferred from S to L
without any mapping.

The result must be derived according to the case mappings in the Unicode character database (this
explicitly includes not only the UnicodeData.txt file, but also the SpecialCasings.txt file that
accompanies it in Unicode 2.1.8 and later).

NOTE 1 The case mapping of some characters may produce multiple characters. In this case the result
String may not be the same length as the source String. Because both toUpperCase and toLowerCase
have context-sensitive behaviour, the functions are not symmetrical. In other words,
s.toUpperCase().toLowerCase() is not necessarily equal to s.toLowerCase().

NOTE 2 The toLowerCase function is intentionally generic; it does not require that its this value be
a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

15.5.4.17 String.prototype.toLocaleLowerCase ( )
================================================

This function works exactly the same as toLowerCase except that its result is intended to yield the
correct result for the host environment’s current locale, rather than a locale-independent result.
There will only be a difference in the few cases (such as Turkish) where the rules for that language
conflict with the regular Unicode case mappings.

NOTE 1 The first parameter to this function is likely to be used in a future version of this
standard; it is recommended that implementations do not use this parameter position for anything
else.

NOTE 2 The toLocaleLowerCase function is intentionally generic; it does not require that its this
value be a String object. Therefore, it can be transferred to other kinds of objects for use as a
method.

15.5.4.18 String.prototype.toUpperCase ( )
==========================================

This function behaves in exactly the same way as String.prototype.toLowerCase, except that
characters are mapped to their uppercase equivalents as specified in the Unicode Character Database.

NOTE The toUpperCase function is intentionally generic; it does not require that its this value be a
String object. Therefore, it can be transferred to other kinds of objects for use as a method.

15.5.4.19 String.prototype.toLocaleUpperCase ( )
================================================

This function works exactly the same as toUpperCase except that its result is intended to yield the
correct result for the host environment’s current locale, rather than a locale-independent result.
There will only be a difference in the few cases (such as Turkish) where the rules for that language
conflict with the regular Unicode case mappings.

NOTE 1 The first parameter to this function is likely to be used in a future version of this
standard; it is recommended that implementations do not use this parameter position for anything
else.

NOTE 2 The toLocaleUpperCase function is intentionally generic; it does not require that its this
value be a String object. Therefore, it can be transferred to other kinds of objects for use as a
method.

15.5.4.20 String.prototype.trim ( )
===================================

The following steps are taken:

1.  Call CheckObjectCoercible passing the this value as its argument.
2.  Let S be the result of calling ToString, giving it the this value as its argument.
3.  Let T be a String value that is a copy of S with both leading and trailing white space removed.
    The definition of white space is the union of WhiteSpace and LineTerminator.
4.  Return T.

NOTE The trim function is intentionally generic; it does not require that its this value be a String
object. Therefore, it can be transferred to other kinds of objects for use as a method.

15.5.5 Properties of String Instances
=====================================

String instances inherit properties from the String prototype object and their [[Class]] internal
property value is "String". String instances also have a [[PrimitiveValue]] internal property, a
length property, and a set of enumerable properties with array index names.

The [[PrimitiveValue]] internal property is the String value represented by this String object. The
array index named properties correspond to the individual characters of the String value. A special
[[GetOwnProperty]] internal method is used to specify the number, values, and attributes of the
array index named properties.

15.5.5.1 length
===============

The number of characters in the String value represented by this String object.

Once a String object is created, this property is unchanging. It has the attributes { [[Writable]]:
false, [[Enumerable]]: false, [[Configurable]]: false }.

15.5.5.2 [[GetOwnProperty]] ( P )
=================================

String objects use a variation of the [[GetOwnProperty]] internal method used for other native
ECMAScript objects (8.12.1). This special internal method provides access to named properties
corresponding to the individual characters of String objects.

Assume S is a String object and P is a String.

When the [[GetOwnProperty]] internal method of S is called with property name P, the following steps
are taken:

1.  Let desc be the result of calling the default [[GetOwnProperty]] internal method (8.12.1) on S
    with argument P.
2.  If desc is not undefined return desc.
3.  If ToString(abs(ToInteger(P))) is not the same value as P, return undefined.
4.  Let str be the String value of the [[PrimitiveValue]] internal property of S.
5.  Let index be ToInteger(P).
6.  Let len be the number of characters in str.
7.  If len ≤ index, return undefined.
8.  Let resultStr be a String of length 1, containing one character from str, specifically the
    character at position index, where the first (leftmost) character in str is considered to be at
    position 0, the next one at position 1, and so on.
9.  Return a Property Descriptor { [[Value]]: resultStr, [[Enumerable]]: true, [[Writable]]: false,
    [[Configurable]]: false }

15.6 Boolean Objects
====================

15.6.1 The Boolean Constructor Called as a Function
===================================================

When Boolean is called as a function rather than as a constructor, it performs a type conversion.

15.6.1.1 Boolean (value)
========================

Returns a Boolean value (not a Boolean object) computed by ToBoolean(value).

15.6.2 The Boolean Constructor
==============================

When Boolean is called as part of a new expression it is a constructor: it initialises the newly
created object.

15.6.2.1 new Boolean (value)
============================

The [[Prototype]] internal property of the newly constructed object is set to the original Boolean
prototype object, the one that is the initial value of Boolean.prototype (15.6.3.1).

The [[Class]] internal property of the newly constructed Boolean object is set to "Boolean".

The [[PrimitiveValue]] internal property of the newly constructed Boolean object is set to
ToBoolean(value).

The [[Extensible]] internal property of the newly constructed object is set to true.

15.6.3 Properties of the Boolean Constructor
============================================

The value of the [[Prototype]] internal property of the Boolean constructor is the Function
prototype object (15.3.4).

Besides the internal properties and the length property (whose value is 1), the Boolean constructor
has the following property:

15.6.3.1 Boolean.prototype
==========================

The initial value of Boolean.prototype is the Boolean prototype object (15.6.4).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.

15.6.4 Properties of the Boolean Prototype Object
=================================================

The Boolean prototype object is itself a Boolean object (its [[Class]] is "Boolean") whose value is
false.

The value of the [[Prototype]] internal property of the Boolean prototype object is the standard
built-in Object prototype object (15.2.4).

15.6.4.1 Boolean.prototype.constructor
======================================

The initial value of Boolean.prototype.constructor is the built-in Boolean constructor.

15.6.4.2 Boolean.prototype.toString ( )
=======================================

The following steps are taken:

1.  Let B be the this value.
2.  If Type(B) is Boolean, then let b be B.
3.  Else if Type(B) is Object and the value of the [[Class]] internal property of B is "Boolean",
    then let b be the value of the [[PrimitiveValue]] internal property of B.
4.  Else throw a TypeError exception.
5.  If b is true, then return "true"; else return "false".

15.6.4.3 Boolean.prototype.valueOf ( )
======================================

The following steps are taken:

1.  Let B be the this value.
2.  If Type(B) is Boolean, then let b be B.
3.  Else if Type(B) is Object and the value of the [[Class]] internal property of B is "Boolean",
    then let b be the value of the [[PrimitiveValue]] internal property of B.
4.  Else throw a TypeError exception.
5.  Return b.

15.6.5 Properties of Boolean Instances
======================================

Boolean instances inherit properties from the Boolean prototype object and their [[Class]] internal
property value is "Boolean". Boolean instances also have a [[PrimitiveValue]] internal property.

The [[PrimitiveValue]] internal property is the Boolean value represented by this Boolean object.

15.7 Number Objects
===================

15.7.1 The Number Constructor Called as a Function
==================================================

When Number is called as a function rather than as a constructor, it performs a type conversion.

15.7.1.1 Number ( [ value ] )
=============================

Returns a Number value (not a Number object) computed by ToNumber(value) if value was supplied, else
returns +0.

15.7.2 The Number Constructor
=============================

When Number is called as part of a new expression it is a constructor: it initialises the newly
created object.

15.7.2.1 new Number ( [ value ] )
=================================

The [[Prototype]] internal property of the newly constructed object is set to the original Number
prototype object, the one that is the initial value of Number.prototype (15.7.3.1).

The [[Class]] internal property of the newly constructed object is set to "Number".

The [[PrimitiveValue]] internal property of the newly constructed object is set to ToNumber(value)
if value was supplied, else to +0.

The [[Extensible]] internal property of the newly constructed object is set to true.

15.7.3 Properties of the Number Constructor
===========================================

The value of the [[Prototype]] internal property of the Number constructor is the Function prototype
object (15.3.4).

Besides the internal properties and the length property (whose value is 1), the Number constructor
has the following properties:

15.7.3.1 Number.prototype
=========================

The initial value of Number.prototype is the Number prototype object (15.7.4).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.

15.7.3.2 Number.MAX_VALUE
=========================

The value of Number.MAX_VALUE is the largest positive finite value of the Number type, which is
approximately 1.7976931348623157 × 10^308.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.

15.7.3.3 Number.MIN_VALUE
=========================

The value of Number.MIN_VALUE is the smallest positive value of the Number type, which is
approximately 5 × 10‑324.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.

15.7.3.4 Number.NaN
===================

The value of Number.NaN is NaN.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.

15.7.3.5 Number.NEGATIVE_INFINITY
=================================

The value of Number.NEGATIVE_INFINITY is −∞.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.

15.7.3.6 Number.POSITIVE_INFINITY
=================================

The value of Number.POSITIVE_INFINITY is +∞.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.

15.7.4 Properties of the Number Prototype Object
================================================

The Number prototype object is itself a Number object (its [[Class]] is "Number") whose value is +0.

The value of the [[Prototype]] internal property of the Number prototype object is the standard
built-in Object prototype object (15.2.4).

Unless explicitly stated otherwise, the methods of the Number prototype object defined below are not
generic and the this value passed to them must be either a Number value or an Object for which the
value of the [[Class]] internal property is "Number".

In the following descriptions of functions that are properties of the Number prototype object, the
phrase “this Number object” refers to either the object that is the this value for the invocation of
the function or, if Type(this value) is Number, an object that is created as if by the expression
new Number(this value) where Number is the standard built-in constructor with that name. Also, the
phrase “this Number value” refers to either the Number value represented by this Number object, that
is, the value of the [[PrimitiveValue]] internal property of this Number object or the this value if
its type is Number. A TypeError exception is thrown if the this value is neither an object for which
the value of the [[Class]] internal property is "Number" or a value whose type is Number.

15.7.4.1 Number.prototype.constructor
=====================================

The initial value of Number.prototype.constructor is the built-in Number constructor.

15.7.4.2 Number.prototype.toString ( [ radix ] )
================================================

The optional radix should be an integer value in the inclusive range 2 to 36. If radix not present
or is undefined the Number 10 is used as the value of radix. If ToInteger(radix) is the Number 10
then this Number value is given as an argument to the ToString abstract operation; the resulting
String value is returned.

If ToInteger(radix) is not an integer between 2 and 36 inclusive throw a RangeError exception. If
ToInteger(radix) is an integer from 2 to 36, but not 10, the result is a String representation of
this Number value using the specified radix. Letters a-z are used for digits with values 10 through
35. The precise algorithm is implementation-dependent if the radix is not 10, however the algorithm
should be a generalisation of that specified in 9.8.1.

The toString function is not generic; it throws a TypeError exception if its this value is not a
Number or a Number object. Therefore, it cannot be transferred to other kinds of objects for use as
a method.

15.7.4.3 Number.prototype.toLocaleString()
==========================================

Produces a String value that represents this Number value formatted according to the conventions of
the host environment’s current locale. This function is implementation-dependent, and it is
permissible, but not encouraged, for it to return the same thing as toString.

NOTE The first parameter to this function is likely to be used in a future version of this standard;
it is recommended that implementations do not use this parameter position for anything else.

15.7.4.4 Number.prototype.valueOf ( )
=====================================

Returns this Number value.

The valueOf function is not generic; it throws a TypeError exception if its this value is not a
Number or a Number object. Therefore, it cannot be transferred to other kinds of objects for use as
a method.

15.7.4.5 Number.prototype.toFixed (fractionDigits)
==================================================

Return a String containing this Number value represented in decimal fixed-point notation with
fractionDigits digits after the decimal point. If fractionDigits is undefined, 0 is assumed.
Specifically, perform the following steps:

1.  Let f be ToInteger(fractionDigits). (If fractionDigits is undefined, this step produces the
    value 0).
2.  If f < 0 or f > 20, throw a RangeError exception.
3.  Let x be this Number value.
4.  If x is NaN, return the String "NaN".
5.  Let s be the empty String.
6.  If x < 0, then
    1.  Let s be "-".
    2.  Let x = –x.

7.  If x ≥ 10^21, then
    1.  Let m = ToString(x).

8.  Else, x < 10^21
    1.  Let n be an integer for which the exact mathematical value of n ÷ 10f – x is as close to
        zero as possible. If there are two such n, pick the larger n.
    2.  If n = 0, let m be the String "0". Otherwise, let m be the String consisting of the digits
        of the decimal representation of n (in order, with no leading zeroes).
    3.  If f ≠ 0, then
        1.  Let k be the number of characters in m.
        2.  If k ≤ f, then
            1.  Let z be the String consisting of f+1–k occurrences of the character ‘0’.
            2.  Let m be the concatenation of Strings z and m.
            3.  Let k = f + 1.

        3.  Let a be the first k–f characters of m, and let b be the remaining f characters of m.
        4.  Let m be the concatenation of the three Strings a, ".", and b.

9.  Return the concatenation of the Strings s and m.

The length property of the toFixed method is 1.

If the toFixed method is called with more than one argument, then the behaviour is undefined (see
clause 15).

An implementation is permitted to extend the behaviour of toFixed for values of fractionDigits less
than 0 or greater than 20. In this case toFixed would not necessarily throw RangeError for such
values.

NOTE The output of toFixed may be more precise than toString for some values because toString only
prints enough significant digits to distinguish the number from adjacent number values. For example,

(1000000000000000128).toString() returns "1000000000000000100",  
while (1000000000000000128).toFixed(0) returns "1000000000000000128".

15.7.4.6 Number.prototype.toExponential (fractionDigits)
========================================================

Return a String containing this Number value represented in decimal exponential notation with one
digit before the significand’s decimal point and fractionDigits digits after the significand’s
decimal point. If fractionDigits is undefined, include as many significand digits as necessary to
uniquely specify the Number (just like in ToString except that in this case the Number is always
output in exponential notation). Specifically, perform the following steps:

1.  Let x be this Number value.
2.  Let f be ToInteger(fractionDigits).
3.  If x is NaN, return the String "NaN".
4.  Let s be the empty String.
5.  If x < 0, then
    1.  Let s be "-".
    2.  Let x = –x.

6.  If x = +∞, then
    1.  Return the concatenation of the Strings s and "Infinity".

7.  If fractionDigits is not undefined and (f < 0 or f > 20), throw a RangeError exception.
8.  If x = 0, then
    1.  Let f = 0.
    2.  Let m be the String consisting of f+1 occurrences of the character ‘0’.
    3.  Let e = 0.

9.  Else, x ≠ 0
    1.  If fractionDigits is not undefined, then
        1.  Let e and n be integers such that 10f ≤ n < 10f+1 and for which the exact mathematical
            value of n × 10e–f – x is as close to zero as possible. If there are two such sets of e
            and n, pick the e and n for which n × 10e–f is larger.

    2.  Else, fractionDigits is undefined
        1.  Let e, n, and f be integers such that f ≥ 0, 10f ≤ n < 10f+1, the number value for n ×
            10e–f is x, and f is as small as possible. Note that the decimal representation of n has
            f+1 digits, n is not divisible by 10, and the least significant digit of n is not
            necessarily uniquely determined by these criteria.

    3.  Let m be the String consisting of the digits of the decimal representation of n (in order,
        with no leading zeroes).

10. If f ≠ 0, then
    1.  Let a be the first character of m, and let b be the remaining f characters of m.
    2.  Let m be the concatenation of the three Strings a, ".", and b.

11. If e = 0, then
    1.  Let c = "+".
    2.  Let d = "0".

12. Else
    1.  If e > 0, then let c = "+".
    2.  Else, e ≤ 0
        1.  Let c = "-".
        2.  Let e = –e.

    3.  Let d be the String consisting of the digits of the decimal representation of e (in order,
        with no leading zeroes).

13. Let m be the concatenation of the four Strings m, "e", c, and d.
14. Return the concatenation of the Strings s and m.

The length property of the toExponential method is 1.

If the toExponential method is called with more than one argument, then the behaviour is undefined
(see clause 15).

An implementation is permitted to extend the behaviour of toExponential for values of fractionDigits
less than 0 or greater than 20. In this case toExponential would not necessarily throw RangeError
for such values.

NOTE For implementations that provide more accurate conversions than required by the rules above, it
is recommended that the following alternative version of step 9.b.i be used as a guideline:

1.  Let e, n, and f be integers such that f ≥ 0, 10f ≤ n < 10f+1, the number value for n × 10e–f is
    x, and f is as small as possible. If there are multiple possibilities for n, choose the value of
    n for which n × 10e–f is closest in value to x. If there are two such possible values of n,
    choose the one that is even.

15.7.4.7 Number.prototype.toPrecision (precision)
=================================================

Return a String containing this Number value represented either in decimal exponential notation with
one digit before the significand’s decimal point and precision–1 digits after the significand’s
decimal point or in decimal fixed notation with precision significant digits. If precision is
undefined, call ToString (9.8.1) instead. Specifically, perform the following steps:

1.  Let x be this Number value.
2.  If precision is undefined, return ToString(x).
3.  Let p be ToInteger(precision).
4.  If x is NaN, return the String "NaN".
5.  Let s be the empty String.
6.  If x < 0, then
    1.  Let s be "-".
    2.  Let x = –x.

7.  If x = +∞, then
    1.  Return the concatenation of the Strings s and "Infinity".

8.  If p < 1 or p > 21, throw a RangeError exception.
9.  If x = 0, then
    1.  Let m be the String consisting of p occurrences of the character ‘0’.
    2.  Let e = 0.

10. Else x ≠ 0,
    1.  Let e and n be integers such that 10p–1 ≤ n < 10p and for which the exact mathematical value
        of n × 10e–p+1 – x is as close to zero as possible. If there are two such sets of e and n,
        pick the e and n for which n × 10e–p+1 is larger.
    2.  Let m be the String consisting of the digits of the decimal representation of n (in order,
        with no leading zeroes).
    3.  If e < –6 or e ≥ p, then
        1.  Let a be the first character of m, and let b be the remaining p–1 characters of m.
        2.  Let m be the concatenation of the three Strings a, ".", and b.
        3.  If e = 0, then
            1.  Let c = "+" and d = "0".

        4.  Else e ≠ 0,
            1.  If e > 0, then
                1.  Let c = "+".

            2.  Else e < 0,
                1.  Let c = "-".
                2.  Let e = –e.

            3.  Let d be the String consisting of the digits of the decimal representation of e (in
                order, with no leading zeroes).

        5.  Let m be the concatenation of the five Strings s, m, "e", c, and d.

11. If e = p–1, then return the concatenation of the Strings s and m.
12. If e ≥ 0, then
    1.  Let m be the concatenation of the first e+1 characters of m, the character ‘.’, and the
        remaining p– (e+1) characters of m.

13. Else e < 0,
    1.  Let m be the concatenation of the String "0.", –(e+1) occurrences of the character ‘0’, and
        the String m.

14. Return the concatenation of the Strings s and m.

The length property of the toPrecision method is 1.

If the toPrecision method is called with more than one argument, then the behaviour is undefined
(see clause 15).

An implementation is permitted to extend the behaviour of toPrecision for values of precision less
than 1 or greater than 21. In this case toPrecision would not necessarily throw RangeError for such
values.

15.7.5 Properties of Number Instances
=====================================

Number instances inherit properties from the Number prototype object and their [[Class]] internal
property value is "Number". Number instances also have a [[PrimitiveValue]] internal property.

The [[PrimitiveValue]] internal property is the Number value represented by this Number object.

15.8 The Math Object
====================

The Math object is a single object that has some named properties, some of which are functions.

The value of the [[Prototype]] internal property of the Math object is the standard built-in Object
prototype object (15.2.4). The value of the [[Class]] internal property of the Math object is
"Math".

The Math object does not have a [[Construct]] internal property; it is not possible to use the Math
object as a constructor with the new operator.

The Math object does not have a [[Call]] internal property; it is not possible to invoke the Math
object as a function.

NOTE In this specification, the phrase “the Number value for x” has a technical meaning defined in
8.5.

15.8.1 Value Properties of the Math Object
==========================================

15.8.1.1 E
==========

The Number value for e, the base of the natural logarithms, which is approximately
2.7182818284590452354.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.

15.8.1.2 LN10
=============

The Number value for the natural logarithm of 10, which is approximately 2.302585092994046.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.

15.8.1.3 LN2
============

The Number value for the natural logarithm of 2, which is approximately 0.6931471805599453.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.

15.8.1.4 LOG2E
==============

The Number value for the base-2 logarithm of e, the base of the natural logarithms; this value is
approximately 1.4426950408889634.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.

NOTE The value of Math.LOG2E is approximately the reciprocal of the value of Math.LN2.

15.8.1.5 LOG10E
===============

The Number value for the base-10 logarithm of e, the base of the natural logarithms; this value is
approximately 0.4342944819032518.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.

NOTE The value of Math.LOG10E is approximately the reciprocal of the value of Math.LN10.

15.8.1.6 PI
===========

The Number value for π, the ratio of the circumference of a circle to its diameter, which is
approximately 3.1415926535897932.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.

15.8.1.7 SQRT1_2
================

The Number value for the square root of ½, which is approximately 0.7071067811865476.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.

NOTE The value of Math.SQRT1_2 is approximately the reciprocal of the value of Math.SQRT2.

15.8.1.8 SQRT2
==============

The Number value for the square root of 2, which is approximately 1.4142135623730951.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.

15.8.2 Function Properties of the Math Object
=============================================

Each of the following Math object functions applies the ToNumber abstract operator to each of its
arguments (in left-to-right order if there is more than one) and then performs a computation on the
resulting Number value(s).

In the function descriptions below, the symbols NaN, −0, +0, −∞ and +∞ refer to the Number values
described in 8.5.

NOTE The behaviour of the functions acos, asin, atan, atan2, cos, exp, log, pow, sin, sqrt, and tan
is not precisely specified here except to require specific results for certain argument values that
represent boundary cases of interest. For other argument values, these functions are intended to
compute approximations to the results of familiar mathematical functions, but some latitude is
allowed in the choice of approximation algorithms. The general intent is that an implementer should
be able to use the same mathematical library for ECMAScript on a given hardware platform that is
available to C programmers on that platform.

Although the choice of algorithms is left to the implementation, it is recommended (but not
specified by this standard) that implementations use the approximation algorithms for IEEE 754
arithmetic contained in fdlibm, the freely distributable mathematical library from Sun Microsystems
(http://www.netlib.org/fdlibm).

15.8.2.1 abs (x)
================

Returns the absolute value of x; the result has the same magnitude as x but has positive sign.

-   If x is NaN, the result is NaN.
-   If x is −0, the result is +0.
-   If x is −∞, the result is +∞.

15.8.2.2 acos (x)
=================

Returns an implementation-dependent approximation to the arc cosine of x. The result is expressed in
radians and ranges from +0 to +π.

-   If x is NaN, the result is NaN.
-   If x is greater than 1, the result is NaN.
-   If x is less than −1, the result is NaN.
-   If x is exactly 1, the result is +0.

15.8.2.3 asin (x)
=================

Returns an implementation-dependent approximation to the arc sine of x. The result is expressed in
radians and ranges from −π/2 to +π/2.

-   If x is NaN, the result is NaN.
-   If x is greater than 1, the result is NaN.
-   If x is less than –1, the result is NaN.
-   If x is +0, the result is +0.
-   If x is −0, the result is −0.

15.8.2.4 atan (x)
=================

Returns an implementation-dependent approximation to the arc tangent of x. The result is expressed
in radians and ranges from −π/2 to +π/2.

-   If x is NaN, the result is NaN.
-   If x is +0, the result is +0.
-   If x is −0, the result is −0.
-   If x is +∞, the result is an implementation-dependent approximation to +π/2.
-   If x is −∞, the result is an implementation-dependent approximation to −π/2.

15.8.2.5 atan2 (y, x)
=====================

Returns an implementation-dependent approximation to the arc tangent of the quotient y/x of the
arguments y and x, where the signs of y and x are used to determine the quadrant of the result. Note
that it is intentional and traditional for the two-argument arc tangent function that the argument
named y be first and the argument named x be second. The result is expressed in radians and ranges
from −π to +π.

