gotoAndPlay JavaScript Style Guide() {

Based on Airbnb JavaScript Style Guide.

A mostly reasonable approach to JavaScript

Other Style Guides

CSS & Sass

Types

1.1 Primitives: When you access a primitive type you work directly on its value.
- string
- number
- boolean
- null
- undefined
- symbol
- bigint
```
const foo = 1;
let bar = foo;

bar = 9;

console.log(foo, bar); // => 1, 9
```

1.2 Complex: When you access a complex type you work on a reference to its value.

object
array
function

const foo = [1, 2];
const bar = foo;

bar[0] = 9;

console.log(foo[0], bar[0]); // => 9, 9

References

2.1 Use const for all of your references; avoid using var. eslint: prefer-const, no-const-assign

Why? This ensures that you can’t reassign your references, which can lead to bugs and difficult to comprehend code.
```
// bad
var a = 1;
var b = 2;

// good
const a = 1;
const b = 2;
```

2.2 If you must reassign references, use let instead of var. eslint: no-var

Why? let is block-scoped rather than function-scoped like var.

// bad
var count = 1;
if (true) {
  count += 1;
}

// good, use the let.
let count = 1;
if (true) {
  count += 1;
}

2.3 Note that both let and const are block-scoped.

// const and let only exist in the blocks they are defined in.
{
  let a = 1;
  const b = 1;
}
console.log(a); // ReferenceError
console.log(b); // ReferenceError

Objects

3.1 Use the literal syntax for object creation. eslint: no-new-object

// bad
const item = new Object();

// good
const item = {};

3.2 Use computed property names when creating objects with dynamic property names.

Why? They allow you to define all the properties of an object in one place.

function getKey(k) {
  return `a key named ${k}`;
}

// bad
const obj = {
  id: 5,
  name: 'San Francisco',
};
obj[getKey('enabled')] = true;

// good
const obj = {
  id: 5,
  name: 'San Francisco',
  [getKey('enabled')]: true,
};

3.3 Use object method shorthand. eslint: object-shorthand

// bad
const atom = {
  value: 1,
  addValue: function (value) {
    return atom.value + value;
  },
};

// good
const atom = {
  value: 1,
  addValue: (value) => {
    return atom.value + value;
  },
};

3.4 Use property value shorthand. eslint: object-shorthand

Why? It is shorter and descriptive.

const lukeSkywalker = 'Luke Skywalker';

// bad
const obj = {
  lukeSkywalker: lukeSkywalker,
};

// good
const obj = {
  lukeSkywalker,
};

3.5 Only quote properties that are invalid identifiers. eslint: quote-props

Why? In general we consider it subjectively easier to read. It improves syntax highlighting, and is also more easily optimized by many JS engines.
```
// bad
const bad = {
  'foo': 3,
  'bar': 4,
  'data-blah': 5,
};

// good
const good = {
  foo: 3,
  bar: 4,
  'data-blah': 5,
};
```

3.6 Do not call Object.prototype methods directly, such as hasOwnProperty, propertyIsEnumerable, and isPrototypeOf. eslint: no-prototype-builtins

Why? These methods may be shadowed by properties on the object in question - consider { hasOwnProperty: false } - or, the object may be a null object (Object.create(null)).

// bad
console.log(object.hasOwnProperty(key));

// good
console.log(Object.prototype.hasOwnProperty.call(object, key));

// best
const has = Object.prototype.hasOwnProperty; // cache the lookup once, in module scope.
console.log(has.call(object, key));
/* or */
import has from 'has'; // https://www.npmjs.com/package/has
console.log(has(object, key));

3.7 Prefer the object spread operator over Object.assign to shallow-copy objects. Use the object rest operator to get a new object with certain properties omitted. eslint: prefer-object-spread

// very bad
const original = { a: 1, b: 2 };
const copy = Object.assign(original, { c: 3 }); // this mutates `original` ಠ_ಠ
delete copy.a; // so does this

// bad
const original = { a: 1, b: 2 };
const copy = Object.assign({}, original, { c: 3 }); // copy => { a: 1, b: 2, c: 3 }

// good
const original = { a: 1, b: 2 };
const copy = { ...original, c: 3 }; // copy => { a: 1, b: 2, c: 3 }

const { a, ...noA } = copy; // noA => { b: 2, c: 3 }

Arrays

4.1 Use the literal syntax for array creation. eslint: no-array-constructor
```
// bad
const items = new Array();

// good
const items = [];
```

4.2 Use Array#push instead of direct assignment to add items to an array.

const someStack = [];

// bad
someStack[someStack.length] = 'abracadabra';

// good
someStack.push('abracadabra');

4.3 Use array spreads ... to copy arrays.

// bad
const len = items.length;
const itemsCopy = [];
let i;

for (i = 0; i < len; i += 1) {
  itemsCopy[i] = items[i];
}

// good
const itemsCopy = [...items];

4.4 To convert an iterable object to an array, use spreads ... instead of Array.from.

const foo = document.querySelectorAll('.foo');

// good
const nodes = Array.from(foo);

// best
const nodes = [...foo];

4.5 Use Array.from for converting an array-like object to an array.

const arrLike = { 0: 'foo', 1: 'bar', 2: 'baz', length: 3 };

// bad
const arr = Array.prototype.slice.call(arrLike);

// good
const arr = Array.from(arrLike);

4.6 Use Array.from instead of spread ... for mapping over iterables, because it avoids creating an intermediate array.
```
// bad
const baz = [...foo].map(bar);

// good
const baz = Array.from(foo, bar);
```

4.7 Use return statements in array method callbacks. It’s ok to omit the return if the function body consists of a single statement returning an expression without side effects, following 8.2. eslint: array-callback-return

// good
[1, 2, 3].map((x) => {
  const y = x + 1;
  return x * y;
});

// good
[1, 2, 3].map((x) => x + 1);

// bad - no returned value means `acc` becomes undefined after the first iteration
[[0, 1], [2, 3], [4, 5]].reduce((acc, item, index) => {
  const flatten = acc.concat(item);
});

// good
[[0, 1], [2, 3], [4, 5]].reduce((acc, item, index) => {
  const flatten = acc.concat(item);
  return flatten;
});

// bad
inbox.filter((msg) => {
  const { subject, author } = msg;
  if (subject === 'Mockingbird') {
    return author === 'Harper Lee';
  } else {
    return false;
  }
});

// good
inbox.filter((msg) => {
  const { subject, author } = msg;
  if (subject === 'Mockingbird') {
    return author === 'Harper Lee';
  }

  return false;
});

4.8 Use line breaks after open and before close array brackets if an array has multiple lines

// bad
const arr = [
  [0, 1], [2, 3], [4, 5],
];

const objectInArray = [{
  id: 1,
}, {
  id: 2,
}];

const numberInArray = [
  1, 2,
];

// good
const arr = [[0, 1], [2, 3], [4, 5]];

const objectInArray = [
  {
    id: 1,
  },
  {
    id: 2,
  },
];

const numberInArray = [
  1,
  2,
];

Destructuring

5.1 Use object destructuring when accessing and using multiple properties of an object. eslint: prefer-destructuring

Why? Destructuring saves you from creating temporary references for those properties, and from repetitive access of the object. Repeating object access creates more repetitive code, requires more reading, and creates more opportunities for mistakes. Destructuring objects also provides a single site of definition of the object structure that is used in the block, rather than requiring reading the entire block to determine what is used.
```
// bad
function getFullName(user) {
  const firstName = user.firstName;
  const lastName = user.lastName;

  return `${firstName} ${lastName}`;
}

// good
function getFullName(user) {
  const { firstName, lastName } = user;
  return `${firstName} ${lastName}`;
}
```

5.2 Use array destructuring. eslint: prefer-destructuring

const arr = [1, 2, 3, 4];

// bad
const first = arr[0];
const second = arr[1];

// good
const [first, second] = arr;

5.3 Use object destructuring for multiple return values, not array destructuring.

Why? You can add new properties over time or change the order of things without breaking call sites.

// bad
function processInput(input) {
  // then a miracle occurs
  return [left, right, top, bottom];
}

// the caller needs to think about the order of return data
const [left, __, top] = processInput(input);

// good
function processInput(input) {
  // then a miracle occurs
  return { left, right, top, bottom };
}

// the caller selects only the data they need
const { left, top } = processInput(input);

Strings

6.1 Use single quotes '' for strings. eslint: quotes

// bad
const name = "Capt. Janeway";

// bad - template literals should contain interpolation or newlines
const name = `Capt. Janeway`;

// good
const name = 'Capt. Janeway';

6.2 Strings that cause the line to go over 100 characters should not be written across multiple lines using string concatenation.

Why? Broken strings are painful to work with and make code less searchable.

// bad
const errorMessage = 'This is a super long error that was thrown because \
of Batman. When you stop to think about how Batman had anything to do \
with this, you would get nowhere \
fast.';

// bad
const errorMessage = 'This is a super long error that was thrown because ' +
  'of Batman. When you stop to think about how Batman had anything to do ' +
  'with this, you would get nowhere fast.';

// good
const errorMessage = 'This is a super long error that was thrown because of Batman. When you stop to think about how Batman had anything to do with this, you would get nowhere fast.';

6.3 When programmatically building up strings, use template strings instead of concatenation. eslint: prefer-template template-curly-spacing

Why? Template strings give you a readable, concise syntax with proper newlines and string interpolation features.

// bad
function sayHi(name) {
  return 'How are you, ' + name + '?';
}

// bad
function sayHi(name) {
  return ['How are you, ', name, '?'].join();
}

// bad
function sayHi(name) {
  return `How are you, ${ name }?`;
}

// good
function sayHi(name) {
  return `How are you, ${name}?`;
}

6.4 Never use eval() on a string, it opens too many vulnerabilities. eslint: no-eval

6.5 Do not unnecessarily escape characters in strings. eslint: no-useless-escape

Why? Backslashes harm readability, thus they should only be present when necessary.
```
// bad
const foo = '\'this\' \i\s \"quoted\"';

// good
const foo = '\'this\' is "quoted"';
const foo = `my name is '${name}'`;
```

Functions

7.1 Avoid function declarations, prefer arrow functions. eslint: func-style

Why? Function declarations are hoisted, which means that it’s easy - too easy - to reference the function before it is defined in the file. This harms readability and maintainability. If you find that a function’s definition is large or complex enough that it is interfering with understanding the rest of the file, then perhaps it’s time to extract it to its own module!
```
// bad
function foo() {
  // ...
}

// good: function expression instead of function declaration
const foo = function () {
  // ...
};

// best: arrow function
const foo = () => {
  // ...
};
```

7.2 Wrap immediately invoked function expressions in parentheses. eslint: wrap-iife

Why? An immediately invoked function expression is a single unit - wrapping both it, and its invocation parens, in parens, cleanly expresses this. Note that in a world with modules everywhere, you almost never need an IIFE.
```
// immediately-invoked function expression (IIFE)
(function () {
  console.log('Welcome to the Internet. Please follow me.');
}());
```

7.3 Never declare a function in a non-function block (if, while, etc). Assign the function to a variable instead. Browsers will allow you to do it, but they all interpret it differently, which is bad news bears. eslint: no-loop-func

7.4 Note: ECMA-262 defines a block as a list of statements. A function declaration is not a statement.

// bad
if (currentUser) {
  function test() {
    console.log('Nope.');
  }
}

// good
let test;
if (currentUser) {
  test = () => {
    console.log('Yup.');
  };
}

7.5 Never name a parameter arguments. This will take precedence over the arguments object that is given to every function scope.