-   If either x or y is NaN, the result is NaN.
-   If y>0 and x is +0, the result is an implementation-dependent approximation to +π/2.
-   If y>0 and x is −0, the result is an implementation-dependent approximation to +π/2.
-   If y is +0 and x>0, the result is +0.
-   If y is +0 and x is +0, the result is +0.
-   If y is +0 and x is −0, the result is an implementation-dependent approximation to +π.
-   If y is +0 and x<0, the result is an implementation-dependent approximation to +π.
-   If y is −0 and x>0, the result is −0.
-   If y is −0 and x is +0, the result is −0.
-   If y is −0 and x is −0, the result is an implementation-dependent approximation to −π.
-   If y is −0 and x<0, the result is an implementation-dependent approximation to −π.
-   If y<0 and x is +0, the result is an implementation-dependent approximation to −π/2.
-   If y<0 and x is −0, the result is an implementation-dependent approximation to −π/2.
-   If y>0 and y is finite and x is +∞, the result is +0.
-   If y>0 and y is finite and x is −∞, the result if an implementation-dependent approximation to
    +π.
-   If y<0 and y is finite and x is +∞, the result is −0.
-   If y<0 and y is finite and x is −∞, the result is an implementation-dependent approximation to
    −π.
-   If y is +∞ and x is finite, the result is an implementation-dependent approximation to +π/2.
-   If y is −∞ and x is finite, the result is an implementation-dependent approximation to −π/2.
-   If y is +∞ and x is +∞, the result is an implementation-dependent approximation to +π/4.
-   If y is +∞ and x is −∞, the result is an implementation-dependent approximation to +3π/4.
-   If y is −∞ and x is +∞, the result is an implementation-dependent approximation to −π/4.
-   If y is −∞ and x is −∞, the result is an implementation-dependent approximation to −3π/4.

15.8.2.6 ceil (x)
=================

Returns the smallest (closest to −∞) Number value that is not less than x and is equal to a
mathematical integer. If x is already an integer, the result is x.

-   If x is NaN, the result is NaN.
-   If x is +0, the result is +0.
-   If x is −0, the result is −0.
-   If x is +∞, the result is +∞.
-   If x is −∞, the result is −∞.
-   If x is less than 0 but greater than -1, the result is −0.

The value of Math.ceil(x) is the same as the value of -Math.floor(-x).

15.8.2.7 cos (x)
================

Returns an implementation-dependent approximation to the cosine of x. The argument is expressed in
radians.

-   If x is NaN, the result is NaN.
-   If x is +0, the result is 1.
-   If x is −0, the result is 1.
-   If x is +∞, the result is NaN.
-   If x is −∞, the result is NaN.

15.8.2.8 exp (x)
================

Returns an implementation-dependent approximation to the exponential function of x (e raised to the
power of x, where e is the base of the natural logarithms).

-   If x is NaN, the result is NaN.
-   If x is +0, the result is 1.
-   If x is −0, the result is 1.
-   If x is +∞, the result is +∞.
-   If x is −∞, the result is +0.

15.8.2.9 floor (x)
==================

Returns the greatest (closest to +∞) Number value that is not greater than x and is equal to a
mathematical integer. If x is already an integer, the result is x.

-   If x is NaN, the result is NaN.
-   If x is +0, the result is +0.
-   If x is −0, the result is −0.
-   If x is +∞, the result is +∞.
-   If x is −∞, the result is −∞.
-   If x is greater than 0 but less than 1, the result is +0.

NOTE The value of Math.floor(x) is the same as the value of -Math.ceil(-x).

15.8.2.10 log (x)
=================

-   Returns an implementation-dependent approximation to the natural logarithm of x.
-   If x is NaN, the result is NaN.
-   If x is less than 0, the result is NaN.
-   If x is +0 or −0, the result is −∞.
-   If x is 1, the result is +0.
-   If x is +∞, the result is +∞.

15.8.2.11 max ( [ value1 [ , value2 [ , … ] ] ] )
=================================================

Given zero or more arguments, calls ToNumber on each of the arguments and returns the largest of the
resulting values.

-   If no arguments are given, the result is −∞.
-   If any value is NaN, the result is NaN.
-   The comparison of values to determine the largest value is done as in 11.8.5 except that +0 is
    considered to be larger than −0.

The length property of the max method is 2.

15.8.2.12 min ( [ value1 [ , value2 [ , … ] ] ] )
=================================================

Given zero or more arguments, calls ToNumber on each of the arguments and returns the smallest of
the resulting values.

-   If no arguments are given, the result is +∞.
-   If any value is NaN, the result is NaN.
-   The comparison of values to determine the smallest value is done as in 11.8.5 except that +0 is
    considered to be larger than −0.

The length property of the min method is 2.

15.8.2.13 pow (x, y)
====================

Returns an implementation-dependent approximation to the result of raising x to the power y.

-   If y is NaN, the result is NaN.
-   If y is +0, the result is 1, even if x is NaN.
-   If y is −0, the result is 1, even if x is NaN.
-   If x is NaN and y is nonzero, the result is NaN.
-   If abs(x)>1 and y is +∞, the result is +∞.
-   If abs(x)>1 and y is −∞, the result is +0.
-   If abs(x)==1 and y is +∞, the result is NaN.
-   If abs(x)==1 and y is −∞, the result is NaN.
-   If abs(x)<1 and y is +∞, the result is +0.
-   If abs(x)<1 and y is −∞, the result is +∞.
-   If x is +∞ and y>0, the result is +∞.
-   If x is +∞ and y<0, the result is +0.
-   If x is −∞ and y>0 and y is an odd integer, the result is −∞.
-   If x is −∞ and y>0 and y is not an odd integer, the result is +∞.
-   If x is −∞ and y<0 and y is an odd integer, the result is −0.
-   If x is −∞ and y<0 and y is not an odd integer, the result is +0.
-   If x is +0 and y>0, the result is +0.
-   If x is +0 and y<0, the result is +∞.
-   If x is −0 and y>0 and y is an odd integer, the result is −0.
-   If x is −0 and y>0 and y is not an odd integer, the result is +0.
-   If x is −0 and y<0 and y is an odd integer, the result is −∞.
-   If x is −0 and y<0 and y is not an odd integer, the result is +∞.
-   If x<0 and x is finite and y is finite and y is not an integer, the result is NaN.

15.8.2.14 random ( )
====================

Returns a Number value with positive sign, greater than or equal to 0 but less than 1, chosen
randomly or pseudo randomly with approximately uniform distribution over that range, using an
implementation-dependent algorithm or strategy. This function takes no arguments.

15.8.2.15 round (x)
===================

Returns the Number value that is closest to x and is equal to a mathematical integer. If two integer
Number values are equally close to x, then the result is the Number value that is closer to +∞. If x
is already an integer, the result is x.

-   If x is NaN, the result is NaN.
-   If x is +0, the result is +0.
-   If x is −0, the result is −0.
-   If x is +∞, the result is +∞.
-   If x is −∞, the result is −∞.
-   If x is greater than 0 but less than 0.5, the result is +0.
-   If x is less than 0 but greater than or equal to -0.5, the result is −0.

NOTE 1 Math.round(3.5) returns 4, but Math.round(–3.5) returns –3.

NOTE 2 The value of Math.round(x) is the same as the value of Math.floor(x+0.5), except when x is −0
or is less than 0 but greater than or equal to -0.5; for these cases Math.round(x) returns −0, but
Math.floor(x+0.5) returns +0.

15.8.2.16 sin (x)
=================

Returns an implementation-dependent approximation to the sine of x. The argument is expressed in
radians.

-   If x is NaN, the result is NaN.
-   If x is +0, the result is +0.
-   If x is −0, the result is −0.
-   If x is +∞ or −∞, the result is NaN.

15.8.2.17 sqrt (x)
==================

Returns an implementation-dependent approximation to the square root of x.

-   If x is NaN, the result is NaN.
-   If x is less than 0, the result is NaN.
-   If x is +0, the result is +0.
-   If x is −0, the result is −0.
-   If x is +∞, the result is +∞.

15.8.2.18 tan (x)
=================

Returns an implementation-dependent approximation to the tangent of x. The argument is expressed in
radians.

-   If x is NaN, the result is NaN.
-   If x is +0, the result is +0.
-   If x is −0, the result is −0.
-   If x is +∞ or −∞, the result is NaN.

15.9 Date Objects
=================

15.9.1 Overview of Date Objects and Definitions of Abstract Operators
=====================================================================

The following functions are abstract operations that operate on time values (defined in 15.9.1.1).
Note that, in every case, if any argument to one of these functions is NaN, the result will be NaN.

15.9.1.1 Time Values and Time Range
===================================

A Date object contains a Number indicating a particular instant in time to within a millisecond.
Such a Number is called a time value. A time value may also be NaN, indicating that the Date object
does not represent a specific instant of time.

Time is measured in ECMAScript in milliseconds since 01 January, 1970 UTC. In time values leap
seconds are ignored. It is assumed that there are exactly 86,400,000 milliseconds per day.
ECMAScript Number values can represent all integers from –9,007,199,254,740,992 to
9,007,199,254,740,992; this range suffices to measure times to millisecond precision for any instant
that is within approximately 285,616 years, either forward or backward, from 01 January, 1970 UTC.

The actual range of times supported by ECMAScript Date objects is slightly smaller: exactly
–100,000,000 days to 100,000,000 days measured relative to midnight at the beginning of 01 January,
1970 UTC. This gives a range of 8,640,000,000,000,000 milliseconds to either side of 01 January,
1970 UTC.

The exact moment of midnight at the beginning of 01 January, 1970 UTC is represented by the value
+0.

15.9.1.2 Day Number and Time within Day
=======================================

A given time value t belongs to day number

Day(t) = floor(t / msPerDay)

where the number of milliseconds per day is

msPerDay = 86400000

The remainder is called the time within the day:

TimeWithinDay(t) = t modulo msPerDay

15.9.1.3 Year Number
====================

ECMAScript uses an extrapolated Gregorian system to map a day number to a year number and to
determine the month and date within that year. In this system, leap years are precisely those which
are (divisible by 4) and ((not divisible by 100) or (divisible by 400)). The number of days in year
number y is therefore defined by

DaysInYear(y) = 365 if (y modulo 4) ≠ 0  
= 366 if (y modulo 4) = 0 and (y modulo 100) ≠ 0  
= 365 if (y modulo 100) = 0 and (y modulo 400) ≠ 0  
= 366 if (y modulo 400) = 0

All non-leap years have 365 days with the usual number of days per month and leap years have an
extra day in February. The day number of the first day of year y is given by:

DayFromYear(y) = 365 × (y−1970) + floor((y−1969)/4) − floor((y−1901)/100) + floor((y−1601)/400)

The time value of the start of a year is:

TimeFromYear(y) = msPerDay × DayFromYear(y)

A time value determines a year by:

YearFromTime(t) = the largest integer y (closest to positive infinity) such that TimeFromYear(y) ≤ t

The leap-year function is 1 for a time within a leap year and otherwise is zero:

InLeapYear(t) = 0 if DaysInYear(YearFromTime(t)) = 365  
= 1 if DaysInYear(YearFromTime(t)) = 366

15.9.1.4 Month Number
=====================

Months are identified by an integer in the range 0 to 11, inclusive. The mapping MonthFromTime(t)
from a time value t to a month number is defined by:

MonthFromTime(t) = 0 if 0 ≤ DayWithinYear(t) < 31  
 = 1 if 31 ≤ DayWithinYear (t) < 59+InLeapYear(t)  
 = 2 if 59+InLeapYear(t) ≤ DayWithinYear (t) < 90+InLeapYear(t)  
 = 3 if 90+InLeapYear(t) ≤ DayWithinYear (t) < 120+InLeapYear(t)  
 = 4 if 120+InLeapYear(t) ≤ DayWithinYear (t) < 151+InLeapYear(t)  
 = 5 if 151+InLeapYear(t) ≤ DayWithinYear (t) < 181+InLeapYear(t)  
 = 6 if 181+InLeapYear(t) ≤ DayWithinYear (t) < 212+InLeapYear(t)  
 = 7 if 212+InLeapYear(t) ≤ DayWithinYear (t) < 243+InLeapYear(t)  
 = 8 if 243+InLeapYear(t) ≤ DayWithinYear (t) < 273+InLeapYear(t)  
 = 9 if 273+InLeapYear(t) ≤ DayWithinYear (t) < 304+InLeapYear(t)  
 = 10 if 304+InLeapYear(t) ≤ DayWithinYear (t) < 334+InLeapYear(t)  
 = 11 if 334+InLeapYear(t) ≤ DayWithinYear (t) < 365+InLeapYear(t)

where

DayWithinYear(t) = Day(t)−DayFromYear(YearFromTime(t))

A month value of 0 specifies January; 1 specifies February; 2 specifies March; 3 specifies April;
4 specifies May; 5 specifies June; 6 specifies July; 7 specifies August; 8 specifies September; 9
specifies October; 10 specifies November; and 11 specifies December. Note that MonthFromTime(0) = 0,
corresponding to Thursday, 01 January, 1970.

15.9.1.5 Date Number
====================

A date number is identified by an integer in the range 1 through 31, inclusive. The mapping
DateFromTime(t) from a time value t to a month number is defined by:

DateFromTime(t) = DayWithinYear(t)+1 if MonthFromTime(t)=0  
 = DayWithinYear(t)−30 if MonthFromTime(t)=1  
 = DayWithinYear(t)−58−InLeapYear(t) if MonthFromTime(t)=2  
 = DayWithinYear(t)−89−InLeapYear(t) if MonthFromTime(t)=3  
 = DayWithinYear(t)−119−InLeapYear(t) if MonthFromTime(t)=4  
 = DayWithinYear(t)−150−InLeapYear(t) if MonthFromTime(t)=5  
 = DayWithinYear(t)−180−InLeapYear(t) if MonthFromTime(t)=6  
 = DayWithinYear(t)−211−InLeapYear(t) if MonthFromTime(t)=7  
 = DayWithinYear(t)−242−InLeapYear(t) if MonthFromTime(t)=8  
 = DayWithinYear(t)−272−InLeapYear(t) if MonthFromTime(t)=9  
 = DayWithinYear(t)−303−InLeapYear(t) if MonthFromTime(t)=10  
 = DayWithinYear(t)−333−InLeapYear(t) if MonthFromTime(t)=11

15.9.1.6 Week Day
=================

The weekday for a particular time value t is defined as

WeekDay(t) = (Day(t) + 4) modulo 7

A weekday value of 0 specifies Sunday; 1 specifies Monday; 2 specifies Tuesday; 3 specifies
Wednesday; 4 specifies Thursday; 5 specifies Friday; and 6 specifies Saturday. Note that WeekDay(0)
= 4, corresponding to Thursday, 01 January, 1970.

15.9.1.7 Local Time Zone Adjustment
===================================

An implementation of ECMAScript is expected to determine the local time zone adjustment. The local
time zone adjustment is a value LocalTZA measured in milliseconds which when added to UTC represents
the local standard time. Daylight saving time is not reflected by LocalTZA. The value LocalTZA does
not vary with time but depends only on the geographic location.

15.9.1.8 Daylight Saving Time Adjustment
========================================

An implementation of ECMAScript is expected to determine the daylight saving time algorithm. The
algorithm to determine the daylight saving time adjustment DaylightSavingTA(t), measured in
milliseconds, must depend only on four things:

(1) the time since the beginning of the year

t – TimeFromYear(YearFromTime(t))

(2) whether t is in a leap year

InLeapYear(t)

(3) the week day of the beginning of the year

WeekDay(TimeFromYear(YearFromTime(t)))

and (4) the geographic location.

The implementation of ECMAScript should not try to determine whether the exact time was subject to
daylight saving time, but just whether daylight saving time would have been in effect if the current
daylight saving time algorithm had been used at the time. This avoids complications such as taking
into account the years that the locale observed daylight saving time year round.

If the host environment provides functionality for determining daylight saving time, the
implementation of ECMAScript is free to map the year in question to an equivalent year (same
leap-year-ness and same starting week day for the year) for which the host environment provides
daylight saving time information. The only restriction is that all equivalent years should produce
the same result.

15.9.1.9 Local Time
===================

Conversion from UTC to local time is defined by

LocalTime(t) = t + LocalTZA + DaylightSavingTA(t)

Conversion from local time to UTC is defined by

UTC(t) = t – LocalTZA – DaylightSavingTA(t – LocalTZA)

Note that UTC(LocalTime(t)) is not necessarily always equal to t.

15.9.1.10 Hours, Minutes, Second, and Milliseconds
==================================================

The following functions are useful in decomposing time values:

HourFromTime(t) = floor(t / msPerHour) modulo HoursPerDay

MinFromTime(t) = floor(t / msPerMinute) modulo MinutesPerHour

SecFromTime(t) = floor(t / msPerSecond) modulo SecondsPerMinute

msFromTime(t) = t modulo msPerSecond

where

HoursPerDay = 24

MinutesPerHour = 60

SecondsPerMinute = 60

msPerSecond = 1000

msPerMinute = 60000 = msPerSecond × SecondsPerMinute

msPerHour = 3600000 = msPerMinute × MinutesPerHour

15.9.1.11 MakeTime (hour, min, sec, ms)
=======================================

The operator MakeTime calculates a number of milliseconds from its four arguments, which must be
ECMAScript Number values. This operator functions as follows:

1.  If hour is not finite or min is not finite or sec is not finite or ms is not finite, return NaN.
2.  Let h be ToInteger(hour).
3.  Let m be ToInteger(min).
4.  Let s be ToInteger(sec).
5.  Let milli be ToInteger(ms).
6.  Let t be h * msPerHour + m * msPerMinute + s * msPerSecond + milli, performing the arithmetic
    according to IEEE 754 rules (that is, as if using the ECMAScript operators * and +).
7.  Return t.

15.9.1.12 MakeDay (year, month, date)
=====================================

The operator MakeDay calculates a number of days from its three arguments, which must be ECMAScript
Number values. This operator functions as follows:

1.  If year is not finite or month is not finite or date is not finite, return NaN.
2.  Let y be ToInteger(year).
3.  Let m be ToInteger(month).
4.  Let dt be ToInteger(date).
5.  Let ym be y + floor(m /12).
6.  Let mn be m modulo 12.
7.  Find a value t such that YearFromTime(t) == ym and MonthFromTime(t) == mn and DateFromTime(t) ==
    1; but if this is not possible (because some argument is out of range), return NaN.
8.  Return Day(t) + dt − 1.

15.9.1.13 MakeDate (day, time)
==============================

The operator MakeDate calculates a number of milliseconds from its two arguments, which must be
ECMAScript Number values. This operator functions as follows:

1.  If day is not finite or time is not finite, return NaN.
2.  Return day × msPerDay + time.

15.9.1.14 TimeClip (time)
=========================

The operator TimeClip calculates a number of milliseconds from its argument, which must be an
ECMAScript Number value. This operator functions as follows:

1.  If time is not finite, return NaN.
2.  If abs(time) > 8.64 x 10^15, return NaN.
3.  Return an implementation-dependent choice of either ToInteger(time) or ToInteger(time) + (+0).
    (Adding a positive zero converts −0 to +0.)

NOTE The point of step 3 is that an implementation is permitted a choice of internal representations
of time values, for example as a 64-bit signed integer or as a 64-bit floating-point value.
Depending on the implementation, this internal representation may or may not distinguish −0 and +0.

15.9.1.15 Date Time String Format
=================================

ECMAScript defines a string interchange format for date-times based upon a simplification of the ISO
8601 Extended Format. The format is as follows: YYYY-MM-DDTHH:mm:ss.sssZ

Where the fields are as follows:

YYYY is the decimal digits of the year 0000 to 9999 in the Gregorian calendar.

- “-” (hyphen) appears literally twice in the string.

MM is the month of the year from 01 (January) to 12 (December).

DD is the day of the month from 01 to 31.

T “T” appears literally in the string, to indicate the beginning of the time element.

HH is the number of complete hours that have passed since midnight as two decimal digits from 00 to
24.

: “:” (colon) appears literally twice in the string.

mm is the number of complete minutes since the start of the hour as two decimal digits from 00 to
59.

ss is the number of complete seconds since the start of the minute as two decimal digits from 00 to
59.

. “.” (dot) appears literally in the string.

sss is the number of complete milliseconds since the start of the second as three decimal digits.

Z is the time zone offset specified as “Z” (for UTC) or either “+” or “-” followed by a time
expression HH:mm

This format includes date-only forms:

~~~~ {.Note}
YYYYYYYY-MMYYYY-MM-DD
~~~~

It also includes “date-time” forms that consist of one of the above date-only forms immediately
followed by one of the following time forms with an optional time zone offset appended:

    THH:mmTHH:mm:ssTHH:mm:ss.sss

All numbers must be base 10. If the MM or DD fields are absent “01” is used as the value. If the HH,
mm, or ss fields are absent “00” is used as the value and the value of an absent sss field is “000”.
The value of an absent time zone offset is “Z”.

Illegal values (out-of-bounds as well as syntax errors) in a format string means that the format
string is not a valid instance of this format.

NOTE 1 As every day both starts and ends with midnight, the two notations 00:00 and 24:00 are
available to distinguish the two midnights that can be associated with one date. This means that the
following two notations refer to exactly the same point in time: 1995-02-04T24:00 and
1995-02-05T00:00

NOTE 2 There exists no international standard that specifies abbreviations for civil time zones like
CET, EST, etc. and sometimes the same abbreviation is even used for two very different time zones.
For this reason, ISO 8601 and this format specifies numeric representations of date and time.

15.9.1.15.1 Extended years
==========================

ECMAScript requires the ability to specify 6 digit years (extended years); approximately 285,426
years, either forward or backward, from 01 January, 1970 UTC. To represent years before 0 or after
9999, ISO 8601 permits the expansion of the year representation, but only by prior agreement between
the sender and the receiver. In the simplified ECMAScript format such an expanded year
representation shall have 2 extra year digits and is always prefixed with a + or – sign. The year 0
is considered positive and hence prefixed with a + sign.

NOTE Examples of extended years:

-283457-03-21T15:00:59.008Z   283458 B.C.  
-000001-01-01T00:00:00Z          2 B.C.  
+000000-01-01T00:00:00Z         1 B.C.  
+000001-01-01T00:00:00Z         1 A.D.  
+001970-01-01T00:00:00Z         1970 A.D.  
+002009-12-15T00:00:00Z         2009 A.D.  
+287396-10-12T08:59:00.992Z 287396 A.D.

15.9.2 The Date Constructor Called as a Function
================================================

When Date is called as a function rather than as a constructor, it returns a String representing the
current time (UTC).

NOTE The function call Date(…) is not equivalent to the object creation expression new Date(…) with
the same arguments.

15.9.2.1 Date ( [ year [, month [, date [, hours [, minutes [, seconds [, ms ] ] ] ] ] ] ] )
============================================================================================

All of the arguments are optional; any arguments supplied are accepted but are completely ignored. A
String is created and returned as if by the expression (new Date()).toString() where Date is the
standard built-in constructor with that name and toString is the standard built-in method
Date.prototype.toString.

15.9.3 The Date Constructor
===========================

When Date is called as part of a new expression, it is a constructor: it initialises the newly
created object.

15.9.3.1 new Date (year, month [, date [, hours [, minutes [, seconds [, ms ] ] ] ] ] )
=======================================================================================

When Date is called with two to seven arguments, it computes the date from year, month, and
(optionally) date, hours, minutes, seconds and ms.

The [[Prototype]] internal property of the newly constructed object is set to the original Date
prototype object, the one that is the initial value of Date.prototype (15.9.4.1).

The [[Class]] internal property of the newly constructed object is set to "Date".

The [[Extensible]] internal property of the newly constructed object is set to true.

The [[PrimitiveValue]] internal property of the newly constructed object is set as follows:

1.  Let y be ToNumber(year).
2.  Let m be ToNumber(month).
3.  If date is supplied then let dt be ToNumber(date); else let dt be 1.
4.  If hours is supplied then let h be ToNumber(hours); else let h be 0.
5.  If minutes is supplied then let min be ToNumber(minutes); else let min be 0.
6.  If seconds is supplied then let s be ToNumber(seconds); else let s be 0.
7.  If ms is supplied then let milli be ToNumber(ms); else let milli be 0.
8.  If y is not NaN and 0 ≤ ToInteger(y) ≤ 99, then let yr be 1900+ToInteger(y); otherwise, let yr
    be y.
9.  Let finalDate be MakeDate(MakeDay(yr, m, dt), MakeTime(h, min, s, milli)).
10. Set the [[PrimitiveValue]] internal property of the newly constructed object to
    TimeClip(UTC(finalDate)).

15.9.3.2 new Date (value)
=========================

The [[Prototype]] internal property of the newly constructed object is set to the original Date
prototype object, the one that is the initial value of Date.prototype (15.9.4.1).

The [[Class]] internal property of the newly constructed object is set to "Date".

The [[Extensible]] internal property of the newly constructed object is set to true.