// bad
function foo(name, options, arguments) {
  // ...
}

// good
function foo(name, options, args) {
  // ...
}

7.6 Never use arguments, opt to use rest syntax ... instead. eslint: prefer-rest-params

Why? ... is explicit about which arguments you want pulled. Plus, rest arguments are a real Array, and not merely Array-like like arguments.
```
// bad
function concatenateAll() {
  const args = Array.prototype.slice.call(arguments);
  return args.join('');
}

// good
function concatenateAll(...args) {
  return args.join('');
}
```

7.7 Use default parameter syntax rather than mutating function arguments.

// really bad
function handleThings(opts) {
  // No! We shouldn’t mutate function arguments.
  // Double bad: if opts is falsy it'll be set to an object which may
  // be what you want but it can introduce subtle bugs.
  opts = opts || {};
  // ...
}

// still bad
function handleThings(opts) {
  if (opts === void 0) {
    opts = {};
  }
  // ...
}

// good
function handleThings(opts = {}) {
  // ...
}

7.8 Avoid side effects with default parameters.

Why? They are confusing to reason about.

var b = 1;
// bad
function count(a = b++) {
  console.log(a);
}
count();  // 1
count();  // 2
count(3); // 3
count();  // 3

7.9 Always put default parameters last. eslint: default-param-last

// bad
function handleThings(opts = {}, name) {
  // ...
}

// good
function handleThings(name, opts = {}) {
  // ...
}

7.10 Add as few parameters as possible.

Having many parameters on a function makes refactoring difficult. If a function has lots of parameters and it cannot be broken down, consider using an options object to combine multiple parameters into one.
```
// bad
function handleThings(name, a, b, c, d, e) {
  // ...
}

// good
function handleThings(name, opts = {}) {
  const {a, b, c, d, e} = opts;
  // ...
}
```

7.11 Never use the Function constructor to create a new function. eslint: no-new-func

Why? Creating a function in this way evaluates a string similarly to eval(), which opens vulnerabilities.
```
// bad
var add = new Function('a', 'b', 'return a + b');

// still bad
var subtract = Function('a', 'b', 'return a - b');
```

7.12 Spacing in a function signature. eslint: space-before-function-paren space-before-blocks

Why? Consistency is good, and you shouldn’t have to add or remove a space when adding or removing a name.
```
// bad
const f = function(){};
const g = function (){};
const h = function() {};

// good
const x = function () {};
const y = function a() {};
```

7.13 Avoid mutating parameters. eslint: no-param-reassign

Why? Manipulating objects passed in as parameters can cause unwanted variable side effects in the original caller.

// bad - the caller wouldn't expect order to be modified
function isFree(order) {
    order.total = calculateTotal(order);
    return order.total === 0;
}

// good
function isFree(order) {
    const total = calculateTotal(order);
    return total === 0;
}

Exceptions are allowed if they are communicated clearly.

// good - the name makes it clear that we modify the lineItem
function setLineItemTotals(lineItem, totalWithoutVat) {
    lineItem.totalWithoutVat = totalWithoutVat;
    lineItem.vatAmount = calculateVat(totalWithoutVat);
    lineItem.totalWithVat = totalWithoutVat + lineItem.vatAmount;
}

7.14 Never reassign parameters. eslint: no-param-reassign

Why? Reassigning parameters can lead to unexpected behavior, especially when accessing the arguments object. It can also cause optimization issues, especially in V8.

// bad
function f1(a) {
  a = 1;
  // ...
}

function f2(a) {
  if (!a) { a = 1; }
  // ...
}

// good
function f3(a) {
  const b = a || 1;
  // ...
}

function f4(a = 1) {
  // ...
}

7.15 Prefer the use of the spread operator ... to call variadic functions. eslint: prefer-spread

Why? It’s cleaner, you don’t need to supply a context, and you can not easily compose new with apply.

// bad
const x = [1, 2, 3, 4, 5];
console.log.apply(console, x);

// good
const x = [1, 2, 3, 4, 5];
console.log(...x);

// bad
new (Function.prototype.bind.apply(Date, [null, 2016, 8, 5]));

// good
new Date(...[2016, 8, 5]);

7.16 Functions with multiline signatures, or invocations, should be indented just like every other multiline list in this guide: with each item on a line by itself, with a trailing comma on the last item. eslint: function-paren-newline

// bad
function foo(bar,
             baz,
             quux) {
  // ...
}

// good
function foo(
  bar,
  baz,
  quux,
) {
  // ...
}

// bad
console.log(foo,
  bar,
  baz);

// good
console.log(
  foo,
  bar,
  baz,
);

Arrow Functions

8.1 When you must use an anonymous function (as when passing an inline callback), use arrow function notation. eslint: prefer-arrow-callback, arrow-spacing

Why? It creates a version of the function that executes in the context of this, which is usually what you want, and is a more concise syntax.

Why not? If you have a fairly complicated function, you might move that logic out into its own named function expression.
```
// bad
[1, 2, 3].map(function (x) {
  const y = x + 1;
  return x * y;
});

// good
[1, 2, 3].map((x) => {
  const y = x + 1;
  return x * y;
});
```

8.2 If the function body consists of a single statement returning an expression without side effects, omit the braces and use the implicit return. Otherwise, keep the braces and use a return statement. eslint: arrow-parens, arrow-body-style

Why? Syntactic sugar. It reads well when multiple functions are chained together.

// bad
[1, 2, 3].map((number) => {
  const nextNumber = number + 1;
  `A string containing the ${nextNumber}.`;
});

// good
[1, 2, 3].map((number) => `A string containing the ${number + 1}.`);

// good
[1, 2, 3].map((number) => {
  const nextNumber = number + 1;
  return `A string containing the ${nextNumber}.`;
});

// good
[1, 2, 3].map((number, index) => ({
  [index]: number,
}));

// No implicit return with side effects
function foo(callback) {
  const val = callback();
  if (val === true) {
    // Do something if callback returns true
  }
}

let bool = false;

// bad
foo(() => bool = true);

// good
foo(() => {
  bool = true;
});

8.3 In case the expression spans over multiple lines, wrap it in parentheses for better readability.

Why? It shows clearly where the function starts and ends.