The [[PrimitiveValue]] internal property of the newly constructed object is set as follows:

1.  Let v be ToPrimitive(value).
2.  If Type(v) is String, then
    1.  Parse v as a date, in exactly the same manner as for the parse method (15.9.4.2); let V be
        the time value for this date.

3.  Else, let V be ToNumber(v).
4.  Set the [[PrimitiveValue]] internal property of the newly constructed object to TimeClip(V) and
    return.

15.9.3.3 new Date ( )
=====================

The [[Prototype]] internal property of the newly constructed object is set to the original Date
prototype object, the one that is the initial value of Date.prototype (15.9.4.1).

The [[Class]] internal property of the newly constructed object is set to "Date".

The [[Extensible]] internal property of the newly constructed object is set to true.

The [[PrimitiveValue]] internal property of the newly constructed object is set to the time value
(UTC) identifying the current time.

15.9.4 Properties of the Date Constructor
=========================================

The value of the [[Prototype]] internal property of the Date constructor is the Function prototype
object (15.3.4).

Besides the internal properties and the length property (whose value is 7), the Date constructor has
the following properties:

15.9.4.1 Date.prototype
=======================

The initial value of Date.prototype is the built-in Date prototype object (15.9.5).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.

15.9.4.2 Date.parse (string)
============================

The parse function applies the ToString operator to its argument and interprets the resulting String
as a date and time; it returns a Number, the UTC time value corresponding to the date and time. The
String may be interpreted as a local time, a UTC time, or a time in some other time zone, depending
on the contents of the String. The function first attempts to parse the format of the String
according to the rules called out in Date Time String Format (15.9.1.15). If the String does not
conform to that format the function may fall back to any implementation-specific heuristics or
implementation-specific date formats. Unrecognisable Strings or dates containing illegal element
values in the format String shall cause Date.parse to return NaN.

If x is any Date object whose milliseconds amount is zero within a particular implementation of
ECMAScript, then all of the following expressions should produce the same numeric value in that
implementation, if all the properties referenced have their initial values:

    x.valueOf()

    Date.parse(x.toString())

    Date.parse(x.toUTCString())

    Date.parse(x.toISOString())

However, the expression

Date.parse(x.toLocaleString())

is not required to produce the same Number value as the preceding three expressions and, in general,
the value produced by Date.parse is implementation-dependent when given any String value that does
not conform to the Date Time String Format (15.9.1.15) and that could not be produced in that
implementation by the toString or toUTCString method.

15.9.4.3 Date.UTC (year, month [, date [, hours [, minutes [, seconds [, ms ] ] ] ] ] )
=======================================================================================

When the UTC function is called with fewer than two arguments, the behaviour is
implementation-dependent. When the UTC function is called with two to seven arguments, it computes
the date from year, month and (optionally) date, hours, minutes, seconds and ms. The following steps
are taken:

1.  Let y be ToNumber(year).
2.  Let m be ToNumber(month).
3.  If date is supplied then let dt be ToNumber(date); else let dt be 1.
4.  If hours is supplied then let h be ToNumber(hours); else let h be 0.
5.  If minutes is supplied then let min be ToNumber(minutes); else let min be 0.
6.  If seconds is supplied then let s be ToNumber(seconds); else let s be 0.
7.  If ms is supplied then let milli be ToNumber(ms); else let milli be 0.
8.  If y is not NaN and 0 ≤ ToInteger(y) ≤ 99, then let yr be 1900+ToInteger(y); otherwise, let yr
    be y.
9.  Return TimeClip(MakeDate(MakeDay(yr, m, dt), MakeTime(h, min, s, milli))).

The length property of the UTC function is 7.

NOTE The UTC function differs from the Date constructor in two ways: it returns a time value as a
Number, rather than creating a Date object, and it interprets the arguments in UTC rather than as
local time.

15.9.4.4 Date.now ( )
=====================

The now function return a Number value that is the time value designating the UTC date and time of
the occurrence of the call to now.

15.9.5 Properties of the Date Prototype Object
==============================================

The Date prototype object is itself a Date object (its [[Class]] is "Date") whose [[PrimitiveValue]]
is NaN.

The value of the [[Prototype]] internal property of the Date prototype object is the standard
built-in Object prototype object (15.2.4).

In following descriptions of functions that are properties of the Date prototype object, the phrase
“this Date object” refers to the object that is the this value for the invocation of the function.
Unless explicitly noted otherwise, none of these functions are generic; a TypeError exception is
thrown if the this value is not an object for which the value of the [[Class]] internal property is
"Date". Also, the phrase “this time value” refers to the Number value for the time represented by
this Date object, that is, the value of the [[PrimitiveValue]] internal property of this Date
object.

15.9.5.1 Date.prototype.constructor
===================================

The initial value of Date.prototype.constructor is the built-in Date constructor.

15.9.5.2 Date.prototype.toString ( )
====================================

This function returns a String value. The contents of the String are implementation-dependent, but
are intended to represent the Date in the current time zone in a convenient, human-readable form.

NOTE For any Date value d whose milliseconds amount is zero, the result of Date.parse(d.toString())
is equal to d.valueOf(). See 15.9.4.2.

15.9.5.3 Date.prototype.toDateString ( )
========================================

This function returns a String value. The contents of the String are implementation-dependent, but
are intended to represent the “date” portion of the Date in the current time zone in a convenient,
human-readable form.

15.9.5.4 Date.prototype.toTimeString ( )
========================================

This function returns a String value. The contents of the String are implementation-dependent, but
are intended to represent the “time” portion of the Date in the current time zone in a convenient,
human-readable form.

15.9.5.5 Date.prototype.toLocaleString ( )
==========================================

This function returns a String value. The contents of the String are implementation-dependent, but
are intended to represent the Date in the current time zone in a convenient, human-readable form
that corresponds to the conventions of the host environment’s current locale.

NOTE The first parameter to this function is likely to be used in a future version of this standard;
it is recommended that implementations do not use this parameter position for anything else.

15.9.5.6 Date.prototype.toLocaleDateString ( )
==============================================

This function returns a String value. The contents of the String are implementation-dependent, but
are intended to represent the “date” portion of the Date in the current time zone in a convenient,
human-readable form that corresponds to the conventions of the host environment’s current locale.

NOTE The first parameter to this function is likely to be used in a future version of this standard;
it is recommended that implementations do not use this parameter position for anything else.

15.9.5.7 Date.prototype.toLocaleTimeString ( )
==============================================

This function returns a String value. The contents of the String are implementation-dependent, but
are intended to represent the “time” portion of the Date in the current time zone in a convenient,
human-readable form that corresponds to the conventions of the host environment’s current locale.

NOTE The first parameter to this function is likely to be used in a future version of this standard;
it is recommended that implementations do not use this parameter position for anything else.

15.9.5.8 Date.prototype.valueOf ( )
===================================

The valueOf function returns a Number, which is this time value.

15.9.5.9 Date.prototype.getTime ( )
===================================

1.  Return this time value.

15.9.5.10 Date.prototype.getFullYear ( )
========================================

1.  Let t be this time value.
2.  If t is NaN, return NaN.
3.  Return YearFromTime(LocalTime(t)).

15.9.5.11 Date.prototype.getUTCFullYear ( )
===========================================

1.  Let t be this time value.
2.  If t is NaN, return NaN.
3.  Return YearFromTime(t).

15.9.5.12 Date.prototype.getMonth ( )
=====================================

1.  Let t be this time value.
2.  If t is NaN, return NaN.
3.  Return MonthFromTime(LocalTime(t)).

15.9.5.13 Date.prototype.getUTCMonth ( )
========================================

1.  Let t be this time value.
2.  If t is NaN, return NaN.
3.  Return MonthFromTime(t).

15.9.5.14 Date.prototype.getDate ( )
====================================

1.  Let t be this time value.
2.  If t is NaN, return NaN.
3.  Return DateFromTime(LocalTime(t)).

15.9.5.15 Date.prototype.getUTCDate ( )
=======================================

1.  Let t be this time value.
2.  If t is NaN, return NaN.
3.  Return DateFromTime(t).

15.9.5.16 Date.prototype.getDay ( )
===================================

1.  Let t be this time value.
2.  If t is NaN, return NaN.
3.  Return WeekDay(LocalTime(t)).

15.9.5.17 Date.prototype.getUTCDay ( )
======================================

1.  Let t be this time value.
2.  If t is NaN, return NaN.
3.  Return WeekDay(t).

15.9.5.18 Date.prototype.getHours ( )
=====================================

1.  Let t be this time value.
2.  If t is NaN, return NaN.
3.  Return HourFromTime(LocalTime(t)).

15.9.5.19 Date.prototype.getUTCHours ( )
========================================

1.  Let t be this time value.
2.  If t is NaN, return NaN.
3.  Return HourFromTime(t).

15.9.5.20 Date.prototype.getMinutes ( )
=======================================

1.  Let t be this time value.
2.  If t is NaN, return NaN.
3.  Return MinFromTime(LocalTime(t)).

15.9.5.21 Date.prototype.getUTCMinutes ( )
==========================================

1.  Let t be this time value.
2.  If t is NaN, return NaN.
3.  Return MinFromTime(t).

15.9.5.22 Date.prototype.getSeconds ( )
=======================================

1.  Let t be this time value.
2.  If t is NaN, return NaN.
3.  Return SecFromTime(LocalTime(t)).

15.9.5.23 Date.prototype.getUTCSeconds ( )
==========================================

1.  Let t be this time value.
2.  If t is NaN, return NaN.
3.  Return SecFromTime(t).

15.9.5.24 Date.prototype.getMilliseconds ( )
============================================

1.  Let t be this time value.
2.  If t is NaN, return NaN.
3.  Return msFromTime(LocalTime(t)).

15.9.5.25 Date.prototype.getUTCMilliseconds ( )
===============================================

1.  Let t be this time value.
2.  If t is NaN, return NaN.
3.  Return msFromTime(t).

15.9.5.26 Date.prototype.getTimezoneOffset ( )
==============================================

Returns the difference between local time and UTC time in minutes.

1.  Let t be this time value.
2.  If t is NaN, return NaN.
3.  Return (t − LocalTime(t)) / msPerMinute.

15.9.5.27 Date.prototype.setTime (time)
=======================================

1.  Let v be TimeClip(ToNumber(time)).
2.  Set the [[PrimitiveValue]] internal property of this Date object to v.
3.  Return v.

15.9.5.28 Date.prototype.setMilliseconds (ms)
=============================================

1.  Let t be the result of LocalTime(this time value).
2.  Let time be MakeTime(HourFromTime(t), MinFromTime(t), SecFromTime(t), ToNumber(ms)).
3.  Let u be TimeClip(UTC(MakeDate(Day(t), time))).
4.  Set the [[PrimitiveValue]] internal property of this Date object to u.
5.  Return u.

15.9.5.29 Date.prototype.setUTCMilliseconds (ms)
================================================

1.  Let t be this time value.
2.  Let time be MakeTime(HourFromTime(t), MinFromTime(t), SecFromTime(t), ToNumber(ms)).
3.  Let v be TimeClip(MakeDate(Day(t), time)).
4.  Set the [[PrimitiveValue]] internal property of this Date object to v.
5.  Return v.

15.9.5.30 Date.prototype.setSeconds (sec [, ms ] )
==================================================

If ms is not specified, this behaves as if ms were specified with the value getMilliseconds().

1.  Let t be the result of LocalTime(this time value).
2.  Let s be ToNumber(sec).
3.  If ms is not specified, then let milli be msFromTime(t); otherwise, let milli be ToNumber(ms).
4.  Let date be MakeDate(Day(t), MakeTime(HourFromTime(t), MinFromTime(t), s, milli)).
5.  Let u be TimeClip(UTC(date)).
6.  Set the [[PrimitiveValue]] internal property of this Date object to u.
7.  Return u.

The length property of the setSeconds method is 2.

15.9.5.31 Date.prototype.setUTCSeconds (sec [, ms ] )
=====================================================

If ms is not specified, this behaves as if ms were specified with the value getUTCMilliseconds().

1.  Let t be this time value.
2.  Let s be ToNumber(sec).
3.  If ms is not specified, then let milli be msFromTime(t); otherwise, let milli be ToNumber(ms).
4.  Let date be MakeDate(Day(t), MakeTime(HourFromTime(t), MinFromTime(t), s, milli)).
5.  Let v be TimeClip(date).
6.  Set the [[PrimitiveValue]] internal property of this Date object to v.
7.  Return v.

The length property of the setUTCSeconds method is 2.

15.9.5.32 Date.prototype.setMinutes (min [, sec [, ms ] ] )
===========================================================

If sec is not specified, this behaves as if sec were specified with the value getSeconds().

If ms is not specified, this behaves as if ms were specified with the value getMilliseconds().

1.  Let t be the result of LocalTime(this time value).
2.  Let m be ToNumber(min).
3.  If sec is not specified, then let s be SecFromTime(t); otherwise, let s be ToNumber(sec).
4.  If ms is not specified, then let milli be msFromTime(t); otherwise, let milli be ToNumber(ms).
5.  Let date be MakeDate(Day(t), MakeTime(HourFromTime(t), m, s, milli)).
6.  Let u be TimeClip(UTC(date)).
7.  Set the [[PrimitiveValue]] internal property of this Date object to u.
8.  Return u.

The length property of the setMinutes method is 3.

15.9.5.33 Date.prototype.setUTCMinutes (min [, sec [, ms ] ] )
==============================================================

If sec is not specified, this behaves as if sec were specified with the value getUTCSeconds().

If ms is not specified, this function behaves as if ms were specified with the value return by
getUTCMilliseconds().

1.  Let t be this time value.
2.  Let m be ToNumber(min).
3.  If sec is not specified, then let s be SecFromTime(t); otherwise, let s be ToNumber(sec).
4.  If ms is not specified, then let milli be msFromTime(t); otherwise, let milli be ToNumber(ms).
5.  Let date be MakeDate(Day(t), MakeTime(HourFromTime(t), m, s, milli)).
6.  Let v be TimeClip(date).
7.  Set the [[PrimitiveValue]] internal property of this Date object to v.
8.  Return v.

The length property of the setUTCMinutes method is 3.

15.9.5.34 Date.prototype.setHours (hour [, min [, sec [, ms ] ] ] )
===================================================================

If min is not specified, this behaves as if min were specified with the value getMinutes().

If sec is not specified, this behaves as if sec were specified with the value getSeconds().

If ms is not specified, this behaves as if ms were specified with the value getMilliseconds().

1.  Let t be the result of LocalTime(this time value).
2.  Let h be ToNumber(hour).
3.  If min is not specified, then let m be MinFromTime(t); otherwise, let m be ToNumber(min).
4.  If If sec is not specified, then let s be SecFromTime(t); otherwise, let s be ToNumber(sec).
5.  If ms is not specified, then let milli be msFromTime(t); otherwise, let milli be ToNumber(ms).
6.  Let date be MakeDate(Day(t), MakeTime(h, m, s, milli)).
7.  Let u be TimeClip(UTC(date)).
8.  Set the [[PrimitiveValue]] internal property of this Date object to u.
9.  Return u.

The length property of the setHours method is 4.

15.9.5.35 Date.prototype.setUTCHours (hour [, min [, sec [, ms ] ] ] )
======================================================================

If min is not specified, this behaves as if min were specified with the value getUTCMinutes().

If sec is not specified, this behaves as if sec were specified with the value getUTCSeconds().

If ms is not specified, this behaves as if ms were specified with the value getUTCMilliseconds().

1.  Let t be this time value.
2.  Let h be ToNumber(hour).
3.  If min is not specified, then let m be MinFromTime(t); otherwise, let m be ToNumber(min).
4.  If sec is not specified, then let s be SecFromTime(t); otherwise, let s be ToNumber(sec).
5.  If ms is not specified, then let milli be msFromTime(t); otherwise, let milli be ToNumber(ms).
6.  Let newDate be MakeDate(Day(t), MakeTime(h, m, s, milli)).
7.  Let v be TimeClip(newDate).
8.  Set the [[PrimitiveValue]] internal property of this Date object to v.
9.  Return v.

The length property of the setUTCHours method is 4.

15.9.5.36 Date.prototype.setDate (date)
=======================================

1.  Let t be the result of LocalTime(this time value).
2.  Let dt be ToNumber(date).
3.  Let newDate be MakeDate(MakeDay(YearFromTime(t), MonthFromTime(t), dt), TimeWithinDay(t)).
4.  Let u be TimeClip(UTC(newDate)).
5.  Set the [[PrimitiveValue]] internal property of this Date object to u.
6.  Return u.

15.9.5.37 Date.prototype.setUTCDate (date)
==========================================

1.  Let t be this time value.
2.  Let dt be ToNumber(date).
3.  Let newDate be MakeDate(MakeDay(YearFromTime(t), MonthFromTime(t), dt), TimeWithinDay(t)).
4.  Let v be TimeClip(newDate).
5.  Set the [[PrimitiveValue]] internal property of this Date object to v.
6.  Return v.

15.9.5.38 Date.prototype.setMonth (month [, date ] )
====================================================

If date is not specified, this behaves as if date were specified with the value getDate().

1.  Let t be the result of LocalTime(this time value).
2.  Let m be ToNumber(month).
3.  If date is not specified, then let dt be DateFromTime(t); otherwise, let dt be ToNumber(date).
4.  Let newDate be MakeDate(MakeDay(YearFromTime(t), m, dt), TimeWithinDay(t)).
5.  Let u be TimeClip(UTC(newDate)).
6.  Set the [[PrimitiveValue]] internal property of this Date object to u.
7.  Return u.

The length property of the setMonth method is 2.

15.9.5.39 Date.prototype.setUTCMonth (month [, date ] )
=======================================================

If date is not specified, this behaves as if date were specified with the value getUTCDate().

1.  Let t be this time value.
2.  Let m be ToNumber(month).
3.  If date is not specified, then let dt be DateFromTime(t); otherwise, let dt be ToNumber(date).
4.  Let newDate be MakeDate(MakeDay(YearFromTime(t), m, dt), TimeWithinDay(t)).
5.  Let v be TimeClip(newDate).
6.  Set the [[PrimitiveValue]] internal property of this Date object to v.
7.  Return v.

The length property of the setUTCMonth method is 2.

15.9.5.40 Date.prototype.setFullYear (year [, month [, date ] ] )
=================================================================

If month is not specified, this behaves as if month were specified with the value getMonth().

If date is not specified, this behaves as if date were specified with the value getDate().

1.  Let t be the result of LocalTime(this time value); but if this time value is NaN, let t be +0.
2.  Let y be ToNumber(year).
3.  If month is not specified, then let m be MonthFromTime(t); otherwise, let m be ToNumber(month).
4.  If date is not specified, then let dt be DateFromTime(t); otherwise, let dt be ToNumber(date).
5.  Let newDate be MakeDate(MakeDay(y, m, dt), TimeWithinDay(t)).
6.  Let u be TimeClip(UTC(newDate)).
7.  Set the [[PrimitiveValue]] internal property of this Date object to u.
8.  Return u.

The length property of the setFullYear method is 3.

15.9.5.41 Date.prototype.setUTCFullYear (year [, month [, date ] ] )
====================================================================

If month is not specified, this behaves as if month were specified with the value getUTCMonth().

If date is not specified, this behaves as if date were specified with the value getUTCDate().

1.  Let t be this time value; but if this time value is NaN, let t be +0.
2.  Let y be ToNumber(year).
3.  If month is not specified, then let m be MonthFromTime(t); otherwise, let m be ToNumber(month).
4.  If date is not specified, then let dt be DateFromTime(t); otherwise, let dt be ToNumber(date).
5.  Let newDate be MakeDate(MakeDay(y, m, dt), TimeWithinDay(t)).
6.  Let v be TimeClip(newDate).
7.  Set the [[PrimitiveValue]] internal property of this Date object to v.
8.  Return v.

The length property of the setUTCFullYear method is 3.

15.9.5.42 Date.prototype.toUTCString ( )
========================================

This function returns a String value. The contents of the String are implementation-dependent, but
are intended to represent the Date in a convenient, human-readable form in UTC.

NOTE The intent is to produce a String representation of a date that is more readable than the
format specified in 15.9.1.15. It is not essential that the chosen format be unambiguous or easily
machine parsable. If an implementation does not have a preferred human-readable format it is
recommended to use the format defined in 15.9.1.15 but with a space rather than a “T” used to
separate the date and time elements.

15.9.5.43 Date.prototype.toISOString ( )
========================================

This function returns a String value represent the instance in time represented by this Date object.
The format of the String is the Date Time string format defined in 15.9.1.15. All fields are present
in the String. The time zone is always UTC, denoted by the suffix Z. If the time value of this
object is not a finite Number a RangeError exception is thrown.

15.9.5.44 Date.prototype.toJSON ( key )
=======================================

This function provides a String representation of a Date object for use by JSON.stringify (15.12.3).

When the toJSON method is called with argument key, the following steps are taken:

1.  Let O be the result of calling ToObject, giving it the this value as its argument.
2.  Let tv be ToPrimitive(O, hint Number).
3.  If tv is a Number and is not finite, return null.
4.  Let toISO be the result of calling the [[Get]] internal method of O with argument "toISOString".
5.  If IsCallable(toISO) is false, throw a TypeError exception.
6.  Return the result of calling the [[Call]] internal method of toISO with O as the this value and
    an empty argument list.

NOTE 1 The argument is ignored.

NOTE 2 The toJSON function is intentionally generic; it does not require that its this value be a
Date object. Therefore, it can be transferred to other kinds of objects for use as a method.
However, it does require that any such object have a toISOString method. An object is free to use
the argument key to filter its stringification.

15.9.6 Properties of Date Instances
===================================

Date instances inherit properties from the Date prototype object and their [[Class]] internal
property value is "Date". Date instances also have a [[PrimitiveValue]] internal property.

The [[PrimitiveValue]] internal property is time value represented by this Date object.

15.10 RegExp (Regular Expression) Objects
=========================================

A RegExp object contains a regular expression and the associated flags.

NOTE The form and functionality of regular expressions is modelled after the regular expression
facility in the Perl 5 programming language.

15.10.1 Patterns
================

The RegExp constructor applies the following grammar to the input pattern String. An error occurs if
the grammar cannot interpret the String as an expansion of Pattern.

Syntax
------

Pattern ::

Disjunction

Disjunction ::

Alternative

Alternative | Disjunction

Alternative ::

[empty]

Alternative Term

Term ::

Assertion

Atom

Atom Quantifier

Assertion ::

^

$

\ b

\ B

( ? = Disjunction )

( ? ! Disjunction )

Quantifier ::

QuantifierPrefix

QuantifierPrefix ?

QuantifierPrefix ::

*

+

?

{ DecimalDigits }

{ DecimalDigits , }

{ DecimalDigits , DecimalDigits }

Atom ::

PatternCharacter

.

\ AtomEscape

CharacterClass

( Disjunction )

( ? : Disjunction )

PatternCharacter ::

SourceCharacter but not one of

^ $ \ . * + ? ( ) [ ] { } |

AtomEscape ::

DecimalEscape

CharacterEscape

CharacterClassEscape

CharacterEscape ::

ControlEscape

c ControlLetter

HexEscapeSequence

UnicodeEscapeSequence

IdentityEscape

ControlEscape :: one of

f n r t v

ControlLetter :: one of

a b c d e f g h i j k l m n o p q r s t u v w x y z

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

IdentityEscape ::

SourceCharacter but not IdentifierPart

<ZWJ>

<ZWNJ>

DecimalEscape ::

DecimalIntegerLiteral [lookahead ∉ DecimalDigit]

CharacterClassEscape :: one of

d D s S w W

CharacterClass ::

[ [lookahead ∉ {^}] ClassRanges ]

[ ^ ClassRanges ]

ClassRanges ::

[empty]

NonemptyClassRanges

NonemptyClassRanges ::

ClassAtom

ClassAtom NonemptyClassRangesNoDash

ClassAtom - ClassAtom ClassRanges

NonemptyClassRangesNoDash ::

ClassAtom

ClassAtomNoDash NonemptyClassRangesNoDash

ClassAtomNoDash - ClassAtom ClassRanges

ClassAtom ::

-

ClassAtomNoDash

ClassAtomNoDash ::

SourceCharacter but not one of \ or ] or -

\ ClassEscape

ClassEscape ::

DecimalEscape

b

CharacterEscape

CharacterClassEscape

15.10.2 Pattern Semantics
=========================

A regular expression pattern is converted into an internal procedure using the process described
below. An implementation is encouraged to use more efficient algorithms than the ones listed below,
as long as the results are the same. The internal procedure is used as the value of a RegExp
object’s [[Match]] internal property.

15.10.2.1 Notation
==================

The descriptions below use the following variables:

-   Input is the String being matched by the regular expression pattern. The notation input[n] means
    the nth character of input, where n can range between 0 (inclusive) and InputLength (exclusive).