// bad
['get', 'post', 'put'].map((httpMethod) => Object.prototype.hasOwnProperty.call(
    httpMagicObjectWithAVeryLongName,
    httpMethod,
  )
);

// good
['get', 'post', 'put'].map((httpMethod) => (
  Object.prototype.hasOwnProperty.call(
    httpMagicObjectWithAVeryLongName,
    httpMethod,
  )
));

8.4 Always include parentheses around arguments for clarity and consistency. eslint: arrow-parens

Why? Minimizes diff churn when adding or removing arguments.

// bad
[1, 2, 3].map(x => x * x);

// good
[1, 2, 3].map((x) => x * x);

// bad
[1, 2, 3].map(number => (
  `A long string with the ${number}. It’s so long that we don’t want it to take up space on the .map line!`
));

// good
[1, 2, 3].map((number) => (
  `A long string with the ${number}. It’s so long that we don’t want it to take up space on the .map line!`
));

// bad
[1, 2, 3].map(x => {
  const y = x + 1;
  return x * y;
});

// good
[1, 2, 3].map((x) => {
  const y = x + 1;
  return x * y;
});

8.5 Avoid confusing arrow function syntax (=>) with comparison operators (<=, >=). eslint: no-confusing-arrow

// bad
const itemHeight = (item) => item.height <= 256 ? item.largeSize : item.smallSize;

// bad
const itemHeight = (item) => item.height >= 256 ? item.largeSize : item.smallSize;

// good
const itemHeight = (item) => (item.height <= 256 ? item.largeSize : item.smallSize);

// good
const itemHeight = (item) => {
  const { height, largeSize, smallSize } = item;
  return height <= 256 ? largeSize : smallSize;
};

8.6 Enforce the location of arrow function bodies with implicit returns. eslint: implicit-arrow-linebreak

// bad
(foo) =>
  bar;

(foo) =>
  (bar);

// good
(foo) => bar;
(foo) => (bar);
(foo) => (
   bar
)

Classes & Constructors

9.1 Always use class. Avoid manipulating prototype directly.

Why? class syntax is more concise and easier to reason about.

// bad
function Queue(contents = []) {
  this.queue = [...contents];
}
Queue.prototype.pop = function () {
  const value = this.queue[0];
  this.queue.splice(0, 1);
  return value;
};

// good
class Queue {
  constructor(contents = []) {
    this.queue = [...contents];
  }
  pop() {
    const value = this.queue[0];
    this.queue.splice(0, 1);
    return value;
  }
}

9.2 Use extends for inheritance.

Why? It is a built-in way to inherit prototype functionality without breaking instanceof.

// bad
const inherits = require('inherits');
function PeekableQueue(contents) {
  Queue.apply(this, contents);
}
inherits(PeekableQueue, Queue);
PeekableQueue.prototype.peek = function () {
  return this.queue[0];
};

// good
class PeekableQueue extends Queue {
  peek() {
    return this.queue[0];
  }
}

9.3 Methods can return this to help with method chaining.

// bad
Jedi.prototype.jump = function () {
  this.jumping = true;
  return true;
};

Jedi.prototype.setHeight = function (height) {
  this.height = height;
};

const luke = new Jedi();
luke.jump(); // => true
luke.setHeight(20); // => undefined

// good
class Jedi {
  jump() {
    this.jumping = true;
    return this;
  }

  setHeight(height) {
    this.height = height;
    return this;
  }
}

const luke = new Jedi();

luke.jump()
  .setHeight(20);

9.4 It’s okay to write a custom toString() method, just make sure it works successfully and causes no side effects.

class Jedi {
  constructor(options = {}) {
    this.name = options.name || 'no name';
  }

  getName() {
    return this.name;
  }

  toString() {
    return `Jedi - ${this.getName()}`;
  }
}

9.5 Classes have a default constructor if one is not specified. An empty constructor function or one that just delegates to a parent class is unnecessary. eslint: no-useless-constructor

// bad
class Jedi {
  constructor() {}

  getName() {
    return this.name;
  }
}

// bad
class Rey extends Jedi {
  constructor(...args) {
    super(...args);
  }
}

// good
class Rey extends Jedi {
  constructor(...args) {
    super(...args);
    this.name = 'Rey';
  }
}

9.6 Avoid duplicate class members. eslint: no-dupe-class-members

Why? Duplicate class member declarations will silently prefer the last one - having duplicates is almost certainly a bug.

// bad
class Foo {
  bar() { return 1; }
  bar() { return 2; }
}

// good
class Foo {
  bar() { return 1; }
}

// good
class Foo {
  bar() { return 2; }
}

9.7 Class methods should use this or be made into a static method unless an external library or framework requires to use specific non-static methods. Being an instance method should indicate that it behaves differently based on properties of the receiver. eslint: class-methods-use-this

// bad
class Foo {
  bar() {
    console.log('bar');
  }
}

// good - this is used
class Foo {
  bar() {
    console.log(this.bar);
  }
}

// good - constructor is exempt
class Foo {
  constructor() {
    // ...
  }
}

// good - static methods aren't expected to use this
class Foo {
  static bar() {
    console.log('bar');
  }
}

9.8 Don't use return statements inside constructors. We always expect a constructor to return the constructed instance. eslint: no-constructor-return

// bad
class gotoAndDevelop {
  constructor(env) {
    // Do something with env
    // ...
    return this.getCompilers();
  }
  // ...
}
const compilers = new gotoAndDevelop(env);

// good
class gotoAndDevelop {
  constructor(env) {
    // Do something with env
    // Don't return anything
  }
  // ...
}
const dev = new gotoAndDevelop(env);
const compilers = dev.getCompilers();

Modules

10.1 Always use modules (import/export) over a non-standard module system. You can always transpile to your preferred module system.

Why? Modules are the future, let’s start using the future now.