-   InputLength is the number of characters in the Input String.

-   NcapturingParens is the total number of left capturing parentheses (i.e. the total number of
    times the Atom :: ( Disjunction ) production is expanded) in the pattern. A left capturing
    parenthesis is any ( pattern character that is matched by the ( terminal of the Atom :: (
    Disjunction ) production.

-   IgnoreCase is the setting of the RegExp object’s ignoreCase property.

-   Multiline is the setting of the RegExp object’s multiline property.

Furthermore, the descriptions below use the following internal data structures:

-   A CharSet is a mathematical set of characters.

-   A State is an ordered pair (endIndex, captures) where endIndex is an integer and captures is an
    internal array of NcapturingParens values. States are used to represent partial match states in
    the regular expression matching algorithms. The endIndex is one plus the index of the last input
    character matched so far by the pattern, while captures holds the results of capturing
    parentheses. The nth element of captures is either a String that represents the value obtained
    by the nth set of capturing parentheses or undefined if the nth set of capturing parentheses
    hasn’t been reached yet. Due to backtracking, many States may be in use at any time during the
    matching process.

-   A MatchResult is either a State or the special token failure that indicates that the match
    failed.

-   A Continuation procedure is an internal closure (i.e. an internal procedure with some arguments
    already bound to values) that takes one State argument and returns a MatchResult result. If an
    internal closure references variables bound in the function that creates the closure, the
    closure uses the values that these variables had at the time the closure was created. The
    Continuation attempts to match the remaining portion (specified by the closure’s already-bound
    arguments) of the pattern against the input String, starting at the intermediate state given by
    its State argument. If the match succeeds, the Continuation returns the final State that it
    reached; if the match fails, the Continuation returns failure.

-   A Matcher procedure is an internal closure that takes two arguments – a State and a Continuation
    – and returns a MatchResult result. A Matcher attempts to match a middle subpattern (specified
    by the closure’s already-bound arguments) of the pattern against the input String, starting at
    the intermediate state given by its State argument. The Continuation argument should be a
    closure that matches the rest of the pattern. After matching the subpattern of a pattern to
    obtain a new State, the Matcher then calls Continuation on that new State to test if the rest of
    the pattern can match as well. If it can, the Matcher returns the State returned by
    Continuation; if not, the Matcher may try different choices at its choice points, repeatedly
    calling Continuation until it either succeeds or all possibilities have been exhausted.

-   An AssertionTester procedure is an internal closure that takes a State argument and returns a
    Boolean result. The assertion tester tests a specific condition (specified by the closure’s
    already-bound arguments) against the current place in the input String and returns true if the
    condition matched or false if not.

-   An EscapeValue is either a character or an integer. An EscapeValue is used to denote the
    interpretation of a DecimalEscape escape sequence: a character ch means that the escape sequence
    is interpreted as the character ch, while an integer n means that the escape sequence is
    interpreted as a backreference to the nth set of capturing parentheses.

15.10.2.2 Pattern
=================

The production Pattern :: Disjunction evaluates as follows:

1.  Evaluate Disjunction to obtain a Matcher m.
2.  Return an internal closure that takes two arguments, a String str and an integer index, and
    performs the following:
    1.  Let Input be the given String str. This variable will be used throughout the algorithms in
        15.10.2.
    2.  Let InputLength be the length of Input. This variable will be used throughout the algorithms
        in 15.10.2.
    3.  Let c be a Continuation that always returns its State argument as a successful MatchResult.
    4.  Let cap be an internal array of NcapturingParens undefined values, indexed 1 through
        NcapturingParens.
    5.  Let x be the State (index, cap).
    6.  Call m(x, c) and return its result.

NOTE A Pattern evaluates (“compiles”) to an internal procedure value. RegExp.prototype.exec can then
apply this procedure to a String and an offset within the String to determine whether the pattern
would match starting at exactly that offset within the String, and, if it does match, what the
values of the capturing parentheses would be. The algorithms in 15.10.2 are designed so that
compiling a pattern may throw a SyntaxError exception; on the other hand, once the pattern is
successfully compiled, applying its result internal procedure to find a match in a String cannot
throw an exception (except for any host-defined exceptions that can occur anywhere such as
out-of-memory).

15.10.2.3 Disjunction
=====================

The production Disjunction :: Alternative evaluates by evaluating Alternative to obtain a Matcher
and returning that Matcher.

The production Disjunction :: Alternative | Disjunction evaluates as follows:

1.  Evaluate Alternative to obtain a Matcher m1.
2.  Evaluate Disjunction to obtain a Matcher m2.
3.  Return an internal Matcher closure that takes two arguments, a State x and a Continuation c, and
    performs the following:
    1.  Call m1(x, c) and let r be its result.
    2.  If r isn’t failure, return r.
    3.  Call m2(x, c) and return its result.

NOTE The | regular expression operator separates two alternatives. The pattern first tries to match
the left Alternative (followed by the sequel of the regular expression); if it fails, it tries to
match the right Disjunction (followed by the sequel of the regular expression). If the left
Alternative, the right Disjunction, and the sequel all have choice points, all choices in the sequel
are tried before moving on to the next choice in the left Alternative. If choices in the left
Alternative are exhausted, the right Disjunction is tried instead of the left Alternative. Any
capturing parentheses inside a portion of the pattern skipped by | produce undefined values instead
of Strings. Thus, for example,

    /a|ab/.exec("abc")

returns the result "a" and not "ab". Moreover,

    /((a)|(ab))((c)|(bc))/.exec("abc")

returns the array

    ["abc", "a", "a", undefined, "bc", undefined, "bc"]

and not

    ["abc", "ab", undefined, "ab", "c", "c", undefined]

15.10.2.4 Alternative
=====================

The production Alternative :: [empty] evaluates by returning a Matcher that takes two arguments, a
State x and a Continuation c, and returns the result of calling c(x).

The production Alternative :: Alternative Term evaluates as follows:

1.  Evaluate Alternative to obtain a Matcher m1.
2.  Evaluate Term to obtain a Matcher m2.
3.  Return an internal Matcher closure that takes two arguments, a State x and a Continuation c, and
    performs the following:
    1.  Create a Continuation d that takes a State argument y and returns the result of calling
        m2(y, c).
    2.  Call m1(x, d) and return its result.

NOTE Consecutive Terms try to simultaneously match consecutive portions of the input String. If the
left Alternative, the right Term, and the sequel of the regular expression all have choice points,
all choices in the sequel are tried before moving on to the next choice in the right Term, and all
choices in the right Term are tried before moving on to the next choice in the left Alternative.

15.10.2.5 Term
==============

The production Term :: Assertion evaluates by returning an internal Matcher closure that takes two
arguments, a State x and a Continuation c, and performs the following:

1.  Evaluate Assertion to obtain an AssertionTester t.
2.  Call t(x) and let r be the resulting Boolean value.
3.  If r is false, return failure.
4.  Call c(x) and return its result.

The production Term :: Atom evaluates by evaluating Atom to obtain a Matcher and returning that
Matcher.

The production Term :: Atom Quantifier evaluates as follows:

1.  Evaluate Atom to obtain a Matcher m.
2.  Evaluate Quantifier to obtain the three results: an integer min, an integer (or ∞) max, and
    Boolean greedy.
3.  If max is finite and less than min, then throw a SyntaxError exception.
4.  Let parenIndex be the number of left capturing parentheses in the entire regular expression that
    occur to the left of this production expansion’s Term. This is the total number of times the
    Atom :: ( Disjunction ) production is expanded prior to this production’s Term plus the total
    number of Atom :: ( Disjunction ) productions enclosing this Term.
5.  Let parenCount be the number of left capturing parentheses in the expansion of this production’s
    Atom. This is the total number of Atom :: ( Disjunction ) productions enclosed by this
    production’s Atom.
6.  Return an internal Matcher closure that takes two arguments, a State x and a Continuation c, and
    performs the following:
    1.  Call RepeatMatcher(m, min, max, greedy, x, c, parenIndex, parenCount) and return its result.

The abstract operation RepeatMatcher takes eight parameters, a Matcher m, an integer min, an integer
(or ∞) max, a Boolean greedy, a State x, a Continuation c, an integer parenIndex, and an integer
parenCount, and performs the following:

1.  If max is zero, then call c(x) and return its result.
2.  Create an internal Continuation closure d that takes one State argument y and performs the
    following:
    1.  If min is zero and y’s endIndex is equal to x’s endIndex, then return failure.
    2.  If min is zero then let min2 be zero; otherwise let min2 be min–1.
    3.  If max is ∞, then let max2 be ∞; otherwise let max2 be max–1.
    4.  Call RepeatMatcher(m, min2, max2, greedy, y, c, parenIndex, parenCount) and return its
        result.

3.  Let cap be a fresh copy of x’s captures internal array.
4.  For every integer k that satisfies parenIndex < k and k ≤ parenIndex+parenCount, set cap[k] to
    undefined.
5.  Let e be x’s endIndex.
6.  Let xr be the State (e, cap).
7.  If min is not zero, then call m(xr, d) and return its result.
8.  If greedy is false, then
    1.  Call c(x) and let z be its result.
    2.  If z is not failure, return z.
    3.  Call m(xr, d) and return its result.

9.  Call m(xr, d) and let z be its result.
10. If z is not failure, return z.
11. Call c(x) and return its result.

NOTE 1 An Atom followed by a Quantifier is repeated the number of times specified by the Quantifier.
A Quantifier can be non-greedy, in which case the Atom pattern is repeated as few times as possible
while still matching the sequel, or it can be greedy, in which case the Atom pattern is repeated as
many times as possible while still matching the sequel. The Atom pattern is repeated rather than the
input String that it matches, so different repetitions of the Atom can match different input
substrings.

NOTE 2 If the Atom and the sequel of the regular expression all have choice points, the Atom is
first matched as many (or as few, if non-greedy) times as possible. All choices in the sequel are
tried before moving on to the next choice in the last repetition of Atom. All choices in the last
(nth) repetition of Atom are tried before moving on to the next choice in the next-to-last (n–1)st
repetition of Atom; at which point it may turn out that more or fewer repetitions of Atom are now
possible; these are exhausted (again, starting with either as few or as many as possible) before
moving on to the next choice in the (n-1)st repetition of Atom and so on.

Compare

    /a[a-z]{2,4}/.exec("abcdefghi")

which returns "abcde" with

    /a[a-z]{2,4}?/.exec("abcdefghi")

which returns "abc".

Consider also

    /(aa|aabaac|ba|b|c)*/.exec("aabaac")

which, by the choice point ordering above, returns the array

    ["aaba", "ba"]

and not any of:

    ["aabaac", "aabaac"]

    ["aabaac", "c"]

The above ordering of choice points can be used to write a regular expression that calculates the
greatest common divisor of two numbers (represented in unary notation). The following example
calculates the gcd of 10 and 15:

    "aaaaaaaaaa,aaaaaaaaaaaaaaa".replace(/^(a+)\1*,\1+$/,"$1")

which returns the gcd in unary notation "aaaaa".

NOTE 3 Step 4 of the RepeatMatcher clears Atom’s captures each time Atom is repeated. We can see its
behaviour in the regular expression

    /(z)((a+)?(b+)?(c))*/.exec("zaacbbbcac")

which returns the array

    ["zaacbbbcac", "z", "ac", "a", undefined, "c"]

and not

    ["zaacbbbcac", "z", "ac", "a", "bbb", "c"]

because each iteration of the outermost * clears all captured Strings contained in the quantified
Atom, which in this case includes capture Strings numbered 2, 3, 4, and 5.

NOTE 4 Step 1 of the RepeatMatcher’s d closure states that, once the minimum number of repetitions
has been satisfied, any more expansions of Atom that match the empty String are not considered for
further repetitions. This prevents the regular expression engine from falling into an infinite loop
on patterns such as:

    /(a*)*/.exec("b")

or the slightly more complicated:

    /(a*)b\1+/.exec("baaaac")

which returns the array

    ["b", ""]

15.10.2.6 Assertion
===================

The production Assertion :: ^ evaluates by returning an internal AssertionTester closure that takes
a State argument x and performs the following:

1.  Let e be x’s endIndex.
2.  If e is zero, return true.
3.  If Multiline is false, return false.
4.  If the character Input[e–1] is one of LineTerminator, return true.
5.  Return false.

The production Assertion :: $ evaluates by returning an internal AssertionTester closure that takes
a State argument x and performs the following:

1.  Let e be x’s endIndex.
2.  If e is equal to InputLength, return true.
3.  If multiline is false, return false.
4.  If the character Input[e] is one of LineTerminator, return true.
5.  Return false.

The production Assertion :: \ b evaluates by returning an internal AssertionTester closure that
takes a State argument x and performs the following:

1.  Let e be x’s endIndex.
2.  Call IsWordChar(e–1) and let a be the Boolean result.
3.  Call IsWordChar(e) and let b be the Boolean result.
4.  If a is true and b is false, return true.
5.  If a is false and b is true, return true.
6.  Return false.

The production Assertion :: \ B evaluates by returning an internal AssertionTester closure that
takes a State argument x and performs the following:

1.  Let e be x’s endIndex.
2.  Call IsWordChar(e–1) and let a be the Boolean result.
3.  Call IsWordChar(e) and let b be the Boolean result.
4.  If a is true and b is false, return false.
5.  If a is false and b is true, return false.
6.  Return true.

The production Assertion :: ( ? = Disjunction ) evaluates as follows:

1.  Evaluate Disjunction to obtain a Matcher m.
2.  Return an internal Matcher closure that takes two arguments, a State x and a Continuation c, and
    performs the following steps:
    1.  Let d be a Continuation that always returns its State argument as a successful MatchResult.
    2.  Call m(x, d) and let r be its result.
    3.  If r is failure, return failure.
    4.  Let y be r’s State.
    5.  Let cap be y’s captures internal array.
    6.  Let xe be x’s endIndex.
    7.  Let z be the State (xe, cap).
    8.  Call c(z) and return its result.

The production Assertion :: ( ? ! Disjunction ) evaluates as follows:

1.  Evaluate Disjunction to obtain a Matcher m.
2.  Return an internal Matcher closure that takes two arguments, a State x and a Continuation c, and
    performs the following steps:
    1.  Let d be a Continuation that always returns its State argument as a successful MatchResult.
    2.  Call m(x, d) and let r be its result.
    3.  If r isn’t failure, return failure.
    4.  Call c(x) and return its result.

The abstract operation IsWordChar takes an integer parameter e and performs the following:

1.  If e == –1 or e == InputLength, return false.
2.  Let c be the character Input[e].
3.  If c is one of the sixty-three characters below, return true.

  --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- ---
  a   b   c   d   e   f   g   h   i   j   k   l   m   n   o   p   q   r   s   t   u   v   w   x   y   z
  A   B   C   D   E   F   G   H   I   J   K   L   M   N   O   P   Q   R   S   T   U   V   W   X   Y   Z
  0   1   2   3   4   5   6   7   8   9   _                                                           
  --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- ---

1.  Return false.

15.10.2.7 Quantifier
====================

The production Quantifier :: QuantifierPrefix evaluates as follows:

1.  Evaluate QuantifierPrefix to obtain the two results: an integer min and an integer (or ∞) max.
2.  Return the three results min, max, and true.

The production Quantifier :: QuantifierPrefix ? evaluates as follows:

1.  Evaluate QuantifierPrefix to obtain the two results: an integer min and an integer (or ∞) max.
2.  Return the three results min, max, and false.

The production QuantifierPrefix :: * evaluates by returning the two results 0 and ∞.

The production QuantifierPrefix :: + evaluates by returning the two results 1 and ∞.

The production QuantifierPrefix :: ? evaluates by returning the two results 0 and 1.

The production QuantifierPrefix :: { DecimalDigits } evaluates as follows:

1.  Let i be the MV of DecimalDigits (see 7.8.3).
2.  Return the two results i and i.

The production QuantifierPrefix :: { DecimalDigits , } evaluates as follows:

1.  Let i be the MV of DecimalDigits.
2.  Return the two results i and ∞.

The production QuantifierPrefix :: { DecimalDigits , DecimalDigits } evaluates as follows:

1.  Let i be the MV of the first DecimalDigits.
2.  Let j be the MV of the second DecimalDigits.
3.  Return the two results i and j.

15.10.2.8 Atom
==============

The production Atom :: PatternCharacter evaluates as follows:

1.  Let ch be the character represented by PatternCharacter.
2.  Let A be a one-element CharSet containing the character ch.
3.  Call CharacterSetMatcher(A, false) and return its Matcher result.

The production Atom :: . evaluates as follows:

1.  Let A be the set of all characters except LineTerminator.
2.  Call CharacterSetMatcher(A, false) and return its Matcher result.

The production Atom :: \ AtomEscape evaluates by evaluating AtomEscape to obtain a Matcher and
returning that Matcher.

The production Atom :: CharacterClass evaluates as follows:

1.  Evaluate CharacterClass to obtain a CharSet A and a Boolean invert.
2.  Call CharacterSetMatcher(A, invert) and return its Matcher result.

The production Atom :: ( Disjunction ) evaluates as follows:

1.  Evaluate Disjunction to obtain a Matcher m.
2.  Let parenIndex be the number of left capturing parentheses in the entire regular expression that
    occur to the left of this production expansion’s initial left parenthesis. This is the total
    number of times the Atom :: ( Disjunction ) production is expanded prior to this production’s
    Atom plus the total number of Atom :: ( Disjunction ) productions enclosing this Atom.
3.  Return an internal Matcher closure that takes two arguments, a State x and a Continuation c, and
    performs the following steps:
    1.  Create an internal Continuation closure d that takes one State argument y and performs the
        following steps:
        1.  Let cap be a fresh copy of y’s captures internal array.
        2.  Let xe be x’s endIndex.
        3.  Let ye be y’s endIndex.
        4.  Let s be a fresh String whose characters are the characters of Input at positions xe
            (inclusive) through ye (exclusive).
        5.  Set cap[parenIndex+1] to s.
        6.  Let z be the State (ye, cap).
        7.  Call c(z) and return its result.

    2.  Call m(x, d) and return its result.

The production Atom :: ( ? : Disjunction ) evaluates by evaluating Disjunction to obtain a Matcher
and returning that Matcher.

The abstract operation CharacterSetMatcher takes two arguments, a CharSet A and a Boolean flag
invert, and performs the following:

1.  Return an internal Matcher closure that takes two arguments, a State x and a Continuation c, and
    performs the following steps:
    1.  Let e be x’s endIndex.
    2.  If e == InputLength, return failure.
    3.  Let ch be the character Input[e].
    4.  Let cc be the result of Canonicalize(ch).
    5.  If invert is false, then
        1.  If there does not exist a member a of set A such that Canonicalize(a) == cc, return
            failure.

    6.  Else invert is true,
        1.  If there exists a member a of set A such that Canonicalize(a) == cc, return failure.

    7.  Let cap be x’s captures internal array.
    8.  Let y be the State (e+1, cap).
    9.  Call c(y) and return its result.

The abstract operation Canonicalize takes a character parameter ch and performs the following steps:

1.  If IgnoreCase is false, return ch.
2.  Let u be ch converted to upper case as if by calling the standard built-in method
    String.prototype.toUpperCase on the one-character String ch.
3.  If u does not consist of a single character, return ch.
4.  Let cu be u’s character.
5.  If ch’s code unit value is greater than or equal to decimal 128 and cu’s code unit value is less
    than decimal 128, then return ch.
6.  Return cu.

NOTE 1 Parentheses of the form ( Disjunction ) serve both to group the components of the Disjunction
pattern together and to save the result of the match. The result can be used either in a
backreference (\ followed by a nonzero decimal number), referenced in a replace String, or returned
as part of an array from the regular expression matching internal procedure. To inhibit the
capturing behaviour of parentheses, use the form (?: Disjunction ) instead.

NOTE 2 The form (?= Disjunction ) specifies a zero-width positive lookahead. In order for it to
succeed, the pattern inside Disjunction must match at the current position, but the current position
is not advanced before matching the sequel. If Disjunction can match at the current position in
several ways, only the first one is tried. Unlike other regular expression operators, there is no
backtracking into a (?= form (this unusual behaviour is inherited from Perl). This only matters when
the Disjunction contains capturing parentheses and the sequel of the pattern contains backreferences
to those captures.

For example,

    /(?=(a+))/.exec("baaabac")

matches the empty String immediately after the first b and therefore returns the array:

    ["", "aaa"]

To illustrate the lack of backtracking into the lookahead, consider:

    /(?=(a+))a*b\1/.exec("baaabac")

This expression returns

    ["aba", "a"]

and not:

    ["aaaba", "a"]

NOTE 3 The form (?! Disjunction ) specifies a zero-width negative lookahead. In order for it to
succeed, the pattern inside Disjunction must fail to match at the current position. The current
position is not advanced before matching the sequel. Disjunction can contain capturing parentheses,
but backreferences to them only make sense from within Disjunction itself. Backreferences to these
capturing parentheses from elsewhere in the pattern always return undefined because the negative
lookahead must fail for the pattern to succeed. For example,

    /(.*?)a(?!(a+)b\2c)\2(.*)/.exec("baaabaac")

looks for an a not immediately followed by some positive number n of a’s, a b, another n a’s
(specified by the first \2) and a c. The second \2 is outside the negative lookahead, so it matches
against undefined and therefore always succeeds. The whole expression returns the array:

    ["baaabaac", "ba", undefined, "abaac"]

In case-insignificant matches all characters are implicitly converted to upper case immediately
before they are compared. However, if converting a character to upper case would expand that
character into more than one character (such as converting "ß" (\u00DF) into "SS"), then the
character is left as-is instead. The character is also left as-is if it is not an ASCII character
but converting it to upper case would make it into an ASCII character. This prevents Unicode
characters such as \u0131 and \u017F from matching regular expressions such as /[a‑z]/i, which are
only intended to match ASCII letters. Furthermore, if these conversions were allowed, then /[^\W]/i
would match each of a, b, …, h, but not i or s.

15.10.2.9 AtomEscape
====================

The production AtomEscape :: DecimalEscape evaluates as follows:

1.  Evaluate DecimalEscape to obtain an EscapeValue E.
2.  If E is a character, then
    1.  Let ch be E’s character.
    2.  Let A be a one-element CharSet containing the character ch.
    3.  Call CharacterSetMatcher(A, false) and return its Matcher result.

3.  E must be an integer. Let n be that integer.
4.  If n=0 or n>NCapturingParens then throw a SyntaxError exception.
5.  Return an internal Matcher closure that takes two arguments, a State x and a Continuation c, and
    performs the following:
    1.  Let cap be x’s captures internal array.
    2.  Let s be cap[n].
    3.  If s is undefined, then call c(x) and return its result.
    4.  Let e be x’s endIndex.
    5.  Let len be s’s length.
    6.  Let f be e+len.
    7.  If f>InputLength, return failure.
    8.  If there exists an integer i between 0 (inclusive) and len (exclusive) such that
        Canonicalize(s[i]) is not the same character as Canonicalize(Input [e+i]), then return
        failure.
    9.  Let y be the State (f, cap).
    10. Call c(y) and return its result.

The production AtomEscape :: CharacterEscape evaluates as follows:

1.  Evaluate CharacterEscape to obtain a character ch.
2.  Let A be a one-element CharSet containing the character ch.
3.  Call CharacterSetMatcher(A, false) and return its Matcher result.

The production AtomEscape :: CharacterClassEscape evaluates as follows:

1.  Evaluate CharacterClassEscape to obtain a CharSet A.
2.  Call CharacterSetMatcher(A, false) and return its Matcher result.

NOTE An escape sequence of the form \ followed by a nonzero decimal number n matches the result of
the nth set of capturing parentheses (see 15.10.2.11). It is an error if the regular expression has
fewer than n capturing parentheses. If the regular expression has n or more capturing parentheses
but the nth one is undefined because it has not captured anything, then the backreference always
succeeds.

15.10.2.10 CharacterEscape
==========================

The production CharacterEscape :: ControlEscape evaluates by returning the character according to
Table 23.

Table 23 — ControlEscape Character Values

  ControlEscape   Code Unit   Name                   Symbol
  --------------- ----------- ---------------------- --------
  t               \u0009      horizontal tab         <HT>
  n               \u000A      line feed (new line)   <LF>
  v               \u000B      vertical tab           <VT>
  f               \u000C      form feed              <FF>
  r               \u000D      carriage return        <CR>

The production CharacterEscape :: c ControlLetter evaluates as follows:

1.  Let ch be the character represented by ControlLetter.
2.  Let i be ch’s code unit value.
3.  Let j be the remainder of dividing i by 32.
4.  Return the character whose code unit value is j.

The production CharacterEscape :: HexEscapeSequence evaluates by evaluating the CV of the
HexEscapeSequence (see 7.8.4) and returning its character result.

The production CharacterEscape :: UnicodeEscapeSequence evaluates by evaluating the CV of the
UnicodeEscapeSequence (see 7.8.4) and returning its character result.

The production CharacterEscape :: IdentityEscape evaluates by returning the character represented by
IdentityEscape.

15.10.2.11 DecimalEscape
========================

The production DecimalEscape :: DecimalIntegerLiteral [lookahead ∉ DecimalDigit] evaluates as
follows:

1.  Let i be the MV of DecimalIntegerLiteral.
2.  If i is zero, return the EscapeValue consisting of a <NUL> character (Unicode value 0000).
3.  Return the EscapeValue consisting of the integer i.

The definition of “the MV of DecimalIntegerLiteral” is in 7.8.3.

NOTE If \ is followed by a decimal number n whose first digit is not 0, then the escape sequence is
considered to be a backreference. It is an error if n is greater than the total number of left
capturing parentheses in the entire regular expression. \0 represents the <NUL> character and cannot
be followed by a decimal digit.

15.10.2.12 CharacterClassEscape
===============================

The production CharacterClassEscape :: d evaluates by returning the ten-element set of characters
containing the characters 0 through 9 inclusive.

The production CharacterClassEscape :: D evaluates by returning the set of all characters not
included in the set returned by CharacterClassEscape :: d .

The production CharacterClassEscape :: s evaluates by returning the set of characters containing the
characters that are on the right-hand side of the WhiteSpace (7.2) or LineTerminator (7.3)
productions.

The production CharacterClassEscape :: S evaluates by returning the set of all characters not
included in the set returned by CharacterClassEscape :: s .

The production CharacterClassEscape :: w evaluates by returning the set of characters containing the
sixty-three characters:

  --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- ---
  a   b   c   d   e   f   g   h   i   j   k   l   m   n   o   p   q   r   s   t   u   v   w   x   y   z
  A   B   C   D   E   F   G   H   I   J   K   L   M   N   O   P   Q   R   S   T   U   V   W   X   Y   Z
  0   1   2   3   4   5   6   7   8   9   _                                                           
  --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- ---

The production CharacterClassEscape :: W evaluates by returning the set of all characters not
included in the set returned by CharacterClassEscape :: w .

15.10.2.13 CharacterClass
=========================

The production CharacterClass :: [ [lookahead ∉ {^}] ClassRanges ] evaluates by evaluating
ClassRanges to obtain a CharSet and returning that CharSet and the Boolean false.

The production CharacterClass :: [ ^ ClassRanges ] evaluates by evaluating ClassRanges to obtain a
CharSet and returning that CharSet and the Boolean true.

15.10.2.14 ClassRanges
======================

The production ClassRanges :: [empty] evaluates by returning the empty CharSet.

The production ClassRanges :: NonemptyClassRanges evaluates by evaluating NonemptyClassRanges to
obtain a CharSet and returning that CharSet.

15.10.2.15 NonemptyClassRanges
==============================

The production NonemptyClassRanges :: ClassAtom evaluates by evaluating ClassAtom to obtain a
CharSet and returning that CharSet.

The production NonemptyClassRanges :: ClassAtom NonemptyClassRangesNoDash evaluates as follows:

1.  Evaluate ClassAtom to obtain a CharSet A.
2.  Evaluate NonemptyClassRangesNoDash to obtain a CharSet B.
3.  Return the union of CharSets A and B.

The production NonemptyClassRanges :: ClassAtom - ClassAtom ClassRanges evaluates as follows:

1.  Evaluate the first ClassAtom to obtain a CharSet A.
2.  Evaluate the second ClassAtom to obtain a CharSet B.
3.  Evaluate ClassRanges to obtain a CharSet C.
4.  Call CharacterRange(A, B) and let D be the resulting CharSet.
5.  Return the union of CharSets D and C.

The abstract operation CharacterRange takes two CharSet parameters A and B and performs the
following:

1.  If A does not contain exactly one character or B does not contain exactly one character then
    throw a SyntaxError exception.
2.  Let a be the one character in CharSet A.
3.  Let b be the one character in CharSet B.
4.  Let i be the code unit value of character a.
5.  Let j be the code unit value of character b.
6.  If i > j then throw a SyntaxError exception.
7.  Return the set containing all characters numbered i through j, inclusive.

15.10.2.16 NonemptyClassRangesNoDash
====================================

The production NonemptyClassRangesNoDash :: ClassAtom evaluates by evaluating ClassAtom to obtain a
CharSet and returning that CharSet.

The production NonemptyClassRangesNoDash :: ClassAtomNoDash NonemptyClassRangesNoDash evaluates as
follows:

1.  Evaluate ClassAtomNoDash to obtain a CharSet A.
2.  Evaluate NonemptyClassRangesNoDash to obtain a CharSet B.
3.  Return the union of CharSets A and B.

The production NonemptyClassRangesNoDash :: ClassAtomNoDash - ClassAtom ClassRanges evaluates as
follows:

1.  Evaluate ClassAtomNoDash to obtain a CharSet A.
2.  Evaluate ClassAtom to obtain a CharSet B.
3.  Evaluate ClassRanges to obtain a CharSet C.
4.  Call CharacterRange(A, B) and let D be the resulting CharSet.
5.  Return the union of CharSets D and C.

NOTE 1 ClassRanges can expand into single ClassAtoms and/or ranges of two ClassAtoms separated by
dashes. In the latter case the ClassRanges includes all characters between the first ClassAtom and
the second ClassAtom, inclusive; an error occurs if either ClassAtom does not represent a single
character (for example, if one is \w) or if the first ClassAtom’s code unit value is greater than
the second ClassAtom’s code unit value.

NOTE 2 Even if the pattern ignores case, the case of the two ends of a range is significant in
determining which characters belong to the range. Thus, for example, the pattern /[E-F]/i matches
only the letters E, F, e, and f, while the pattern /[E-f]/i matches all upper and lower-case ASCII
letters as well as the symbols [, \, ], ^, _, and `.

NOTE 3 A - character can be treated literally or it can denote a range. It is treated literally if
it is the first or last character of ClassRanges, the beginning or end limit of a range
specification, or immediately follows a range specification.

15.10.2.17 ClassAtom
====================

The production ClassAtom :: - evaluates by returning the CharSet containing the one character -.

The production ClassAtom :: ClassAtomNoDash evaluates by evaluating ClassAtomNoDash to obtain a
CharSet and returning that CharSet.

15.10.2.18 ClassAtomNoDash
==========================

The production ClassAtomNoDash :: SourceCharacter but not one of \ or ] or - evaluates by returning
a one-element CharSet containing the character represented by SourceCharacter.

The production ClassAtomNoDash :: \ ClassEscape evaluates by evaluating ClassEscape to obtain a
CharSet and returning that CharSet.

15.10.2.19 ClassEscape
======================

The production ClassEscape :: DecimalEscape evaluates as follows:

1.  Evaluate DecimalEscape to obtain an EscapeValue E.
2.  If E is not a character then throw a SyntaxError exception.
3.  Let ch be E’s character.
4.  Return the one-element CharSet containing the character ch.

The production ClassEscape :: b evaluates by returning the CharSet containing the one character <BS>
(Unicode value 0008).

The production ClassEscape :: CharacterEscape evaluates by evaluating CharacterEscape to obtain a
character and returning a one-element CharSet containing that character.

The production ClassEscape :: CharacterClassEscape evaluates by evaluating CharacterClassEscape to
obtain a CharSet and returning that CharSet.

NOTE A ClassAtom can use any of the escape sequences that are allowed in the rest of the regular
expression except for \b, \B, and backreferences. Inside a CharacterClass, \b means the backspace
character, while \B and backreferences raise errors. Using a backreference inside a ClassAtom causes
an error.

15.10.3 The RegExp Constructor Called as a Function
===================================================

15.10.3.1 RegExp(pattern, flags)
================================

If pattern is an object R whose [[Class]] internal property is "RegExp" and flags is undefined, then
return R unchanged. Otherwise call the standard built-in RegExp constructor (15.10.4.1) as if by the
expression new         RegExp(pattern, flags) and return the object constructed by that constructor.

15.10.4 The RegExp Constructor
==============================

When RegExp is called as part of a new expression, it is a constructor: it initialises the newly
created object.

15.10.4.1 new RegExp(pattern, flags)
====================================

If pattern is an object R whose [[Class]] internal property is "RegExp" and flags is undefined, then
let P be the pattern used to construct R and let F be the flags used to construct R. If pattern is
an object R whose [[Class]] internal property is "RegExp" and flags is not undefined, then throw a
TypeError exception. Otherwise, let P be the empty String if pattern is undefined and
ToString(pattern) otherwise, and let F be the empty String if flags is undefined and ToString(flags)
otherwise.

If the characters of P do not have the syntactic form Pattern, then throw a SyntaxError exception.
Otherwise let the newly constructed object have a [[Match]] internal property obtained by evaluating
(“compiling”) the characters of P as a Pattern as described in 15.10.2.

If F contains any character other than "g", "i", or "m", or if it contains the same character more
than once, then throw a SyntaxError exception.

If a SyntaxError exception is not thrown, then:

Let S be a String in the form of a Pattern equivalent to P, in which certain characters are escaped
as described below. S may or may not be identical to P or pattern; however, the internal procedure
that would result from evaluating S as a Pattern must behave identically to the internal procedure
given by the constructed object’s [[Match]] internal property.

The characters / occurring in the pattern shall be escaped in S as necessary to ensure that the
String value formed by concatenating the Strings "/", S, "/", and F can be parsed (in an appropriate
lexical context) as a RegularExpressionLiteral that behaves identically to the constructed regular
expression. For example, if P is "/", then S could be "\/" or "\u002F", among other possibilities,
but not "/", because /// followed by F would be parsed as a SingleLineComment rather than a
RegularExpressionLiteral. If P is the empty String, this specification can be met by letting S be
"(?:)".

The following properties of the newly constructed object are data properties with the attributes
that are specified in 15.10.7. The [[Value]] of each property is set as follows:

The source property of the newly constructed object is set to S.

The global property of the newly constructed object is set to a Boolean value that is true if F
contains the character "g" and false otherwise.

The ignoreCase property of the newly constructed object is set to a Boolean value that is true if F
contains the character "i" and false otherwise.

The multiline property of the newly constructed object is set to a Boolean value that is true if F
contains the character "m" and false otherwise.

The lastIndex property of the newly constructed object is set to 0.

The [[Prototype]] internal property of the newly constructed object is set to the standard built-in
RegExp prototype object as specified in 15.10.6.

The [[Class]] internal property of the newly constructed object is set to "RegExp".

NOTE If pattern is a StringLiteral, the usual escape sequence substitutions are performed before the
String is processed by RegExp. If pattern must contain an escape sequence to be recognised by
RegExp, any backslash \ characters must be escaped within the StringLiteral to prevent them being
removed when the contents of the StringLiteral are formed.

15.10.5 Properties of the RegExp Constructor
============================================

The value of the [[Prototype]] internal property of the RegExp constructor is the standard built-in
Function prototype object (15.3.4).

Besides the internal properties and the length property (whose value is 2), the RegExp constructor
has the following properties:

15.10.5.1 RegExp.prototype
==========================

The initial value of RegExp.prototype is the RegExp prototype object (15.10.6).

This property shall have the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.

15.10.6 Properties of the RegExp Prototype Object
=================================================

The value of the [[Prototype]] internal property of the RegExp prototype object is the standard
built-in Object prototype object (15.2.4). The RegExp prototype object is itself a regular
expression object; its [[Class]] is "RegExp". The initial values of the RegExp prototype object’s
data properties (15.10.7) are set as if the object was created by the expression new       RegExp()
where RegExp is that standard built-in constructor with that name.

The RegExp prototype object does not have a valueOf property of its own; however, it inherits the
valueOf property from the Object prototype object.

In the following descriptions of functions that are properties of the RegExp prototype object, the
phrase “this RegExp object” refers to the object that is the this value for the invocation of the
function; a TypeError exception is thrown if the this value is not an object or an object for which
the value of the [[Class]] internal property is not "RegExp".

15.10.6.1 RegExp.prototype.constructor
======================================

The initial value of RegExp.prototype.constructor is the standard built-in RegExp constructor.

15.10.6.2 RegExp.prototype.exec(string)
=======================================

Performs a regular expression match of string against the regular expression and returns an Array
object containing the results of the match, or null if string did not match.

The String ToString(string) is searched for an occurrence of the regular expression pattern as
follows:

1.  Let R be this RegExp object.
2.  Let S be the value of ToString(string).
3.  Let length be the length of S.
4.  Let lastIndex be the result of calling the [[Get]] internal method of R with argument
    "lastIndex".
5.  Let i be the value of ToInteger(lastIndex).
6.  Let global be the result of calling the [[Get]] internal method of R with argument "global".
7.  If global is false, then let i = 0.
8.  Let matchSucceeded be false.
9.  Repeat, while matchSucceeded is false
    1.  If i < 0 or i > length, then
        1.  Call the [[Put]] internal method of R with arguments "lastIndex", 0, and true.
        2.  Return null.

    2.  Call the [[Match]] internal method of R with arguments S and i.
    3.  If [[Match]] returned failure, then
        1.  Let i = i+1.

    4.  else
        1.  Let r be the State result of the call to [[Match]].
        2.  Set matchSucceeded to true.

10. Let e be r’s endIndex value.
11. If global is true,
    1.  Call the [[Put]] internal method of R with arguments "lastIndex", e, and true.

12. Let n be the length of r’s captures array. (This is the same value as 15.10.2.1’s
    NCapturingParens.)
13. Let A be a new array created as if by the expression new Array() where Array is the standard
    built-in constructor with that name.
14. Let matchIndex be i.
15. Call the [[DefineOwnProperty]] internal method of A with arguments "index", Property Descriptor
    {[[Value]]: matchIndex, [[Writable]: true, [[Enumerable]]: true, [[Configurable]]: true}, and
    true.
16. Call the [[DefineOwnProperty]] internal method of A with arguments "input", Property Descriptor
    {[[Value]]: S, [[Writable]: true, [[Enumerable]]: true, [[Configurable]]: true}, and true.
17. Call the [[DefineOwnProperty]] internal method of A with arguments "length", Property Descriptor
    {[[Value]]: n + 1}, and true.
18. Let matchedSubstr be the matched substring (i.e. the portion of S between offset i inclusive and
    offset e exclusive).
19. Call the [[DefineOwnProperty]] internal method of A with arguments "0", Property Descriptor
    {[[Value]]: matchedSubstr, [[Writable]: true, [[Enumerable]]: true, [[Configurable]]: true}, and
    true.
20. For each integer i such that i > 0 and i ≤ n
    1.  Let captureI be ith element of r’s captures array.
    2.  Call the [[DefineOwnProperty]] internal method of A with arguments ToString(i), Property
        Descriptor {[[Value]]: captureI, [[Writable]: true, [[Enumerable]]: true, [[Configurable]]:
        true}, and true.

21. Return A.

15.10.6.3 RegExp.prototype.test(string)
=======================================

The following steps are taken:

1.  Let match be the result of evaluating the RegExp.prototype.exec (15.10.6.2) algorithm upon this
    RegExp object using string as the argument.
2.  If match is not null, then return true; else return false.

15.10.6.4 RegExp.prototype.toString()
=====================================

Return the String value formed by concatenating the Strings "/", the String value of the source
property of this RegExp object, and "/"; plus "g" if the global property is true, "i" if the
ignoreCase property is true, and "m" if the multiline property is true.

NOTE The returned String has the form of a RegularExpressionLiteral that evaluates to another RegExp
object with the same behaviour as this object.

15.10.7 Properties of RegExp Instances
======================================

RegExp instances inherit properties from the RegExp prototype object and their [[Class]] internal
property value is "RegExp". RegExp instances also have a [[Match]] internal property and a length
property.

The value of the [[Match]] internal property is an implementation dependent representation of the
Pattern of the RegExp object.

RegExp instances also have the following properties.

15.10.7.1 source
================

The value of the source property is a String in the form of a Pattern representing the current
regular expression. This property shall have the attributes { [[Writable]]: false, [[Enumerable]]:
false, [[Configurable]]: false }.

15.10.7.2 global
================

The value of the global property is a Boolean value indicating whether the flags contained the
character “g”. This property shall have the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.

15.10.7.3 ignoreCase
====================

The value of the ignoreCase property is a Boolean value indicating whether the flags contained the
character “i”. This property shall have the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.

15.10.7.4 multiline
===================

The value of the multiline property is a Boolean value indicating whether the flags contained the
character “m”. This property shall have the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.

15.10.7.5 lastIndex
===================

The value of the lastIndex property specifies the String position at which to start the next match.
It is coerced to an integer when used (see 15.10.6.2). This property shall have the attributes
{ [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }.

NOTE Unlike the other standard built-in properties of RegExp instances, lastIndex is writable.

15.11 Error Objects
===================

Instances of Error objects are thrown as exceptions when runtime errors occur. The Error objects may
also serve as base objects for user-defined exception classes.

15.11.1 The Error Constructor Called as a Function
==================================================

When Error is called as a function rather than as a constructor, it creates and initialises a new
Error object. Thus the function call Error(…) is equivalent to the object creation expression new
Error(…) with the same arguments.

15.11.1.1 Error (message)
=========================

The [[Prototype]] internal property of the newly constructed object is set to the original Error
prototype object, the one that is the initial value of Error.prototype (15.11.3.1).

The [[Class]] internal property of the newly constructed object is set to "Error".

The [[Extensible]] internal property of the newly constructed object is set to true.

If the argument message is not undefined, the message own property of the newly constructed object
is set to ToString(message).

15.11.2 The Error Constructor
=============================

When Error is called as part of a new expression, it is a constructor: it initialises the newly
created object.

15.11.2.1 new Error (message)
=============================

The [[Prototype]] internal property of the newly constructed object is set to the original Error
prototype object, the one that is the initial value of Error.prototype (15.11.3.1).

The [[Class]] internal property of the newly constructed Error object is set to "Error".

The [[Extensible]] internal property of the newly constructed object is set to true.

If the argument message is not undefined, the message own property of the newly constructed object
is set to ToString(message).

15.11.3 Properties of the Error Constructor
===========================================

The value of the [[Prototype]] internal property of the Error constructor is the Function prototype
object (15.3.4).

Besides the internal properties and the length property (whose value is 1), the Error constructor
has the following property:

15.11.3.1 Error.prototype
=========================

The initial value of Error.prototype is the Error prototype object (15.11.4).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.

15.11.4 Properties of the Error Prototype Object
================================================

The Error prototype object is itself an Error object (its [[Class]] is "Error").

The value of the [[Prototype]] internal property of the Error prototype object is the standard
built-in Object prototype object (15.2.4).

15.11.4.1 Error.prototype.constructor
=====================================

The initial value of Error.prototype.constructor is the built-in Error constructor.

15.11.4.2 Error.prototype.name
==============================

The initial value of Error.prototype.name is "Error".

15.11.4.3 Error.prototype.message
=================================

The initial value of Error.prototype.message is the empty String.

15.11.4.4 Error.prototype.toString ( )
======================================

The following steps are taken:

1.  Let O be the this value.
2.  If Type(O) is not Object, throw a TypeError exception.
3.  Let name be the result of calling the [[Get]] internal method of O with argument "name".
4.  If name is undefined, then let name be "Error"; else let name be ToString(name).
5.  Let msg be the result of calling the [[Get]] internal method of O with argument "message".
6.  If msg is undefined, then let msg be the empty String; else let msg be ToString(msg).
7.  If msg is undefined, then let msg be the empty String; else let msg be ToString(msg).
8.  If name is the empty String, return msg.
9.  If msg is the empty String, return name.
10. Return the result of concatenating name, ":", a single space character, and msg.

15.11.5 Properties of Error Instances
=====================================

Error instances inherit properties from the Error prototype object and their [[Class]] internal
property value is "Error". Error instances have no special properties.

15.11.6 Native Error Types Used in This Standard
================================================

One of the NativeError objects below is thrown when a runtime error is detected. All of these
objects share the same structure, as described in 15.11.7.

15.11.6.1 EvalError
===================

This exception is not currently used within this specification. This object remains for
compatibility with previous editions of this specification.

15.11.6.2 RangeError
====================

Indicates a numeric value has exceeded the allowable range. See 15.4.2.2, 15.4.5.1, 15.7.4.2,
15.7.4.5, 15.7.4.6, 15.7.4.7, and 15.9.5.43.

15.11.6.3 ReferenceError
========================

Indicate that an invalid reference value has been detected. See 8.7.1, 8.7.2, 10.2.1, 10.2.1.1.4,
10.2.1.2.4, and 11.13.1.

15.11.6.4 SyntaxError
=====================

Indicates that a parsing error has occurred. See 11.1.5, 11.3.1, 11.3.2, 11.4.1, 11.4.4, 11.4.5,
11.13.1, 11.13.2, 12.2.1, 12.10.1, 12.14.1, 13.1, 15.1.2.1, 15.3.2.1, 15.10.2.2, 15.10.2.5,
15.10.2.9, 15.10.2.15, 15.10.2.19, 15.10.4.1, and 15.12.2.

15.11.6.5 TypeError
===================

Indicates the actual type of an operand is different than the expected type. See 8.6.2, 8.7.2,
8.10.5, 8.12.5, 8.12.7, 8.12.8, 8.12.9, 9.9, 9.10, 10.2.1, 10.2.1.1.3, 10.6, 11.2.2, 11.2.3, 11.4.1,
11.8.6, 11.8.7, 11.3.1, 13.2, 13.2.3, 15, 15.2.3.2, 15.2.3.3, 15.2.3.4, 15.2.3.5, 15.2.3.6,
15.2.3.7, 15.2.3.8, 15.2.3.9, 15.2.3.10, 15.2.3.11, 15.2.3.12, 15.2.3.13, 15.2.3.14, 15.2.4.3,
15.3.4.2, 15.3.4.3, 15.3.4.4, 15.3.4.5, 15.3.4.5.2, 15.3.4.5.3, 15.3.5, 15.3.5.3, 15.3.5.4,
15.4.4.3, 15.4.4.11, 15.4.4.16, 15.4.4.17, 15.4.4.18, 15.4.4.19, 15.4.4.20, 15.4.4.21, 15.4.4.22,
15.4.5.1, 15.5.4.2, 15.5.4.3, 15.6.4.2, 15.6.4.3, 15.7.4, 15.7.4.2, 15.7.4.4, 15.9.5, 15.9.5.44,
15.10.4.1, 15.10.6, 15.11.4.4 and 15.12.3.

15.11.6.6 URIError
==================

Indicates that one of the global URI handling functions was used in a way that is incompatible with
its definition. See 15.1.3.

15.11.7 NativeError Object Structure
====================================

When an ECMAScript implementation detects a runtime error, it throws an instance of one of the
NativeError objects defined in 15.11.6. Each of these objects has the structure described below,
differing only in the name used as the constructor name instead of NativeError, in the name property
of the prototype object, and in the implementation-defined message property of the prototype object.

For each error object, references to NativeError in the definition should be replaced with the
appropriate error object name from 15.11.6.

15.11.7.1 NativeError Constructors Called as Functions
======================================================

When a NativeError constructor is called as a function rather than as a constructor, it creates and
initialises a new object. A call of the object as a function is equivalent to calling it as a
constructor with the same arguments.

15.11.7.2 NativeError (message)
===============================

The [[Prototype]] internal property of the newly constructed object is set to the prototype object
for this error constructor. The [[Class]] internal property of the newly constructed object is set
to "Error". The [[Extensible]] internal property of the newly constructed object is set to true.

If the argument message is not undefined, the message own property of the newly constructed object
is set to ToString(message).

15.11.7.3 The NativeError Constructors
======================================

When a NativeError constructor is called as part of a new expression, it is a constructor: it
initialises the newly created object.

15.11.7.4 new NativeError (message)
===================================

The [[Prototype]] internal property of the newly constructed object is set to the prototype object
for this NativeError constructor. The [[Class]] internal property of the newly constructed object is
set to "Error". The [[Extensible]] internal property of the newly constructed object is set to true.

If the argument message is not undefined, the message own property of the newly constructed object
is set to ToString(message).

15.11.7.5 Properties of the NativeError Constructors
====================================================

The value of the [[Prototype]] internal property of a NativeError constructor is the Function
prototype object (15.3.4).

Besides the internal properties and the length property (whose value is 1), each NativeError
constructor has the following property:

15.11.7.6 NativeError.prototype
===============================

The initial value of NativeError.prototype is a NativeError prototype object (15.11.7.7). Each
NativeError constructor has a separate prototype object.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.

15.11.7.7 Properties of the NativeError Prototype Objects
=========================================================

Each NativeError prototype object is an Error object (its [[Class]] is "Error").

The value of the [[Prototype]] internal property of each NativeError prototype object is the
standard built-in Error prototype object (15.11.4).