// bad
const AirbnbStyleGuide = require('./AirbnbStyleGuide');
module.exports = AirbnbStyleGuide.es6;

// ok
import AirbnbStyleGuide from './AirbnbStyleGuide';
export { AirbnbStyleGuide.es6 as es6 };

// best
import { es6 } from './AirbnbStyleGuide';
export { es6 };

10.2 And do not export directly from an import.

Why? Although the one-liner is concise, having one clear way to import and one clear way to export makes things consistent.
```
// bad
export { es6 } from './AirbnbStyleGuide';

// good
import { es6 } from './AirbnbStyleGuide';
export { es6 };
```

10.3 Only import from a path in one place. eslint: no-duplicate-imports

Why? Having multiple lines that import from the same path can make code harder to maintain.

// bad
import foo from 'foo';
// … some other imports … //
import { named1, named2 } from 'foo';

// good
import foo, { named1, named2 } from 'foo';

// good
import foo, {
  named1,
  named2,
} from 'foo';

10.4 Do not export mutable bindings. eslint: import/no-mutable-exports

Why? Mutation should be avoided in general, but in particular when exporting mutable bindings. While this technique may be needed for some special cases, in general, only constant references should be exported.
```
// bad
let foo = 3;
export { foo };

// good
const foo = 3;
export { foo };
```

10.5 Avoid default exports, always use named exports. eslint: import/no-default-export

Why? There will almost always be named exports in a project, especially in TypeScript code where a single file often exports one or more types in addition to the primary value. For the sake of consistency, avoid mixing default and named exports/imports. Named exports also provide unambiguous variable names for automatic imports, whereas additional conventions are needed with default exports.
```
// bad
export default function foo() {}

// good
export function foo() {}
```

10.6 Put all imports above non-import statements. eslint: import/first

Why? Since imports are hoisted, keeping them all at the top prevents surprising behavior.

// bad
import foo from 'foo';
foo.init();

import bar from 'bar';

// good
import foo from 'foo';
import bar from 'bar';

foo.init();

10.7 Multiline imports should be indented just like multiline array and object literals. eslint: object-curly-newline

Why? The curly braces follow the same indentation rules as every other curly brace block in the style guide, as do the trailing commas.
```
// bad
import {longNameA, longNameB, longNameC, longNameD, longNameE} from 'path';

// good
import {
  longNameA,
  longNameB,
  longNameC,
  longNameD,
  longNameE,
} from 'path';
```

10.8 Disallow Webpack loader syntax in module import statements. eslint: import/no-webpack-loader-syntax

Why? Since using Webpack syntax in the imports couples the code to a module bundler. Prefer using the loader syntax in webpack.config.js.
```
// bad
import fooSass from 'css!sass!foo.scss';
import barCss from 'style!css!bar.css';

// good
import fooSass from 'foo.scss';
import barCss from 'bar.css';
```

Iterators and Generators

11.1 Prefer JavaScript’s higher-order functions instead of iteration.

Why? This enforces our immutable rule. Dealing with pure functions that return values is easier to reason about than side effects.

Use map() / every() / filter() / find() / findIndex() / reduce() / some() / ... to iterate over arrays, and Object.keys() / Object.values() / Object.entries() to produce arrays so you can iterate over objects.

const numbers = [1, 2, 3, 4, 5];

// bad
const increasedByOne = [];
for (let i = 0; i < numbers.length; i++) {
    increasedByOne.push(numbers[i] + 1);
}

// good
const increasedByOne = numbers.map((num) => num + 1);

// bad
const evenNumbers = [];
numbers.forEach((num) => {
    if (num % 2 === 0) {
        evenNumbers.push(num);
    }
});

// good
const evenNumbers = numbers.filter((num) => num % 2 === 0);

// bad
let anyEvenNumbers = false;
for (const num of numbers) {
    if (num % 2 === 0) {
        anyEvenNumbers = true;
        break;
    }
}

// good
const anyEvenNumbers = numbers.some((num) => num % 2 === 0);

// bad
let sum = 0;
for (let num of numbers) {
    sum += num;
}
sum === 15;

// good
const sum = numbers.reduce((total, num) => total + num, 0);
sum === 15;

Avoid side effects in functions that should be pure, such as callbacks passed to methods like .map(), .filter() etc.

// bad - we expect .map() to return a new array without side effects
const increasedByOne = numbers.map((num) => {
    if (num === 3) {
        sendEmail();
    }

    return num + 1;
});

// good - side effect is separated from pure function
const increasedByOne = numbers.map((num) => num + 1);

if (numbers.includes(3)) {
    sendEmail();
}

Do not use for-in loops. eslint: no-restricted-syntax

First, look for an array method that might do what you need.

If you don't find one, use .forEach() or a for-of loop instead.

// bad - don't use for-in
for (let num in numbers) {
    console.log(num);
}

// bad - should use .reduce() like above
let sum = 0;
numbers.forEach((num) => sum += num);

// good - iteration is suitable for side effects
numbers.forEach((num) => console.log(num));

Prefer .forEach() at the end of an array method "chain" over a for-of loop

// bad
for (const num of numbers.filter((num) => num % 2 === 0).map((num) => num * 5)) {
    console.log(num);
}

// good
numbers.filter((num) => num % 2 === 0)
    .map((num) => num * 5)
    .forEach((num) => console.log(num));

11.2 Make sure the function signature of generators is spaced properly. eslint: generator-star-spacing

Why? function and * are part of the same conceptual keyword - * is not a modifier for function, function* is a unique construct, different from function.