15.11.7.8 NativeError.prototype.constructor
===========================================

The initial value of the constructor property of the prototype for a given NativeError constructor
is the NativeError constructor function itself (15.11.7).

15.11.7.9 NativeError.prototype.name
====================================

The initial value of the name property of the prototype for a given NativeError constructor is the
name of the constructor (the name used instead of NativeError).

15.11.7.10 NativeError.prototype.message
========================================

The initial value of the message property of the prototype for a given NativeError constructor is
the empty String.

NOTE The prototypes for the NativeError constructors do not themselves provide a toString function,
but instances of errors will inherit it from the Error prototype object.

15.11.7.11 Properties of NativeError Instances
==============================================

NativeError instances inherit properties from their NativeError prototype object and their [[Class]]
internal property value is "Error". NativeError instances have no special properties.

15.12 The JSON Object
=====================

The JSON object is a single object that contains two functions, parse and stringify, that are used
to parse and construct JSON texts. The JSON Data Interchange Format is described in RFC 4627
<http://www.ietf.org/rfc/rfc4627.txt>. The JSON interchange format used in this specification is
exactly that described by RFC 4627 with two exceptions:

-   The top level JSONText production of the ECMAScript JSON grammar may consist of any JSONValue
    rather than being restricted to being a JSONObject or a JSONArray as specified by RFC 4627.

-   Conforming implementations of JSON.parse and JSON.stringify must support the exact interchange
    format described in this specification without any deletions or extensions to the format. This
    differs from RFC 4627 which permits a JSON parser to accept non-JSON forms and extensions.

The value of the [[Prototype]] internal property of the JSON object is the standard built-in Object
prototype object (15.2.4). The value of the [[Class]] internal property of the JSON object is
"JSON". The value of the [[Extensible]] internal property of the JSON object is set to true.

The JSON object does not have a [[Construct]] internal property; it is not possible to use the JSON
object as a constructor with the new operator.

The JSON object does not have a [[Call]] internal property; it is not possible to invoke the JSON
object as a function.

15.12.1 The JSON Grammar
========================

JSON.stringify produces a String that conforms to the following JSON grammar. JSON.parse accepts a
String that conforms to the JSON grammar.

15.12.1.1 The JSON Lexical Grammar
==================================

JSON is similar to ECMAScript source text in that it consists of a sequence of characters conforming
to the rules of SourceCharacter. The JSON Lexical Grammar defines the tokens that make up a JSON
text similar to the manner that the ECMAScript lexical grammar defines the tokens of an ECMAScript
source text. The JSON Lexical grammar only recognises the white space character specified by the
production JSONWhiteSpace. The JSON lexical grammar shares some productions with the ECMAScript
lexical grammar. All nonterminal symbols of the grammar that do not begin with the characters “JSON”
are defined by productions of the ECMAScript lexical grammar.

Syntax
------

JSONWhiteSpace ::

<TAB>  
<CR>  
<LF>  
<SP>

JSONString ::

" JSONStringCharactersopt "

JSONStringCharacters ::

JSONStringCharacter JSONStringCharactersopt

JSONStringCharacter ::

SourceCharacter but not one of " or \ or U+0000 through U+001F

\ JSONEscapeSequence

JSONEscapeSequence ::

JSONEscapeCharacter

UnicodeEscapeSequence

JSONEscapeCharacter :: one of

" / \ b f n r t

JSONNumber ::

-opt DecimalIntegerLiteral JSONFractionopt ExponentPartopt

JSONFraction ::

. DecimalDigits

JSONNullLiteral ::

NullLiteral

JSONBooleanLiteral ::

BooleanLiteral

15.12.1.2 The JSON Syntactic Grammar
====================================

The JSON Syntactic Grammar defines a valid JSON text in terms of tokens defined by the JSON lexical
grammar. The goal symbol of the grammar is JSONText.

Syntax
------

JSONText :

JSONValue

JSONValue :

JSONNullLiteral

JSONBooleanLiteral

JSONObject

JSONArray

JSONString

JSONNumber

JSONObject :

{ }

{ JSONMemberList }

JSONMember :

JSONString : JSONValue

JSONMemberList :

JSONMember

JSONMemberList , JSONMember

JSONArray :

[ ]

[ JSONElementList ]

JSONElementList :

JSONValue

JSONElementList , JSONValue

15.12.2 parse ( text [ , reviver ] )
====================================

The parse function parses a JSON text (a JSON-formatted String) and produces an ECMAScript value.
The JSON format is a restricted form of ECMAScript literal. JSON objects are realized as ECMAScript
objects. JSON arrays are realized as ECMAScript arrays. JSON strings, numbers, booleans, and null
are realized as ECMAScript Strings, Numbers, Booleans, and null. JSON uses a more limited set of
white space characters than WhiteSpace and allows Unicode code points U+2028 and U+2029 to directly
appear in JSONString literals without using an escape sequence. The process of parsing is similar to
11.1.4 and 11.1.5 as constrained by the JSON grammar.

The optional reviver parameter is a function that takes two parameters, (key and value). It can
filter and transform the results. It is called with each of the key/value pairs produced by the
parse, and its return value is used instead of the original value. If it returns what it received,
the structure is not modified. If it returns undefined then the property is deleted from the result.

1.  Let JText be ToString(text).
2.  Parse JText using the grammars in 15.12.1. Throw a SyntaxError exception if JText did not
    conform to the JSON grammar for the goal symbol JSONText.
3.  Let unfiltered be the result of parsing and evaluating JText as if it was the source text of an
    ECMAScript Program but using JSONString in place of StringLiteral. Note that since JText
    conforms to the JSON grammar this result will be either a primitive value or an object that is
    defined by either an ArrayLiteral or an ObjectLiteral.
4.  If IsCallable(reviver) is true, then
    1.  Let root be a new object created as if by the expression new Object(), where Object is the
        standard built-in constructor with that name.
    2.  Call the [[DefineOwnProperty]] internal method of root with the empty String, the
        PropertyDescriptor {[[Value]]: unfiltered, [[Writable]]: true, [[Enumerable]]: true,
        [[Configurable]]: true}, and false as arguments.
    3.  Return the result of calling the abstract operation Walk, passing root and the empty String.
        The abstract operation Walk is described below.

5.  Else
    1.  Return unfiltered.

The abstract operation Walk is a recursive abstract operation that takes two parameters: a holder
object and the String name of a property in that object. Walk uses the value of reviver that was
originally passed to the above parse function.

1.  Let val be the result of calling the [[Get]] internal method of holder with argument name.
2.  If val is an object, then
    1.  If the [[Class]] internal property of val is "Array"
        1.  Set I to 0.
        2.  Let len be the result of calling the [[Get]] internal method of val with argument
            "length".
        3.  Repeat while I < len,
            1.  Let newElement be the result of calling the abstract operation Walk, passing val and
                ToString(I).
            2.  If newElement is undefined, then
                1.  Call the [[Delete]] internal method of val with ToString(I) and false as
                    arguments.

            3.  Else
                1.  Call the [[DefineOwnProperty]] internal method of val with arguments
                    ToString(I), the Property Descriptor {[[Value]]: newElement, [[Writable]]: true,
                    [[Enumerable]]: true, [[Configurable]]: true}, and false.

            4.  Add 1 to I.

    2.  Else
        1.  Let keys be an internal List of String values consisting of the names of all the own
            properties of val whose [[Enumerable]] attribute is true. The ordering of the Strings
            should be the same as that used by the Object.keys standard built-in function.
        2.  For each String P in keys do,
            1.  Let newElement be the result of calling the abstract operation Walk, passing val and
                P.
            2.  If newElement is undefined, then
                1.  Call the [[Delete]] internal method of val with P and false as arguments.

            3.  Else
                1.  Call the [[DefineOwnProperty]] internal method of val with arguments P, the
                    Property Descriptor {[[Value]]: newElement, [[Writable]]: true, [[Enumerable]]:
                    true, [[Configurable]]: true}, and false.

3.  Return the result of calling the [[Call]] internal method of reviver passing holder as the this
    value and with an argument list consisting of name and val.

It is not permitted for a conforming implementation of JSON.parse to extend the JSON grammars. If an
implementation wishes to support a modified or extended JSON interchange format it must do so by
defining a different parse function.

NOTE In the case where there are duplicate name Strings within an object, lexically preceding values
for the same key shall be overwritten.

15.12.3 stringify ( value [ , replacer [ , space ] ] )
======================================================

The stringify function returns a String in JSON format representing an ECMAScript value. It can take
three parameters. The first parameter is required. The value parameter is an ECMAScript value, which
is usually an object or array, although it can also be a String, Boolean, Number or null. The
optional replacer parameter is either a function that alters the way objects and arrays are
stringified, or an array of Strings and Numbers that acts as a white list for selecting the object
properties that will be stringified. The optional space parameter is a String or Number that allows
the result to have white space injected into it to improve human readability.

These are the steps in stringifying an object:

1.  Let stack be an empty List.
2.  Let indent be the empty String.
3.  Let PropertyList and ReplacerFunction be undefined.
4.  If Type(replacer) is Object, then
    1.  If IsCallable(replacer) is true, then
        1.  Let ReplacerFunction be replacer.

    2.  Else if the [[Class]] internal property of replacer is "Array", then
        1.  Let PropertyList be an empty internal List
        2.  For each value v of a property of replacer that has an array index property name. The
            properties are enumerated in the ascending array index order of their names.
            1.  Let item be undefined.
            2.  If Type(v) is String then let item be v.
            3.  Else if Type(v) is Number then let item be ToString(v).
            4.  Else if Type(v) is Object then,
                1.  If the [[Class]] internal property of v is "String" or "Number" then let item be
                    ToString(v).

            5.  If item is not undefined and item is not currently an element of PropertyList then,
                1.  Append item to the end of PropertyList.

5.  If Type(space) is Object then,
    1.  If the [[Class]] internal property of space is "Number" then,
        1.  Let space be ToNumber(space).

    2.  Else if the [[Class]] internal property of space is "String" then,
        1.  Let space be ToString(space).

6.  If Type(space) is Number
    1.  Let space be min(10, ToInteger(space)).
    2.  Set gap to a String containing space space characters. This will be the empty String if
        space is less than 1.

7.  Else if Type(space) is String
    1.  If the number of characters in space is 10 or less, set gap to space otherwise set gap to a
        String consisting of the first 10 characters of space.

8.  Else
    1.  Set gap to the empty String.

9.  Let wrapper be a new object created as if by the expression new Object(), where Object is the
    standard built-in constructor with that name.
10. Call the [[DefineOwnProperty]] internal method of wrapper with arguments the empty String, the
    Property Descriptor {[[Value]]: value, [[Writable]]: true, [[Enumerable]]: true,
    [[Configurable]]: true}, and false.
11. Return the result of calling the abstract operation Str with the empty String and wrapper.

The abstract operation Str(key, holder) has access to ReplacerFunction from the invocation of the
stringify method. Its algorithm is as follows:

1.  Let value be the result of calling the [[Get]] internal method of holder with argument key.
2.  If Type(value) is Object, then
    1.  Let toJSON be the result of calling the [[Get]] internal method of value with argument
        "toJSON".
    2.  If IsCallable(toJSON) is true
        1.  Let value be the result of calling the [[Call]] internal method of toJSON passing value
            as the this value and with an argument list consisting of key.

3.  If ReplacerFunction is not undefined, then
    1.  Let value be the result of calling the [[Call]] internal method of ReplacerFunction passing
        holder as the this value and with an argument list consisting of key and value.

4.  If Type(value) is Object then,
    1.  If the [[Class]] internal property of value is "Number" then,
        1.  Let value be ToNumber(value).

    2.  Else if the [[Class]] internal property of value is "String" then,
        1.  Let value be ToString(value).

    3.  Else if the [[Class]] internal property of value is "Boolean" then,
        1.  Let value be the value of the [[PrimitiveValue]] internal property of value.

5.  If value is null then return "null".
6.  If value is true then return "true".
7.  If value is false then return "false".
8.  If Type(value) is String, then return the result of calling the abstract operation Quote with
    argument value.
9.  If Type(value) is Number
    1.  If value is finite then return ToString(value).
    2.  Else, return "null".

10. If Type(value) is Object, and IsCallable(value) is false
    1.  If the [[Class]] internal property of value is "Array" then
        1.  Return the result of calling the abstract operation JA with argument value.

    2.  Else, return the result of calling the abstract operation JO with argument value.

11. Return undefined.

The abstract operation Quote(value) wraps a String value in double quotes and escapes characters
within it.

1.  Let product be the double quote character.
2.  For each character C in value
    1.  If C is the double quote character or the backslash character
        1.  Let product be the concatenation of product and the backslash character.
        2.  Let product be the concatenation of product and C.

    2.  Else if C is backspace, formfeed, newline, carriage return, or tab
        1.  Let product be the concatenation of product and the backslash character.
        2.  Let abbrev be the character corresponding to the value of C as follows:
              ----------------- -----
              backspace         “b”
              formfeed          “f”
              newline           “n”
              carriage return   “r”
              tab               “t”
              ----------------- -----

        3.  Let product be the concatenation of product and abbrev.

    3.  Else if C is a control character having a code unit value less than the space character
        1.  Let product be the concatenation of product and the backslash character.
        2.  Let product be the concatenation of product and "u".
        3.  Let hex be the result of converting the numeric code unit value of C to a String of four
            hexadecimal digits.
        4.  Let product be the concatenation of product and hex.

    4.  Else
        1.  Let product be the concatenation of product and C.

3.  Let product be the concatenation of product and the double quote character.
4.  Return product.

The abstract operation JO(value) serializes an object. It has access to the stack, indent, gap,
PropertyList, ReplacerFunction, and space of the invocation of the stringify method.

1.  If stack contains value then throw a TypeError exception because the structure is cyclical.
2.  Append value to stack.
3.  Let stepback be indent.
4.  Let indent be the concatenation of indent and gap.
5.  If PropertyList is not undefined, then
    1.  Let K be PropertyList.

6.  Else
    1.  Let K be an internal List of Strings consisting of the names of all the own properties of
        value whose [[Enumerable]] attribute is true. The ordering of the Strings should be the same
        as that used by the Object.keys standard built-in function.

7.  Let partial be an empty List.
8.  For each element P of K.
    1.  Let strP be the result of calling the abstract operation Str with arguments P and value.
    2.  If strP is not undefined
        1.  Let member be the result of calling the abstract operation Quote with argument P.
        2.  Let member be the concatenation of member and the colon character.
        3.  If gap is not the empty String
            1.  Let member be the concatenation of member and the space character.

        4.  Let member be the concatenation of member and strP.
        5.  Append member to partial.

9.  If partial is empty, then
    1.  Let final be "{}".

10. Else
    1.  If gap is the empty String
        1.  Let properties be a String formed by concatenating all the element Strings of partial
            with each adjacent pair of Strings separated with the comma character. A comma is not
            inserted either before the first String or after the last String.
        2.  Let final be the result of concatenating “{”, properties, and "}".

    2.  Else gap is not the empty String
        1.  Let separator be the result of concatenating the comma character, the line feed
            character, and indent.
        2.  Let properties be a String formed by concatenating all the element Strings of partial
            with each adjacent pair of Strings separated with separator. The separator String is not
            inserted either before the first String or after the last String.
        3.  Let final be the result of concatenating "{", the line feed character, indent,
            properties, the line feed character, stepback, and "}".

11. Remove the last element of stack.
12. Let indent be stepback.
13. Return final.

The abstract operation JA(value) serializes an array. It has access to the stack, indent, gap, and
space of the invocation of the stringify method. The representation of arrays includes only the
elements between zero and array.length – 1 inclusive. Named properties are excluded from the
stringification. An array is stringified as an open left bracket, elements separated by comma, and a
closing right bracket.

1.  If stack contains value then throw a TypeError exception because the structure is cyclical.
2.  Append value to stack.
3.  Let stepback be indent.
4.  Let indent be the concatenation of indent and gap.
5.  Let partial be an empty List.
6.  Let len be the result of calling the [[Get]] internal method of value with argument "length".
7.  Let index be 0.
8.  Repeat while index < len
    1.  Let strP be the result of calling the abstract operation Str with arguments ToString(index)
        and value.
    2.  If strP is undefined
        1.  Append "null" to partial.

    3.  Else
        1.  Append strP to partial.

    4.  Increment index by 1.

9.  If partial is empty ,then
    1.  Let final be "[]".

10. Else
    1.  If gap is the empty String
        1.  Let properties be a String formed by concatenating all the element Strings of partial
            with each adjacent pair of Strings separated with the comma character. A comma is not
            inserted either before the first String or after the last String.
        2.  Let final be the result of concatenating “[”, properties, and "]".

    2.  Else
        1.  Let separator be the result of concatenating the comma character, the line feed
            character, and indent.
        2.  Let properties be a String formed by concatenating all the element Strings of partial
            with each adjacent pair of Strings separated with separator. The separator String is not
            inserted either before the first String or after the last String.
        3.  Let final be the result of concatenating "[", the line feed character, indent,
            properties, the line feed character, stepback, and "]".

11. Remove the last element of stack.
12. Let indent be stepback.
13. Return final.

NOTE 1 JSON structures are allowed to be nested to any depth, but they must be acyclic. If value is
or contains a cyclic structure, then the stringify function must throw a TypeError exception. This
is an example of a value that cannot be stringified:

    a = [];

    a[0] = a;

    my_text = JSON.stringify(a); // This must throw an TypeError.

NOTE 2 Symbolic primitive values are rendered as follows:

-   The null value is rendered in JSON text as the String null.
-   The undefined value is not rendered.
-   The true value is rendered in JSON text as the String true.
-   The false value is rendered in JSON text as the String false.

NOTE 3 String values are wrapped in double quotes. The characters " and \ are escaped with \
prefixes. Control characters are replaced with escape sequences \uHHHH, or with the shorter forms,
\b (backspace), \f (formfeed), \n (newline), \r (carriage return), \t (tab).

NOTE 4 Finite numbers are stringified as if by calling ToString(number). NaN and Infinity regardless
of sign are represented as the String null.

NOTE 5 Values that do not have a JSON representation (such as undefined and functions) do not
produce a String. Instead they produce the undefined value. In arrays these values are represented
as the String null. In objects an unrepresentable value causes the property to be excluded from
stringification.

NOTE 6 An object is rendered as an opening left brace followed by zero or more properties, separated
with commas, closed with a right brace. A property is a quoted String representing the key or
property name, a colon, and then the stringified property value. An array is rendered as an opening
left bracket followed by zero or more values, separated with commas, closed with a right bracket.

16 Errors
=========

An implementation must report most errors at the time the relevant ECMAScript language construct is
evaluated. An early error is an error that can be detected and reported prior to the evaluation of
any construct in the Program containing the error. An implementation must report early errors in a
Program prior to the first evaluation of that Program. Early errors in eval code are reported at the
time eval is called but prior to evaluation of any construct within the eval code. All errors that
are not early errors are runtime errors.

An implementation must treat any instance of the following kinds of errors as an early error:

-   Any syntax error.

-   Attempts to define an ObjectLiteral that has multiple get property assignments with the same
    name or multiple set property assignments with the same name.

-   Attempts to define an ObjectLiteral that has both a data property assignment and a get or set
    property assignment with the same name.

-   Errors in regular expression literals that are not implementation-defined syntax extensions.

-   Attempts in strict mode code to define an ObjectLiteral that has multiple data property
    assignments with the same name.

-   The occurrence of a WithStatement in strict mode code.

-   The occurrence of an Identifier value appearing more than once within a FormalParameterList of
    an individual strict mode FunctionDeclaration or FunctionExpression.

-   Improper uses of return, break, and continue.

-   Attempts to call PutValue on any value for which an early determination can be made that the
    value is not a Reference (for example, executing the assignment statement 3=4).

An implementation shall not treat other kinds of errors as early errors even if the compiler can
prove that a construct cannot execute without error under any circumstances. An implementation may
issue an early warning in such a case, but it should not report the error until the relevant
construct is actually executed.

An implementation shall report all errors as specified, except for the following:

-   An implementation may extend program syntax and regular expression pattern or flag syntax. To
    permit this, all operations (such as calling eval, using a regular expression literal, or using
    the Function or RegExp constructor) that are allowed to throw SyntaxError are permitted to
    exhibit implementation-defined behaviour instead of throwing SyntaxError when they encounter an
    implementation-defined extension to the program syntax or regular expression pattern or flag
    syntax.

-   An implementation may provide additional types, values, objects, properties, and functions
    beyond those described in this specification. This may cause constructs (such as looking up a
    variable in the global scope) to have implementation-defined behaviour instead of throwing an
    error (such as ReferenceError).

-   An implementation may define behaviour other than throwing RangeError for toFixed,
    toExponential, and toPrecision when the fractionDigits or precision argument is outside the
    specified range.

Annex A (informative) Grammar Summary
=====================================

A.1 Lexical Grammar
===================

See clause 6

SourceCharacter ::

any Unicode code unit

See clause 7

InputElementDiv ::

WhiteSpace

LineTerminator

Comment

Token

DivPunctuator

See clause 7

InputElementRegExp ::

WhiteSpace

LineTerminator

Comment

Token

RegularExpressionLiteral

See 7.2

WhiteSpace ::

<TAB>

<VT>

<FF>

<SP>

<NBSP>

<BOM>

<USP>

See 7.3

LineTerminator ::

<LF>

<CR>

<LS>

<PS>

See 7.3

LineTerminatorSequence ::

<LF>

<CR> [lookahead ∉ <LF> ]

<LS>

<PS>

<CR> <LF>

See 7.4

Comment ::

MultiLineComment

SingleLineComment

See 7.4

MultiLineComment ::

/* MultiLineCommentCharsopt */

See 7.4

MultiLineCommentChars ::

MultiLineNotAsteriskChar MultiLineCommentCharsopt

* PostAsteriskCommentCharsopt

See 7.4

PostAsteriskCommentChars ::

MultiLineNotForwardSlashOrAsteriskChar MultiLineCommentCharsopt

* PostAsteriskCommentCharsopt

See 7.4

MultiLineNotAsteriskChar ::

SourceCharacter but not *

See 7.4

MultiLineNotForwardSlashOrAsteriskChar ::

SourceCharacter but not one of / or *

See 7.4

SingleLineComment ::

// SingleLineCommentCharsopt

See 7.4

SingleLineCommentChars ::

SingleLineCommentChar SingleLineCommentCharsopt

See 7.4

SingleLineCommentChar ::

SourceCharacter but not LineTerminator

See 7.5

Token ::

IdentifierName

Punctuator

NumericLiteral

StringLiteral

See 7.6

Identifier ::

IdentifierName but not ReservedWord

See 7.6

IdentifierName ::

IdentifierStart

IdentifierName IdentifierPart

See 7.6

IdentifierStart ::

UnicodeLetter

$

_

\ UnicodeEscapeSequence

See 7.6

IdentifierPart ::

IdentifierStart

UnicodeCombiningMark

UnicodeDigit

UnicodeConnectorPunctuation

<ZWNJ>

<ZWJ>

See 7.6

UnicodeLetter ::

any character in the Unicode categories “Uppercase letter (Lu)”, “Lowercase letter (Ll)”, “Titlecase
letter (Lt)”, “Modifier letter (Lm)”, “Other letter (Lo)”, or “Letter number (Nl)”.

See 7.6

UnicodeCombiningMark ::

any character in the Unicode categories “Non-spacing mark (Mn)” or “Combining spacing mark (Mc)”

See 7.6

UnicodeDigit ::

any character in the Unicode category “Decimal number (Nd)”

See 7.6

UnicodeConnectorPunctuation ::

any character in the Unicode category “Connector punctuation (Pc)”

See 7.6.1

ReservedWord ::

Keyword

FutureReservedWord

NullLiteral

BooleanLiteral

See 7.6.1.1

Keyword :: one of

  ---------- ---------- ------------ --------
  break      do         instanceof   typeof
  case       else       new          var
  catch      finally    return       void
  continue   for        switch       while
  debugger   function   this         with
  default    if         throw        
  delete     in         try          
  ---------- ---------- ------------ --------

See 7.6.1.2

FutureReservedWord :: one of

  ------- -------- ---------- -------
  class   enum     extends    super

  const   export   import     
                              
  ------- -------- ---------- -------

The following tokens are also considered to be FutureReservedWords when parsing strict mode code
(see 10.1.1).

  ------------ --------- ----------- --------
  implements   let       private     public
  interface    package   protected   static
  yield                              
  ------------ --------- ----------- --------

See 7.7

Punctuator :: one of

  ----- ------ ----- ----- ----- -----
  {     }      (     )     [     ]
  .     ;      ,     <     >     <=
  >=    ==     !=    ===   !==   
  +     -      *     %     ++    –
  <<    >>     >>>   &     |     ^
  !     ~      &&    ||    ?     :
  =     +=     -=    *=    %=    <<=
  >>=   >>>=   &=    |=    ^=    
  ----- ------ ----- ----- ----- -----

See 7.7

DivPunctuator :: one of

  --- ---- -- -- -- --
  /   /=            
  --- ---- -- -- -- --

See 7.8

Literal ::

NullLiteral

BooleanLiteral

NumericLiteral

StringLiteral

RegularExpressionLiteral

See 7.8.1

NullLiteral ::

null

See 7.8.2

BooleanLiteral ::

true

false

See 7.8.3

NumericLiteral ::

DecimalLiteral

HexIntegerLiteral

See 7.8.3

DecimalLiteral ::

DecimalIntegerLiteral . DecimalDigitsopt ExponentPartopt

. DecimalDigits ExponentPartopt

DecimalIntegerLiteral ExponentPartopt

See 7.8.3

DecimalIntegerLiteral ::

0

NonZeroDigit DecimalDigitsopt

See 7.8.3

DecimalDigits ::

DecimalDigit

DecimalDigits DecimalDigit

See 7.8.3

DecimalDigit :: one of

0 1 2 3 4 5 6 7 8 9

See 7.8.3

NonZeroDigit :: one of

1 2 3 4 5 6 7 8 9

See 7.8.3

ExponentPart ::

ExponentIndicator SignedInteger

See 7.8.3

ExponentIndicator :: one of

e E

See 7.8.3

SignedInteger ::

DecimalDigits

+ DecimalDigits

- DecimalDigits

See 7.8.3

HexIntegerLiteral ::

0x HexDigit

0X HexDigit

HexIntegerLiteral HexDigit

See 7.8.3

HexDigit :: one of

0 1 2 3 4 5 6 7 8 9 a b c d e f A B C D E F

See 7.8.4

StringLiteral ::

" DoubleStringCharactersopt "

' SingleStringCharactersopt '

See 7.8.4

DoubleStringCharacters ::

DoubleStringCharacter DoubleStringCharactersopt

See 7.8.4

SingleStringCharacters ::

SingleStringCharacter SingleStringCharactersopt

See 7.8.4

DoubleStringCharacter ::

SourceCharacter but not one of " or \ or LineTerminator

\ EscapeSequence

LineContinuation

See 7.8.4

SingleStringCharacter ::

SourceCharacter but not one of ' or \ or LineTerminator

\ EscapeSequence

LineContinuation

See 7.8.4

LineContinuation ::

\ LineTerminatorSequence

See 7.8.4

EscapeSequence ::

CharacterEscapeSequence

0 [lookahead ∉ DecimalDigit]

HexEscapeSequence

UnicodeEscapeSequence

See 7.8.4

CharacterEscapeSequence ::

SingleEscapeCharacter

NonEscapeCharacter

See 7.8.4

SingleEscapeCharacter :: one of

' " \ b f n r t v

See 7.8.4

NonEscapeCharacter ::

SourceCharacter but not one of EscapeCharacter or LineTerminator

See 7.8.4

EscapeCharacter ::

SingleEscapeCharacter

DecimalDigit

x

u

See 7.8.4

HexEscapeSequence ::

x HexDigit HexDigit

See 7.8.4

UnicodeEscapeSequence ::

u HexDigit HexDigit HexDigit HexDigit

See 7.8.5

RegularExpressionLiteral ::

/ RegularExpressionBody / RegularExpressionFlags

See 7.8.5

RegularExpressionBody ::

RegularExpressionFirstChar RegularExpressionChars

See 7.8.5

RegularExpressionChars ::

[empty]

RegularExpressionChars RegularExpressionChar

See 7.8.5

RegularExpressionFirstChar ::

RegularExpressionNonTerminator but not one of * or \ or / or [

RegularExpressionBackslashSequence

RegularExpressionClass

See 7.8.5

RegularExpressionChar ::

RegularExpressionNonTerminator but not \ or / or [

RegularExpressionBackslashSequence

RegularExpressionClass

See 7.8.5

RegularExpressionBackslashSequence ::

\ RegularExpressionNonTerminator

See 7.8.5

RegularExpressionNonTerminator ::

SourceCharacter but not LineTerminator

See 7.8.5

RegularExpressionClass ::

[ RegularExpressionClassChars ]

See 7.8.5

RegularExpressionClassChars ::

[empty]

RegularExpressionClassChars RegularExpressionClassChar

See 7.8.5

RegularExpressionClassChar ::

RegularExpressionNonTerminator but not ] or \

RegularExpressionBackslashSequence

See 7.8.5

RegularExpressionFlags ::

[empty]

RegularExpressionFlags IdentifierPart

A.2 Number Conversions
======================

See 9.3.1

StringNumericLiteral :::

StrWhiteSpaceopt

StrWhiteSpaceopt StrNumericLiteral StrWhiteSpaceopt

See 9.3.1

StrWhiteSpace :::

StrWhiteSpaceChar StrWhiteSpaceopt

See 9.3.1

StrWhiteSpaceChar :::

WhiteSpace

LineTerminator

See 9.3.1

StrNumericLiteral :::

StrDecimalLiteral

HexIntegerLiteral

See 9.3.1

StrDecimalLiteral :::

StrUnsignedDecimalLiteral

+ StrUnsignedDecimalLiteral

- StrUnsignedDecimalLiteral

See 9.3.1

StrUnsignedDecimalLiteral :::

Infinity

DecimalDigits . DecimalDigitsopt ExponentPartopt

. DecimalDigits ExponentPartopt

DecimalDigits ExponentPartopt

See 9.3.1

DecimalDigits :::

DecimalDigit

DecimalDigits DecimalDigit

See 9.3.1

DecimalDigit ::: one of

0 1 2 3 4 5 6 7 8 9

See 9.3.1

ExponentPart :::

ExponentIndicator SignedInteger

See 9.3.1

ExponentIndicator ::: one of

e E

See 9.3.1

SignedInteger :::

DecimalDigits

+ DecimalDigits

- DecimalDigits

See 9.3.1

HexIntegerLiteral :::

0x HexDigit

0X HexDigit

HexIntegerLiteral HexDigit

See 9.3.1

HexDigit ::: one of

0 1 2 3 4 5 6 7 8 9 a b c d e f A B C D E F

A.3 Expressions
===============

See 11.1

PrimaryExpression :

this

Identifier

Literal

ArrayLiteral

ObjectLiteral

( Expression )

See 11.1.4

ArrayLiteral :

[ Elisionopt ]

[ ElementList ]

[ ElementList , Elisionopt ]

See 11.1.4

ElementList :

Elisionopt AssignmentExpression

ElementList , Elisionopt AssignmentExpression

See 11.1.4

Elision :

,

Elision ,

See 11.1.5

ObjectLiteral :

{ }

{ PropertyNameAndValueList }

{ PropertyNameAndValueList , }

See 11.1.5

PropertyNameAndValueList :

PropertyAssignment

PropertyNameAndValueList , PropertyAssignment

See 11.1.5

PropertyAssignment :

PropertyName : AssignmentExpression

get PropertyName ( ) { FunctionBody }

set PropertyName ( PropertySetParameterList ) { FunctionBody }

See 11.1.5

PropertyName :

IdentifierName

StringLiteral

NumericLiteral

See 11.1.5

PropertySetParameterList :

Identifier

See 11.2

MemberExpression :

PrimaryExpression

FunctionExpression

MemberExpression [ Expression ]

MemberExpression . IdentifierName

new MemberExpression Arguments

See 11.2

NewExpression :

MemberExpression

new NewExpression

See 11.2

CallExpression :

MemberExpression Arguments

CallExpression Arguments

CallExpression [ Expression ]

CallExpression . IdentifierName

See 11.2

Arguments :

( )

( ArgumentList )

See 11.2

ArgumentList :

AssignmentExpression

ArgumentList , AssignmentExpression

See 11.2

LeftHandSideExpression :

NewExpression

CallExpression

See 11.3

PostfixExpression :

LeftHandSideExpression

LeftHandSideExpression [no LineTerminator here] ++

LeftHandSideExpression [no LineTerminator here] --

See 11.4

UnaryExpression :

PostfixExpression

delete UnaryExpression

void UnaryExpression

typeof UnaryExpression

++ UnaryExpression

-- UnaryExpression

+ UnaryExpression

- UnaryExpression

~ UnaryExpression

! UnaryExpression

See 11.5

MultiplicativeExpression :

UnaryExpression

MultiplicativeExpression * UnaryExpression

MultiplicativeExpression / UnaryExpression

MultiplicativeExpression % UnaryExpression

See 11.6

AdditiveExpression :

MultiplicativeExpression

AdditiveExpression + MultiplicativeExpression

AdditiveExpression - MultiplicativeExpression

See 11.7

ShiftExpression :

AdditiveExpression

ShiftExpression << AdditiveExpression

ShiftExpression >> AdditiveExpression

ShiftExpression >>> AdditiveExpression

See 11.8

RelationalExpression :

ShiftExpression

RelationalExpression < ShiftExpression

RelationalExpression > ShiftExpression

RelationalExpression <= ShiftExpression

RelationalExpression >= ShiftExpression

RelationalExpression instanceof ShiftExpression

RelationalExpression in ShiftExpression

See 11.8

RelationalExpressionNoIn :

ShiftExpression

RelationalExpressionNoIn < ShiftExpression

RelationalExpressionNoIn > ShiftExpression

RelationalExpressionNoIn <= ShiftExpression

RelationalExpressionNoIn >= ShiftExpression

RelationalExpressionNoIn instanceof ShiftExpression

See 11.9

EqualityExpression :

RelationalExpression

EqualityExpression == RelationalExpression

EqualityExpression != RelationalExpression

EqualityExpression === RelationalExpression

EqualityExpression !== RelationalExpression

See 11.9

EqualityExpressionNoIn :

RelationalExpressionNoIn

EqualityExpressionNoIn == RelationalExpressionNoIn

EqualityExpressionNoIn != RelationalExpressionNoIn

EqualityExpressionNoIn === RelationalExpressionNoIn

EqualityExpressionNoIn !== RelationalExpressionNoIn

See 11.10

BitwiseANDExpression :

EqualityExpression

BitwiseANDExpression & EqualityExpression

See 11.10

BitwiseANDExpressionNoIn :

EqualityExpressionNoIn

BitwiseANDExpressionNoIn & EqualityExpressionNoIn

See 11.10

BitwiseXORExpression :

BitwiseANDExpression

BitwiseXORExpression ^ BitwiseANDExpression

See 11.10

BitwiseXORExpressionNoIn :

BitwiseANDExpressionNoIn

BitwiseXORExpressionNoIn ^ BitwiseANDExpressionNoIn

See 11.10

BitwiseORExpression :

BitwiseXORExpression

BitwiseORExpression | BitwiseXORExpression

See 11.10

BitwiseORExpressionNoIn :

BitwiseXORExpressionNoIn

BitwiseORExpressionNoIn | BitwiseXORExpressionNoIn

See 11.11

LogicalANDExpression :

BitwiseORExpression

LogicalANDExpression && BitwiseORExpression

See 11.11

LogicalANDExpressionNoIn :

BitwiseORExpressionNoIn

LogicalANDExpressionNoIn && BitwiseORExpressionNoIn

See 11.11

LogicalORExpression :

LogicalANDExpression

LogicalORExpression || LogicalANDExpression

See 11.11

LogicalORExpressionNoIn :

LogicalANDExpressionNoIn

LogicalORExpressionNoIn || LogicalANDExpressionNoIn

See 11.12

ConditionalExpression :

LogicalORExpression

LogicalORExpression ? AssignmentExpression : AssignmentExpression

See 11.12

ConditionalExpressionNoIn :

LogicalORExpressionNoIn

LogicalORExpressionNoIn ? AssignmentExpression : AssignmentExpressionNoIn

See 11.13

AssignmentExpression :

ConditionalExpression

LeftHandSideExpression = AssignmentExpression

LeftHandSideExpression AssignmentOperator AssignmentExpression

See 11.13

AssignmentExpressionNoIn :

ConditionalExpressionNoIn

LeftHandSideExpression = AssignmentExpressionNoIn

LeftHandSideExpression AssignmentOperator AssignmentExpressionNoIn

See 11.13

AssignmentOperator : one of

  ---- ---- ---- ---- ---- ----- ----- ------ ---- ---- ----
  *=   /=   %=   +=   -=   <<=   >>=   >>>=   &=   ^=   |=
  ---- ---- ---- ---- ---- ----- ----- ------ ---- ---- ----

See 11.14

Expression :

AssignmentExpression

Expression , AssignmentExpression

See 11.14

ExpressionNoIn :

AssignmentExpressionNoIn

ExpressionNoIn , AssignmentExpressionNoIn

A.4 Statements
==============

See clause 12

Statement :

Block

VariableStatement

EmptyStatement

ExpressionStatement

IfStatement

IterationStatement

ContinueStatement

BreakStatement

ReturnStatement

WithStatement

LabelledStatement

SwitchStatement

ThrowStatement

TryStatement

DebuggerStatement

See 12.1

Block :

{ StatementListopt }

See 12.1

StatementList :

Statement

StatementList Statement

See 12.2

VariableStatement :

var VariableDeclarationList ;

See 12.2

VariableDeclarationList :

VariableDeclaration

VariableDeclarationList , VariableDeclaration

See 12.2

VariableDeclarationListNoIn :

VariableDeclarationNoIn

VariableDeclarationListNoIn , VariableDeclarationNoIn

See 12.2

VariableDeclaration :

Identifier Initialiseropt

See 12.2

VariableDeclarationNoIn :

Identifier InitialiserNoInopt

See 12.2

Initialiser :

= AssignmentExpression

See 12.2

InitialiserNoIn :

= AssignmentExpressionNoIn

See 12.3

EmptyStatement :

;

See 12.4

ExpressionStatement :

[lookahead ∉ {{, function}] Expression ;

See 12.5

IfStatement :

if ( Expression ) Statement else Statement

if ( Expression ) Statement

See 12.6

IterationStatement :

do Statement while ( Expression );

while ( Expression ) Statement

for ( ExpressionNoInopt ; Expressionopt ; Expressionopt ) Statement

for ( var VariableDeclarationListNoIn ; Expressionopt ; Expressionopt ) Statement

for ( LeftHandSideExpression in Expression ) Statement

for ( var VariableDeclarationNoIn in Expression ) Statement

See 12.7

ContinueStatement :

continue ;

continue [no LineTerminator here] Identifier ;

See 12.8

BreakStatement :

break ;

break [no LineTerminator here] Identifier ;

See 12.9

ReturnStatement :

return ;

return [no LineTerminator here] Expression ;

See 12.10

WithStatement :

with ( Expression ) Statement

See 12.11

SwitchStatement :

switch ( Expression ) CaseBlock

See 12.11

CaseBlock :

{ CaseClausesopt }

{ CaseClausesopt DefaultClause CaseClausesopt }

See 12.11

CaseClauses :

CaseClause

CaseClauses CaseClause

See 12.11

CaseClause :

case Expression : StatementListopt

See 12.11

DefaultClause :

default : StatementListopt

See 12.12

LabelledStatement :

Identifier : Statement

See 12.13

ThrowStatement :

throw [no LineTerminator here] Expression ;

See 12.14

TryStatement :

try Block Catch

try Block Finally

try Block Catch Finally

See 12.14

Catch :

catch ( Identifier ) Block

See 12.14

Finally :

finally Block

See 12.15

DebuggerStatement :

debugger ;

A.5 Functions and Programs
==========================

See clause 13

FunctionDeclaration :

function Identifier ( FormalParameterListopt ) { FunctionBody }

See clause 13

FunctionExpression :

function Identifieropt ( FormalParameterListopt ) { FunctionBody }

See clause 13

FormalParameterList :

Identifier

FormalParameterList , Identifier

See clause 13

FunctionBody :

SourceElementsopt

See clause 14

Program :

SourceElementsopt

See clause 14

SourceElements :

SourceElement

SourceElements SourceElement

See clause 14

SourceElement :

Statement

FunctionDeclaration

A.6 Universal Resource Identifier Character Classes
===================================================

See 15.1.3

uri :::

uriCharactersopt

See 15.1.3

uriCharacters :::

uriCharacter uriCharactersopt

See 15.1.3

uriCharacter :::

uriReserved

uriUnescaped

uriEscaped

See 15.1.3

uriReserved ::: one of

; / ? : @ & = + $ ,

See 15.1.3

uriUnescaped :::

uriAlpha

DecimalDigit

uriMark

See 15.1.3

uriEscaped :::

% HexDigit HexDigit

See 15.1.3

uriAlpha ::: one of

a b c d e f g h i j k l m n o p q r s t u v w x y z

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

See 15.1.3

uriMark ::: one of

- _ . ! ~ * ' ( )

A.7 Regular Expressions
=======================

See 15.10.1

Pattern ::

Disjunction

See 15.10.1

Disjunction ::

Alternative

Alternative | Disjunction

See 15.10.1

Alternative ::

[empty]

Alternative Term

See 15.10.1

Term ::

Assertion

Atom

Atom Quantifier

See 15.10.1

Assertion ::

^

$

\ b

\ B

( ? = Disjunction )

( ? ! Disjunction )

See 15.10.1

Quantifier ::

QuantifierPrefix

QuantifierPrefix ?

See 15.10.1

QuantifierPrefix ::

*

+

?

{ DecimalDigits }

{ DecimalDigits , }

{ DecimalDigits , DecimalDigits }

See 15.10.1

Atom ::

PatternCharacter

.

\ AtomEscape

CharacterClass

( Disjunction )

( ? : Disjunction )

See 15.10.1

PatternCharacter ::

SourceCharacter but not one of -

^ $ \ . * + ? ( ) [ ] { } |

See 15.10.1

AtomEscape ::

DecimalEscape

CharacterEscape

CharacterClassEscape

See 15.10.1

CharacterEscape ::

ControlEscape

c ControlLetter

HexEscapeSequence

UnicodeEscapeSequence

IdentityEscape

See 15.10.1

ControlEscape :: one of

f n r t v

See 15.10.1

ControlLetter :: one of

a b c d e f g h i j k l m n o p q r s t u v w x y z

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

See 15.10.1

IdentityEscape ::

SourceCharacter but not IdentifierPart

<ZWJ>

<ZWNJ>

See 15.10.1

DecimalEscape ::

DecimalIntegerLiteral [lookahead ∉ DecimalDigit]

See 15.10.1

CharacterClassEscape :: one of

d D s S w W

See 15.10.1

CharacterClass ::

[ [lookahead ∉ {^}] ClassRanges ]

[ ^ ClassRanges ]

See 15.10.1

ClassRanges ::

[empty]

NonemptyClassRanges

See 15.10.1

NonemptyClassRanges ::

ClassAtom

ClassAtom NonemptyClassRangesNoDash

ClassAtom – ClassAtom ClassRanges

See 15.10.1

NonemptyClassRangesNoDash ::

ClassAtom

ClassAtomNoDash NonemptyClassRangesNoDash

ClassAtomNoDash – ClassAtom ClassRanges

See 15.10.1

ClassAtom ::

-

ClassAtomNoDash

See 15.10.1

ClassAtomNoDash ::

SourceCharacter but not one of \ or ] or -

\ ClassEscape

See 15.10.1

ClassEscape ::

DecimalEscape

b

CharacterEscape

CharacterClassEscape

A.8 JSON
========

A.8.1 JSON Lexical Grammar
==========================

See 15.12.1.1

JSONWhiteSpace ::

<TAB>

<CR>

<LF>

<SP>

See 15.12.1.1

JSONString ::

" JSONStringCharactersopt "

See 15.12.1.1

JSONStringCharacters ::

JSONStringCharacter JSONStringCharactersopt

See 15.12.1.1

JSONStringCharacter ::

SourceCharacter but not one of " or \ or U+0000 through U+001F

\ JSONEscapeSequence

See 15.12.1.1

JSONEscapeSequence ::

JSONEscapeCharacter

UnicodeEscapeSequence

See 15.12.1.1

JSONEscapeCharacter :: one of

" / \ b f n r t

See 15.12.1.1

JSONNumber ::

-opt DecimalIntegerLiteral JSONFractionopt ExponentPartopt

See 15.12.1.1

JSONFraction ::

. DecimalDigits

See 15.12.1.1

JSONNullLiteral ::

NullLiteral

See 15.12.1.1

JSONBooleanLiteral ::

BooleanLiteral

A.8.2 JSON Syntactic Grammar
============================

See 15.12.1.2

JSONText :

JSONValue

See 15.12.1.2

JSONValue :

JSONNullLiteral

JSONBooleanLiteral

JSONObject

JSONArray

JSONString

JSONNumber

See 15.12.1.2

JSONObject :

{ }

{ JSONMemberList }

See 15.12.1.2

JSONMember :

JSONString : JSONValue

See 15.12.1.2

JSONMemberList :

JSONMember

JSONMemberList , JSONMember

See 15.12.1.2

JSONArray :

[ ]

[ JSONElementList ]

See 15.12.1.2

JSONElementList :

JSONValue

JSONElementList , JSONValue

Annex B (informative) Compatibility
===================================

B.1 Additional Syntax
=====================

Past editions of ECMAScript have included additional syntax and semantics for specifying octal
literals and octal escape sequences. These have been removed from this edition of ECMAScript. This
non-normative annex presents uniform syntax and semantics for octal literals and octal escape
sequences for compatibility with some older ECMAScript programs.

B.1.1 Numeric Literals
======================

The syntax and semantics of 7.8.3 can be extended as follows except that this extension is not
allowed for strict mode code:

Syntax
------

NumericLiteral ::

DecimalLiteral

HexIntegerLiteral

OctalIntegerLiteral

OctalIntegerLiteral ::

0 OctalDigit

OctalIntegerLiteral OctalDigit

OctalDigit :: one of

0 1 2 3 4 5 6 7

Semantics

-   The MV of NumericLiteral :: OctalIntegerLiteral is the MV of OctalIntegerLiteral.
-   The MV of OctalDigit :: 0 is 0.
-   The MV of OctalDigit :: 1 is 1.
-   The MV of OctalDigit :: 2 is 2.
-   The MV of OctalDigit :: 3 is 3.
-   The MV of OctalDigit :: 4 is 4.
-   The MV of OctalDigit :: 5 is 5.
-   The MV of OctalDigit :: 6 is 6.
-   The MV of OctalDigit :: 7 is 7.
-   The MV of OctalIntegerLiteral :: 0 OctalDigit is the MV of OctalDigit.
-   The MV of OctalIntegerLiteral :: OctalIntegerLiteral OctalDigit is (the MV of
    OctalIntegerLiteral times 8) plus the MV of OctalDigit.

B.1.2 String Literals
=====================

The syntax and semantics of 7.8.4 can be extended as follows except that this extension is not
allowed for strict mode code:

Syntax
------

EscapeSequence ::

CharacterEscapeSequence

OctalEscapeSequence

HexEscapeSequence

UnicodeEscapeSequence

OctalEscapeSequence ::

OctalDigit [lookahead ∉ DecimalDigit]

ZeroToThree OctalDigit [lookahead ∉ DecimalDigit]

FourToSeven OctalDigit

ZeroToThree OctalDigit OctalDigit

ZeroToThree :: one of

0 1 2 3

FourToSeven :: one of

4 5 6 7

Semantics
---------

-   The CV of EscapeSequence :: OctalEscapeSequence is the CV of the OctalEscapeSequence.

-   The CV of OctalEscapeSequence :: OctalDigit [lookahead ∉ DecimalDigit] is the character whose
    code unit value is the MV of the OctalDigit.

-   The CV of OctalEscapeSequence :: ZeroToThree OctalDigit [lookahead ∉ DecimalDigit] is the
    character whose code unit value is (8 times the MV of the ZeroToThree) plus the MV of the
    OctalDigit.

-   The CV of OctalEscapeSequence :: FourToSeven OctalDigit is the character whose code unit value
    is (8 times the MV of the FourToSeven) plus the MV of the OctalDigit.

-   The CV of OctalEscapeSequence :: ZeroToThree OctalDigit OctalDigit is the character whose code
    unit value is (64 (that is, 8^2) times the MV of the ZeroToThree) plus (8 times the MV of the
    first OctalDigit) plus the MV of the second OctalDigit.

-   The MV of ZeroToThree :: 0 is 0.

-   The MV of ZeroToThree :: 1 is 1.

-   The MV of ZeroToThree :: 2 is 2.

-   The MV of ZeroToThree :: 3 is 3.

-   The MV of FourToSeven :: 4 is 4.

-   The MV of FourToSeven :: 5 is 5.

-   The MV of FourToSeven :: 6 is 6.

-   The MV of FourToSeven :: 7 is 7.

B.2 Additional Properties
=========================

Some implementations of ECMAScript have included additional properties for some of the standard
native objects. This non-normative annex suggests uniform semantics for such properties without
making the properties or their semantics part of this standard.