// bad
function * foo() {
  // ...
}

// bad
const bar = function * () {
  // ...
};

// bad
const baz = function *() {
  // ...
};

// bad
const quux = function*() {
  // ...
};

// bad
function*foo() {
  // ...
}

// bad
function *foo() {
  // ...
}

// very bad
function
*
foo() {
  // ...
}

// very bad
const wat = function
*
() {
  // ...
};

// good
function* foo() {
  // ...
}

// good
const foo = function* () {
  // ...
};

Properties

12.1 Use dot notation when accessing properties. eslint: dot-notation

const luke = {
  jedi: true,
  age: 28,
};

// bad
const isJedi = luke['jedi'];

// good
const isJedi = luke.jedi;

12.2 Use bracket notation [] when accessing properties with a variable.

const luke = {
  jedi: true,
  age: 28,
};

function getProp(prop) {
  return luke[prop];
}

const isJedi = getProp('jedi');

12.3 Use exponentiation operator ** when calculating exponentiations. eslint: no-restricted-properties.
```
// bad
const binary = Math.pow(2, 10);

// good
const binary = 2 ** 10;
```

Variables

13.1 Always use const or let to declare variables. Not doing so will result in global variables. We want to avoid polluting the global namespace. Captain Planet warned us of that. eslint: no-undef prefer-const
```
// bad
superPower = new SuperPower();

// good
const superPower = new SuperPower();
```

13.2 Use one const or let declaration per variable or assignment. eslint: one-var

Why? It’s easier to add new variable declarations this way, and you never have to worry about swapping out a ; for a , or introducing punctuation-only diffs. You can also step through each declaration with the debugger, instead of jumping through all of them at once.

// bad
const items = getItems(),
    goSportsTeam = true,
    dragonball = 'z';

// bad
// (compare to above, and try to spot the mistake)
const items = getItems(),
    goSportsTeam = true;
    dragonball = 'z';

// good
const items = getItems();
const goSportsTeam = true;
const dragonball = 'z';

13.3 Group all your consts and then group all your lets.

Why? This is helpful when later on you might need to assign a variable depending on one of the previously assigned variables.

// bad
let i, len, dragonball,
    items = getItems(),
    goSportsTeam = true;

// bad
let i;
const items = getItems();
let dragonball;
const goSportsTeam = true;
let len;

// good
const goSportsTeam = true;
const items = getItems();
let dragonball;
let i;
let length;

13.4 Assign variables where you need them, but place them in a reasonable place.

Why? let and const are block scoped and not function scoped.

// bad - unnecessary function call
function checkName(hasName) {
  const name = getName();

  if (hasName === 'test') {
    return false;
  }

  if (name === 'test') {
    this.setName('');
    return false;
  }

  return name;
}

// good
function checkName(hasName) {
  if (hasName === 'test') {
    return false;
  }

  const name = getName();

  if (name === 'test') {
    this.setName('');
    return false;
  }

  return name;
}

13.5 Don’t chain variable assignments. eslint: no-multi-assign

Why? Chaining variable assignments creates implicit global variables.

// bad
(function example() {
  // JavaScript interprets this as
  // let a = ( b = ( c = 1 ) );
  // The let keyword only applies to variable a; variables b and c become
  // global variables.
  let a = b = c = 1;
}());

console.log(a); // throws ReferenceError
console.log(b); // 1
console.log(c); // 1

// good
(function example() {
  let a = 1;
  let b = a;
  let c = a;
}());

console.log(a); // throws ReferenceError
console.log(b); // throws ReferenceError
console.log(c); // throws ReferenceError

// the same applies for `const`

13.6 Avoid using unary increments and decrements (++, --). eslint no-plusplus

Why? Per the eslint documentation, unary increment and decrement statements are subject to automatic semicolon insertion and can cause silent errors with incrementing or decrementing values within an application. It is also more expressive to mutate your values with statements like num += 1 instead of num++ or num ++. Disallowing unary increment and decrement statements also prevents you from pre-incrementing/pre-decrementing values unintentionally which can also cause unexpected behavior in your programs.
```
// bad

const array = [1, 2, 3];
let num = 1;
num++;
--num;

let sum = 0;
let truthyCount = 0;
for (let i = 0; i < array.length; i++) {
  let value = array[i];
  sum += value;
  if (value) {
    truthyCount++;
  }
}

// good

const array = [1, 2, 3];
let num = 1;
num += 1;
num -= 1;

const sum = array.reduce((a, b) => a + b, 0);
const truthyCount = array.filter(Boolean).length;
```

13.7 Avoid linebreaks before or after = in an assignment. If your assignment violates max-len, surround the value in parens. eslint operator-linebreak.

Why? Linebreaks surrounding = can obfuscate the value of an assignment.

// bad
const foo =
  superLongLongLongLongLongLongLongLongFunctionName();

// bad
const foo
  = 'superLongLongLongLongLongLongLongLongString';

// good
const foo = (
  superLongLongLongLongLongLongLongLongFunctionName()
);

// good
const foo = 'superLongLongLongLongLongLongLongLongString';

13.8 Disallow unused variables. eslint: no-unused-vars

Why? Variables that are declared and not used anywhere in the code are most likely an error due to incomplete refactoring. Such variables take up space in the code and can lead to confusion by readers.

// bad

var some_unused_var = 42;

// Write-only variables are not considered as used.
var y = 10;
y = 5;

// A read for a modification of itself is not considered as used.
var z = 0;
z = z + 1;

// Unused function arguments.
function getX(x, y) {
    return x;
}

// good

function getXPlusY(x, y) {
  return x + y;
}

var x = 1;
var y = a + 2;

alert(getXPlusY(x, y));

// 'type' is ignored even if unused because it has a rest property sibling.
// This is a form of extracting an object that omits the specified keys.
var { type, ...coords } = data;
// 'coords' is now the 'data' object without its 'type' property.

Hoisting

14.1 var declarations get hoisted to the top of their closest enclosing function scope, their assignment does not. const and let declarations are blessed with a new concept called Temporal Dead Zones (TDZ). It’s important to know why typeof is no longer safe.