B.2.1 escape (string)
=====================

The escape function is a property of the global object. It computes a new version of a String value
in which certain characters have been replaced by a hexadecimal escape sequence.

For those characters being replaced whose code unit value is 0xFF or less, a two-digit escape
sequence of the form %xx is used. For those characters being replaced whose code unit value is
greater than 0xFF, a four-digit escape sequence of the form %uxxxx is used.

When the escape function is called with one argument string, the following steps are taken:

1.  Call ToString(string).
2.  Compute the number of characters in Result(1).
3.  Let R be the empty string.
4.  Let k be 0.
5.  If k equals Result(2), return R.
6.  Get the character (represented as a 16-bit unsigned integer) at position k within Result(1).
7.  If Result(6) is one of the 69 nonblank characters  
    “ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789@*_+-./”then go to step 13.
8.  If Result(6), is less than 256, go to step 11.
9.  Let S be a String containing six characters “%uwxyz” where wxyz are four hexadecimal digits
    encoding the value of Result(6).
10. Go to step 14.
11. Let S be a String containing three characters “%xy” where xy are two hexadecimal digits encoding
    the value of Result(6).
12. Go to step 14.
13. Let S be a String containing the single character Result(6).
14. Let R be a new String value computed by concatenating the previous value of R and S.
15. Increase k by 1.
16. Go to step 5.

NOTE The encoding is partly based on the encoding described in RFC 1738, but the entire encoding
specified in this standard is described above without regard to the contents of RFC 1738. This
encoding does not reflect changes to RFC 1738 made by RFC 3986.

B.2.2 unescape (string)
=======================

The unescape function is a property of the global object. It computes a new version of a String
value in which each escape sequence of the sort that might be introduced by the escape function is
replaced with the character that it represents.

When the unescape function is called with one argument string, the following steps are taken:

1.  Call ToString(string).
2.  Compute the number of characters in Result(1).
3.  Let R be the empty String.
4.  Let k be 0.
5.  If k equals Result(2), return R.
6.  Let c be the character at position k within Result(1).
7.  If c is not %, go to step 18.
8.  If k is greater than Result(2)−6, go to step 14.
9.  If the character at position k+1 within Result(1) is not u, go to step 14.
10. If the four characters at positions k+2, k+3, k+4, and k+5 within Result(1) are not all
    hexadecimal digits, go to step 14.
11. Let c be the character whose code unit value is the integer represented by the four hexadecimal
    digits at positions k+2, k+3, k+4, and k+5 within Result(1).
12. Increase k by 5.
13. Go to step 18.
14. If k is greater than Result(2)−3, go to step 18.
15. If the two characters at positions k+1 and k+2 within Result(1) are not both hexadecimal digits,
    go to step 18.
16. Let c be the character whose code unit value is the integer represented by two zeroes plus the
    two hexadecimal digits at positions k+1 and k+2 within Result(1).
17. Increase k by 2.
18. Let R be a new String value computed by concatenating the previous value of R and c.
19. Increase k by 1.
20. Go to step 5.

B.2.3 String.prototype.substr (start, length)
=============================================

The substr method takes two arguments, start and length, and returns a substring of the result of
converting the this object to a String, starting from character position start and running for
length characters (or through the end of the String if length is undefined). If start is negative,
it is treated as (sourceLength+start) where sourceLength is the length of the String. The result is
a String value, not a String object. The following steps are taken:

1.  Call ToString, giving it the this value as its argument.
2.  Call ToInteger(start).
3.  If length is undefined, use +∞; otherwise call ToInteger(length).
4.  Compute the number of characters in Result(1).
5.  If Result(2) is positive or zero, use Result(2); else use max(Result(4)+Result(2),0).
6.  Compute min(max(Result(3),0), Result(4)–Result(5)).
7.  If Result(6) ≤ 0, return the empty String “”.
8.  Return a String containing Result(6) consecutive characters from Result(1) beginning with the
    character at position Result(5).

The length property of the substr method is 2.

NOTE The substr function is intentionally generic; it does not require that its this value be a
String object. Therefore it can be transferred to other kinds of objects for use as a method.

B.2.4 Date.prototype.getYear ( )
================================

NOTE The getFullYear method is preferred for nearly all purposes, because it avoids the “year 2000
problem.”

When the getYear method is called with no arguments, the following steps are taken:

1.  Let t be this time value.
2.  If t is NaN, return NaN.
3.  Return YearFromTime(LocalTime(t)) − 1900.

B.2.5 Date.prototype.setYear (year)
===================================

NOTE The setFullYear method is preferred for nearly all purposes, because it avoids the “year 2000
problem.”

When the setYear method is called with one argument year, the following steps are taken:

1.  Let t be the result of LocalTime(this time value); but if this time value is NaN, let t be +0.
2.  Call ToNumber(year).
3.  If Result(2) is NaN, set the [[PrimitiveValue]] internal property of the this value to NaN and
    return NaN.
4.  If Result(2) is not NaN and 0 ≤ ToInteger(Result(2)) ≤ 99 then Result(4) is ToInteger(Result(2))
    + 1900. Otherwise, Result(4) is Result(2).
5.  Compute MakeDay(Result(4), MonthFromTime(t), DateFromTime(t)).
6.  Compute UTC(MakeDate(Result(5), TimeWithinDay(t))).
7.  Set the [[PrimitiveValue]] internal property of the this value to TimeClip(Result(6)).
8.  Return the value of the [[PrimitiveValue]] internal property of the this value.

B.2.6 Date.prototype.toGMTString ( )
====================================

NOTE The property toUTCString is preferred. The toGMTString property is provided principally for
compatibility with old code. It is recommended that the toUTCString property be used in new
ECMAScript code.

The Function object that is the initial value of Date.prototype.toGMTString is the same Function
object that is the initial value of Date.prototype.toUTCString.

Annex C (informative) The Strict Mode of ECMAScript
===================================================

The strict mode restriction and exceptions

-   The identifiers
    "implements“,”interface“,”let“,”package“,”private“,”protected“,”public“,”static“, and”yield" are
    classified as FutureReservedWord tokens within strict mode code. (7.6.12).

-   A conforming implementation, when processing strict mode code, may not extend the syntax of
    NumericLiteral (7.8.3) to include OctalIntegerLiteral as described in B.1.1.

-   A conforming implementation, when processing strict mode code (see 10.1.1), may not extend the
    syntax of EscapeSequence to include OctalEscapeSequence as described in B.1.2.

-   Assignment to an undeclared identifier or otherwise unresolvable reference does not create a
    property in the global object. When a simple assignment occurs within strict mode code, its
    LeftHandSide must not evaluate to an unresolvable Reference. If it does a ReferenceError
    exception is thrown (8.7.2). The LeftHandSide also may not be a reference to a data property
    with the attribute value {[[Writable]]:false}, to an accessor property with the attribute value
    {[[Set]]:undefined}, nor to a non-existent property of an object whose [[Extensible]] internal
    property has the value false. In these cases a TypeError exception is thrown (11.13.1).

-   The identifier eval or arguments may not appear as the LeftHandSideExpression of an Assignment
    operator (11.13) or of a PostfixExpression (11.3) or as the UnaryExpression operated upon by a
    Prefix Increment (11.4.4) or a Prefix Decrement (11.4.5) operator.

-   Arguments objects for strict mode functions define non-configurable accessor properties named
    "caller" and "callee" which throw a TypeError exception on access (10.6).

-   Arguments objects for strict mode functions do not dynamically share their array indexed
    property values with the corresponding formal parameter bindings of their functions. (10.6).

-   For strict mode functions, if an arguments object is created the binding of the local identifier
    arguments to the arguments object is immutable and hence may not be the target of an assignment
    expression. (10.5).

-   It is a SyntaxError if strict mode code contains an ObjectLiteral with more than one definition
    of any data property (11.1.5).

-   It is a SyntaxError if the Identifier "eval" or the Identifier "arguments" occurs as the
    Identifier in a PropertySetParameterList of a PropertyAssignment that is contained in strict
    code or if its FunctionBody is strict code (11.1.5).

-   Strict mode eval code cannot instantiate variables or functions in the variable environment of
    the caller to eval. Instead, a new variable environment is created and that environment is used
    for declaration binding instantiation for the eval code (10.4.2).

-   If this is evaluated within strict mode code, then the this value is not coerced to an object. A
    this value of null or undefined is not converted to the global object and primitive values are
    not converted to wrapper objects. The this value passed via a function call (including calls
    made using Function.prototype.apply and Function.prototype.call) do not coerce the passed this
    value to an object (10.4.3, 11.1.1, 15.3.4.3, 15.3.4.4).

-   When a delete operator occurs within strict mode code, a SyntaxError is thrown if its
    UnaryExpression is a direct reference to a variable, function argument, or function
    name(11.4.1).

-   When a delete operator occurs within strict mode code, a TypeError is thrown if the property to
    be deleted has the attribute { [[Configurable]]:false } (11.4.1).

-   It is a SyntaxError if a VariableDeclaration or VariableDeclarationNoIn occurs within strict
    code and its Identifier is eval or arguments (12.2.1).

-   Strict mode code may not include a WithStatement. The occurrence of a WithStatement in such a
    context is an SyntaxError (12.10).

-   It is a SyntaxError if a TryStatement with a Catch occurs within strict code and the Identifier
    of the Catch production is eval or arguments (12.14.1)

-   It is a SyntaxError if the identifier eval or arguments appears within a FormalParameterList of
    a strict mode FunctionDeclaration or FunctionExpression (13.1)

-   A strict mode function may not have two or more formal parameters that have the same name. An
    attempt to create such a function using a FunctionDeclaration, FunctionExpression, or Function
    constructor is a SyntaxError (13.1, 15.3.2).

-   An implementation may not extend, beyond that defined in this specification, the meanings within
    strict mode functions of properties named caller or arguments of function instances. ECMAScript
    code may not create or modify properties with these names on function objects that correspond to
    strict mode functions (10.6, 13.2, 15.3.4.5.3).

-   It is a SyntaxError to use within strict mode code the identifiers eval or arguments as the
    Identifier of a FunctionDeclaration or FunctionExpression or as a formal parameter name (13.1).
    Attempting to dynamically define such a strict mode function using the Function constructor
    (15.3.2) will throw a SyntaxError exception.

Annex D (informative) Corrections and Clarifications in the 5th Edition with Possible 3rd Edition Compatibility Impact
======================================================================================================================

Throughout: In the Edition 3 specification the meaning of phrases such as “as if by the expression
new   Array()” are subject to misinterpretation. In the Edition 5 specification text for all
internal references and invocations of standard built-in objects and methods has been clarified by
making it explicit that the intent is that the actual built-in object is to be used rather than the
current dynamic value of the correspondingly named property.

11.8.2, 11.8.3, 11.8.5: ECMAScript generally uses a left to right evaluation order, however the
Edition 3 specification language for the > and <= operators resulted in a partial right to left
order. The specification has been corrected for these operators such that it now specifies a full
left to right evaluation order. However, this change of order is potentially observable if
side-effects occur during the evaluation process.

11.1.4: Edition 5 clarifies the fact that a trailing comma at the end of an ArrayInitialiser does
not add to the length of the array. This is not a semantic change from Edition 3 but some
implementations may have previously misinterpreted this.

11.2.3: Edition 5 reverses the order of steps 2 and 3 of the algorithm. The original order as
specified in Editions 1 through 3 was incorrectly specified such that side-effects of evaluating
Arguments could affect the result of evaluating MemberExpression.

12.4: In Edition 3, an object is created, as if by new Object()to serve as the scope for resolving
the name of the exception parameter passed to a catch clause of a try statement. If the actual
exception object is a function and it is called from within the catch clause, the scope object will
be passed as the this value of the call. The body of the function can then define new properties on
its this value and those property names become visible identifiers bindings within the scope of the
catch clause after the function returns. In Edition 5, when an exception parameter is called as a
function, undefined is passed as the this value.

13: In Edition 3, the algorithm for the production FunctionExpression with an Identifier adds an
object created as if by new Object() to the scope chain to serve as a scope for looking up the name
of the function. The identifier resolution rules (10.1.4 in Edition 3) when applied to such an
object will, if necessary, follow the object’s prototype chain when attempting to resolve an
identifier. This means all the properties of Object.prototype are visible as identifiers within that
scope. In practice most implementations of Edition 3 have not implemented this semantics. Edition 5
changes the specified semantics by using a Declarative Environment Record to bind the name of the
function.

14: In Edition 3, the algorithm for the production SourceElements : SourceElements SourceElement did
not correctly propagate statement result values in the same manner as Block. This could result in
the eval function producing an incorrect result when evaluating a Program text. In practice most
implementations of Edition 3 have implemented the correct propagation rather than what was specified
in Edition 5.

15.10.6: RegExp.prototype is now a RegExp object rather than an instance of Object. The value of its
[[Class]] internal property which is observable using Object.prototype.toString is now “RegExp”
rather than “Object”.

Annex E (informative) Additions and Changes in the 5th Edition that Introduce Incompatibilities with the 3rd Edition
====================================================================================================================

7.1: Unicode format control characters are no longer stripped from ECMAScript source text before
processing. In Edition 5, if such a character appears in a StringLiteral or RegularExpressionLiteral
the character will be incorporated into the literal where in Edition 3 the character would not be
incorporated into the literal.

7.2: Unicode character <BOM> is now treated as whitespace and its presence in the middle of what
appears to be an identifier could result in a syntax error which would not have occurred in Edition
3

7.3: Line terminator characters that are preceded by an escape sequence are now allowed within a
string literal token. In Edition 3 a syntax error would have been produced.

7.8.5: Regular expression literals now return a unique object each time the literal is evaluated.
This change is detectable by any programs that test the object identity of such literal values or
that are sensitive to the shared side effects.

7.8.5: Edition 5 requires early reporting of any possible RegExp constructor errors that would be
produced when converting a RegularExpressionLiteral to a RegExp object. Prior to Edition 5
implementations were permitted to defer the reporting of such errors until the actual execution time
creation of the object.

7.8.5: In Edition 5 unescaped “/” characters may appear as a CharacterClass in a regular expression
literal. In Edition 3 such a character would have been interpreted as the final character of the
literal.

10.4.2: In Edition 5, indirect calls to the eval function use the global environment as both the
variable environment and lexical environment for the eval code. In Edition 3, the variable and
lexical environments of the caller of an indirect eval was used as the environments for the eval
code.

15.4.4: In Edition 5 all methods of Array.prototype are intentionally generic. In Edition 3 toString
and toLocaleString were not generic and would throw a TypeError exception if applied to objects that
were not instances of Array.

10.6: In Edition 5 the array indexed properties of argument objects that correspond to actual formal
parameters are enumerable. In Edition 3, such properties were not enumerable.

10.6: In Edition 5 the value of the [[Class]] internal property of an arguments object is
"Arguments". In Edition 3, it was "Object". This is observable if toString is called as a method of
an arguments object.

12.6.4: for-in statements no longer throw a TypeError if the in expression evaluates to null or
undefined. Instead, the statement behaves as if the value of the expression was an object with no
enumerable properties.

15: In Edition 5, the following new properties are defined on built-in objects that exist in Edition
3: Object.getPrototypeOf, Object.getOwnPropertyDescriptor, Object.getOwnPropertyNames,
Object.create, Object.defineProperty, Object.defineProperties, Object.seal, Object.freeze,
Object.preventExtensions, Object.isSealed, Object.isFrozen, Object.isExtensible, Object.keys,
Function.prototype.bind, Array.prototype.indexOf, Array.prototype.lastIndexOf,
Array.prototype.every, Array.prototype.some, Array.prototype.forEach, Array.prototype.map,
Array.prototype.filter, Array.prototype.reduce, Array.prototype.reduceRight, String.prototype.trim,
Date.now, Date.prototype.toISOString, Date.prototype.toJSON.

15: Implementations are now required to ignore extra arguments to standard built-in methods unless
otherwise explicitly specified. In Edition 3 the handling of extra arguments was unspecified and
implementations were explicitly allowed to throw a TypeError exception.

15.1.1: The value properties NaN, Infinity, and undefined of the Global Object have been changed to
be read-only properties.

15.1.2.1. Implementations are no longer permitted to restrict the use of eval in ways that are not a
direct call. In addition, any invocation of eval that is not a direct call uses the global
environment as its variable environment rather than the caller’s variable environment.

15.1.2.2: The specification of the function parseInt no longer allows implementations to treat
Strings beginning with a 0 character as octal values.

15.3.4.3: In Edition 3, a TypeError is thrown if the second argument passed to
Function.prototype.apply is neither an array object nor an arguments object. In Edition 5, the
second argument may be any kind of generic array-like object that has a valid length property.

15.3.4.3, 15.3.4.4: In Edition 3 passing undefined or null as the first argument to either
Function.prototype.apply or Function.prototype.call causes the global object to be passed to the
indirectly invoked target function as the this value. If the first argument is a primitive value the
result of calling ToObject on the primitive value is passed as the this value. In Edition 5, these
transformations are not performed and the actual first argument value is passed as the this value.
This difference will normally be unobservable to existing ECMAScript Edition 3 code because a
corresponding transformation takes place upon activation of the target function. However, depending
upon the implementation, this difference may be observable by host object functions called using
apply or call. In addition, invoking a standard built-in function in this manner with null or
undefined passed as the this value will in many cases cause behaviour in Edition 5 implementations
that differ from Edition 3 behaviour. In particular, in Edition 5 built-in functions that are
specified to actually use the passed this value as an object typically throw a TypeError exception
if passed null or undefined as the this value.

15.3.5.2: In Edition 5, the prototype property of Function instances is not enumerable. In Edition
3, this property was enumerable.

15.5.5.2: In Edition 5, the individual characters of a String object’s [[PrimitiveValue] may be
accessed as array indexed properties of the String object. These properties are non-writable and
non-configurable and shadow any inherited properties with the same names. In Edition 3, these
properties did not exist and ECMAScript code could dynamically add and remove writable properties
with such names and could access inherited properties with such names.

15.9.4.2: Date.parse is now required to first attempt to parse its argument as an ISO format string.
Programs that use this format but depended upon implementation specific behaviour (including
failure) may behave differently.

15.10.2.12: In Edition 5, \s now additionally matches <BOM>.

15.10.4.1: In Edition 3, the exact form of the String value of the source property of an object
created by the RegExp constructor is implementation defined. In Edition 5, the String must conform
to certain specified requirements and hence may be different from that produced by an Edition 3
implementation.

15.10.6.4: In Edition 3, the result of RegExp.prototype.toString need not be derived from the value
of the RegExp object’s source property. In Edition 5 the result must be derived from the source
property in a specified manner and hence may be different from the result produced by an Edition 3
implementation.

15.11.2.1, 15.11.4.3: In Edition 5, if an initial value for the message property of an Error object
is not specified via the Error constructor the initial value of the property is the empty String. In
Edition 3, such an initial value is implementation defined.

15.11.4.4: In Edition 3, the result of Error.prototype.toString is implementation defined. In
Edition 5, the result is fully specified and hence may differ from some Edition 3 implementations.

15.12: In Edition 5, the name JSON is defined in the global environment. In Edition 3, testing for
the presence of that name will show it to be undefined unless it is defined by the program or
implementation.

Annex F (informative) Technically Significant Corrections and Clarifications in the 5.1 Edition
===============================================================================================

7.8.4: CV definitions added for DoubleStringCharacter :: LineContinuation and SingleStringCharacter
:: LineContinuation .

10.2.1.1.3: The argument S is not ignored. It controls whether an exception is thrown when
attempting to set an immutable binding.

10.2.1.2.2: In algorithm step 5, true is passed as the last argument to [[DefineOwnProperty]].

10.5: Former algorithm step 5.e is now 5.f and a new step 5.e was added to restore compatibility
with 3rd Edition when redefining global functions.

11.5.3: In the final bullet item, use of IEEE 754 round-to-nearest mode is specified.

12.6.3: Missing ToBoolean restored in step 3.a.ii of both algorithms.

12.6.4: Additional final sentences in each of the last two paragraphs clarify certain property
enumeration requirements.

12.7, 12.8, 12.9: BNF modified to clarify that a continue or break statement without an Identifier
or a return statement without an Expression may have a LineTerminator before the semi-colon.

12.14: Step 3 of algorithm 1 and step 2.a of algorithm 3 are corrected such that the value field of
B is passed as a parameter rather than B itself.

15.1.2.2: In step 2 of algorithm, clarify that S may be the empty string.

15.1.2.3: In step 2 of algorithm clarify that trimmedString may be the empty string.

15.1.3: Added notes clarifying that ECMAScript’s URI syntax is based upon RFC 2396 and not the newer
RFC 3986. In the algorithm for Decode, a step was removed that immediately preceded the current step
4.d.vii.10.a because it tested for a condition that cannot occur.

15.2.3.7: Corrected use of variable P in steps 5 and 6 of algorithm.

15.2.4.2: Edition 5 handling of undefined and null as this value caused existing code to fail.
Specification modified to maintain compatibility with such code. New steps 1 and 2 added to the
algorithm.

15.3.4.3: Steps 5 and 7 of Edition 5 algorithm have been deleted because they imposed requirements
upon the argArray argument that are inconsistent with other uses of generic array-like objects.

15.4.4.12: In step 9.a, incorrect reference to relativeStart was replaced with a reference to
actualStart.

15.4.4.15: Clarified that the default value for fromIndex is the length minus 1 of the array.

15.4.4.18: In step 9 of the algorithm, undefined is now the specified return value.

15.4.4.22: In step 9.c.ii the first argument to the [[Call]] internal method has been changed to
undefined for consistency with the definition of Array.prototype.reduce.

15.4.5.1: In Algorithm steps 3.l.ii and 3.l.iii the variable name was inverted resulting in an
incorrectly inverted test.

15.5.4.9: Normative requirement concerning canonically equivalent strings deleted from paragraph
following algorithm because it is listed as a recommendation in NOTE 2.

15.5.4.14: In split algorithm step 11.a and 13.a, the positional order of the arguments to
SplitMatch was corrected to match the actual parameter signature of SplitMatch. In step
13.a.iii.7.d, lengthA replaces A.length.

15.5.5.2: In first paragraph, removed the implication that the individual character property access
had “array index” semantics. Modified algorithm steps 3 and 5 such that they do not enforce “array
index” requirement.

15.9.1.15: Specified legal value ranges for fields that lacked them. Eliminated “time-only” formats.
Specified default values for all optional fields.

15.10.2.2: The step numbers of the algorithm for the internal closure produced by step 2 were
incorrectly numbered in a manner that implied that they were steps of the outer algorithm.

15.10.2.6: In the abstract operation IsWordChar the first character in the list in step 3 is “a”
rather than “A”.

15.10.2.8: In the algorithm for the closure returned by the abstract operation CharacterSetMatcher,
the variable defined by step 3 and passed as an argument in step 4 was renamed to ch in order to
avoid a name conflict with a formal parameter of the closure.

15.10.6.2: Step 9.e was deleted because It performed an extra increment of i.

15.11.1.1: Removed requirement that the message own property is set to the empty String when the
message argument is undefined.

15.11.1.2: Removed requirement that the message own property is set to the empty String when the
message argument is undefined.

15.11.4.4: Steps 6-10 modified/added to correctly deal with missing or empty message property value.

15.11.1.2: Removed requirement that the message own property is set to the empty String when the
message argument is undefined.

15.12.3: In step 10.b.iii of the JA internal operation, the last element of the concatenation is
“]”.

B.2.1: Added to NOTE that the encoding is based upon RFC 1738 rather than the newer RFC 3986.

Annex C: An item was added corresponding to 7.6.12 regarding FutureReservedWords in strict mode.

Bibliography
============

1.  IEEE Std 754-2008: IEEE Standard for Floating-Point Arithmetic. Institute of Electrical and
    Electronic Engineers, New York (2008)
2.  The Unicode Consortium. The Unicode Standard, Version 3.0, defined by: The Unicode Standard,
    Version 3.0 (Reading, MA, Addison-Wesley, 2000. ISBN 0-201-61633-5)
3.  Unicode Inc. (2010), Unicode Technical Report #15: Unicode Normalization Forms
4.  ISO 8601:2004(E) Data elements and interchange formats – Information interchange –
    Representation of dates and times
5.  RFC 1738 “Uniform Resource Locators (URL)”, available at <http://tools.ietf.org/html/rfc1738>
6.  RFC 2396 “Uniform Resource Identifiers (URI): Generic Syntax”, available at
    <http://tools.ietf.org/html/rfc2396>
7.  RFC 3629 “UTF-8, a transformation format of ISO 10646”, available at
    <http://tools.ietf.org/html/rfc3629>
8.  RFC 4627 “The application/json Media Type for JavaScript Object Notation (JSON)” , available at
    <http://tools.ietf.org/html/rfc4627>