// we know this wouldn’t work (assuming there
// is no notDefined global variable)
function example() {
  console.log(notDefined); // => throws a ReferenceError
}

// creating a variable declaration after you
// reference the variable will work due to
// variable hoisting. Note: the assignment
// value of `true` is not hoisted.
function example() {
  console.log(declaredButNotAssigned); // => undefined
  var declaredButNotAssigned = true;
}

// the interpreter is hoisting the variable
// declaration to the top of the scope,
// which means our example could be rewritten as:
function example() {
  let declaredButNotAssigned;
  console.log(declaredButNotAssigned); // => undefined
  declaredButNotAssigned = true;
}

// using const and let
function example() {
  console.log(declaredButNotAssigned); // => throws a ReferenceError
  console.log(typeof declaredButNotAssigned); // => throws a ReferenceError
  const declaredButNotAssigned = true;
}

14.2 Anonymous function expressions hoist their variable name, but not the function assignment.

function example() {
  console.log(anonymous); // => undefined

  anonymous(); // => TypeError anonymous is not a function

  var anonymous = function () {
    console.log('anonymous function expression');
  };
}

14.3 Named function expressions hoist the variable name, not the function name or the function body.

function example() {
  console.log(named); // => undefined

  named(); // => TypeError named is not a function

  superPower(); // => ReferenceError superPower is not defined

  var named = function superPower() {
    console.log('Flying');
  };
}

// the same is true when the function name
// is the same as the variable name.
function example() {
  console.log(named); // => undefined

  named(); // => TypeError named is not a function

  var named = function named() {
    console.log('named');
  };
}

14.4 Function declarations hoist their name and the function body.

function example() {
  superPower(); // => Flying

  function superPower() {
    console.log('Flying');
  }
}

For more information refer to JavaScript Scoping & Hoisting by Ben Cherry.

Comparison Operators & Equality

15.1 Use === and !== over == and !=. eslint: eqeqeq

15.2 Conditional statements such as the if statement evaluate their expression using coercion with the ToBoolean abstract method and always follow these simple rules:
- Objects evaluate to true
- Undefined evaluates to false
- Null evaluates to false
- Booleans evaluate to the value of the boolean
- Numbers evaluate to false if +0, -0, or NaN, otherwise true
- Strings evaluate to false if an empty string '', otherwise true
```
if ([0] && []) {
  // true
  // an array (even an empty one) is an object, objects will evaluate to true
}
```

15.3 Use shortcuts in conditions.

// bad
if (isValid === true) {
  // ...
}

// good
if (isValid) {
  // ...
}

// bad - the value might also be null or undefined
if (name !== '') {
  // ...
}

// good - this avoids empty strings and all other falsy values
if (name) {
  // ...
}

// bad
if (collection.length > 0) {
  // ...
}

// good
if (collection.length) {
  // ...
}

// ok - negative numbers are truthy, only 0 is falsy
if (possiblyNegativeNumber > 0) {
  // ...
}

// bad - can be misleading since an empty array is truthy in JS, but
// falsy in PHP, for example
if (collection) {
  // ...
}

// best - ensures the array is neither falsy nor empty
if (collection?.length) {
  // ...
}

15.4 For more information see Truth Equality and JavaScript by Angus Croll.

15.5 Use braces to create blocks in case and default clauses that contain lexical declarations (e.g. let, const, function, and class). eslint: no-case-declarations

Why? Lexical declarations are visible in the entire switch block but only get initialized when assigned, which only happens when its case is reached. This causes problems when multiple case clauses attempt to define the same thing.

// bad
switch (foo) {
  case 1:
    let x = 1;
    break;
  case 2:
    const y = 2;
    break;
  case 3:
    function f() {
      // ...
    }
    break;
  default:
    class C {}
}

// good
switch (foo) {
  case 1: {
    let x = 1;
    break;
  }
  case 2: {
    const y = 2;
    break;
  }
  case 3: {
    function f() {
      // ...
    }
    break;
  }
  case 4:
    bar();
    break;
  default: {
    class C {}
  }
}

15.6 Ternaries should not be nested and generally be single line expressions. eslint: no-nested-ternary

// bad
const foo = maybe1 > maybe2
  ? "bar"
  : value1 > value2 ? "baz" : null;

// split into 2 separated ternary expressions
const maybeNull = value1 > value2 ? 'baz' : null;

// better
const foo = maybe1 > maybe2
  ? 'bar'
  : maybeNull;

// best
const foo = maybe1 > maybe2 ? 'bar' : maybeNull;

15.7 Avoid unneeded ternary statements. eslint: no-unneeded-ternary

// bad
const bar = c ? true : false;
const baz = c ? false : true;
const foo = a ? a : b;

// good
const bar = !!c;
const baz = !c;

// good if you want to use b when a is any falsy value (including 0 and '')
const foo = a || b;

// good if you want to use b ONLY when a is undefined or null
const foo = a ?? b;

// bad
const values = values ? values.map((value) => value.isValid()) : [];

// good (assuming values is either an array or undefined or null)
const values = (values ?? []).map((value) => value.isValid());

15.8 Use the optional chaining operator ?. instead of a chain of &&.

// bad
const authorName = comment && comment.post && comment.post.author && comment.post.author.name;

// good
const authorName = comment?.post?.author?.name;

// bad
if (data && data.order && data.order.isCompleted()) {

// good
if (data?.order?.isCompleted()) {

// good
for (const item of data?.order?.getItems() ?? []) {

15.9 When mixing operators, enclose them in parentheses. The only exception is the standard arithmetic operators: +, -, and ** since their precedence is broadly understood. We recommend enclosing / and * in parentheses because their precedence can be ambiguous when they are mixed. eslint: no-mixed-operators

Why? This improves readability and clarifies the developer’s intention.

// bad
const foo = a && b < 0 || c > 0 || d + 1 === 0;

// bad
const bar = a ** b - 5 % d;

// bad
// one may be confused into thinking (a || b) && c
if (a || b && c) {
  return d;
}

// bad
const bar = a + b / c * d;

// good
const foo = (a && b < 0) || c > 0 || (d + 1 === 0);

// good
const bar = a ** b - (5 % d);

// good
if (a || (b && c)) {
  return d;
}

// good
const bar = a + (b / c) * d;

Blocks

16.1 Use braces with all blocks. eslint: curly

// bad
if (test)
  return false;

// bad
if (test) return false;

// good
if (test) {
  return false;
}

// bad
function foo() { return false; }

// good
function bar() {
  return false;
}

16.2 If you’re using multiline blocks with if and else, put else on the same line as your if block’s closing brace. eslint: brace-style

// bad
if (test) {
  thing1();
  thing2();
}
else {
  thing3();
}

// good
if (test) {
  thing1();
  thing2();
} else {
  thing3();
}

16.3 If an if block always executes a return statement, the subsequent else block is unnecessary. A return in an else if block following an if block that contains a return can be separated into multiple if blocks. eslint: no-else-return

// bad
function foo() {
  if (x) {
    return x;
  } else {
    return y;
  }
}

// bad
function cats() {
  if (x) {
    return x;
  } else if (y) {
    return y;
  }
}

// bad
function dogs() {
  if (x) {
    return x;
  } else {
    if (y) {
      return y;
    }
  }
}

// good
function foo() {
  if (x) {
    return x;
  }

  return y;
}

// good
function cats() {
  if (x) {
    return x;
  }

  if (y) {
    return y;
  }
}

// good
function dogs(x) {
  if (x) {
    if (z) {
      return y;
    }
  } else {
    return z;
  }
}

Control Statements

17.1 In case your control statement (if, while etc.) gets too long or exceeds the maximum line length, each (grouped) condition could be put into a new line. The logical operator should begin the line.

Why? Requiring operators at the beginning of the line keeps the operators aligned and follows a pattern similar to method chaining. This also improves readability by making it easier to visually follow complex logic.

// bad
if ((foo === 123 || bar === 'abc') && doesItLookGoodWhenItBecomesThatLong() && isThisReallyHappening()) {
  thing1();
}

// bad
if (foo === 123 &&
  bar === 'abc') {
  thing1();
}

// bad
if (foo === 123
  && bar === 'abc') {
  thing1();
}

// bad
if (
  foo === 123 &&
  bar === 'abc'
) {
  thing1();
}

// good
if (
  foo === 123
  && bar === 'abc'
) {
  thing1();
}

// good
if (
  (foo === 123 || bar === 'abc')
  && doesItLookGoodWhenItBecomesThatLong()
  && isThisReallyHappening()
) {
  thing1();
}

// good
if (foo === 123 && bar === 'abc') {
  thing1();
}

17.2 Don't use selection operators in place of control statements.

// bad
!isRunning && startRunning();

// good
if (!isRunning) {
  startRunning();
}

Comments

18.1 Use /** ... */ for multiline comments.

// bad
// make() returns a new element
// based on the passed in tag name
//
// @param {String} tag
// @return {Element} element
function make(tag) {

  // ...

  return element;
}

// good
/**
 * make() returns a new element
 * based on the passed-in tag name
 */
function make(tag) {

  // ...

  return element;
}

18.2 Use // for single line comments. Place single line comments on a newline above the subject of the comment. Put an empty line before the comment unless it’s on the first line of a block.

// bad
const active = true;  // is current tab

// good
// is current tab
const active = true;

// bad
function getType() {
  console.log('fetching type...');
  // set the default type to 'no type'
  const type = this.type || 'no type';

  return type;
}

// good
function getType() {
  console.log('fetching type...');

  // set the default type to 'no type'
  const type = this.type || 'no type';

  return type;
}

// also good
function getType() {
  // set the default type to 'no type'
  const type = this.type || 'no type';

  return type;
}

18.3 Start all comments with a space to make it easier to read. eslint: spaced-comment

// bad
//is current tab
const active = true;

// good
// is current tab
const active = true;

// bad
/**
 *make() returns a new element
 *based on the passed-in tag name
 */
function make(tag) {

  // ...

  return element;
}

// good
/**
 * make() returns a new element
 * based on the passed-in tag name
 */
function make(tag) {

  // ...

  return element;
}

18.4 Prefixing your comments with FIXME or TODO helps other developers quickly understand if you’re pointing out a problem that needs to be revisited, or if you’re suggesting a solution to the problem that needs to be implemented. These are different than regular comments because they are actionable. The actions are FIXME: -- need to figure this out or TODO: -- need to implement.

18.5 Use // FIXME: to annotate problems.

class Calculator extends Abacus {
  constructor() {
    super();

    // FIXME: shouldn’t use a global here
    total = 0;
  }
}

18.6 Use // TODO: to annotate solutions to problems.

class Calculator extends Abacus {
  constructor() {
    super();

    // TODO: total should be configurable by an options param
    this.total = 0;
  }
}

Whitespace

19.1 Use soft tabs (space character) set to 4 spaces. eslint: indent

// bad
function bar() {
∙let name;
}

// bad
function baz() {
∙∙let name;
}

// good
function foo() {
∙∙∙∙let name;
}

19.2 Place 1 space before the leading brace. eslint: space-before-blocks

// bad
function test(){
  console.log('test');
}

// good
function test() {
  console.log('test');
}

// bad
dog.set('attr',{
  age: '1 year',
  breed: 'Bernese Mountain Dog',
});

// good
dog.set('attr', {
  age: '1 year',
  breed: 'Bernese Mountain Dog',
});

19.3 Place 1 space before the opening parenthesis in control statements (if, while etc.). Place no space between the argument list and the function name in function calls and declarations. eslint: keyword-spacing

// bad
if(isJedi) {
  fight ();
}

// good
if (isJedi) {
  fight();
}

// bad
function fight () {
  console.log ('Swooosh!');
}

// good
function fight() {
  console.log('Swooosh!');
}

19.4 Set off operators with spaces. eslint: space-infix-ops
```
// bad
const x=y+5;

// good
const x = y + 5;
```

19.5 End files with a single newline character. eslint: eol-last

// bad
import { es6 } from './AirbnbStyleGuide';
  // ...
export { es6 };

// bad
import { es6 } from './AirbnbStyleGuide';
  // ...
export { es6 };↵
↵

// good
import { es6 } from './AirbnbStyleGuide';
  // ...
export { es6 };↵

19.6 Use indentation when making long method chains (more than 2 method chains). Use a leading dot, which emphasizes that the line is a method call, not a new statement. eslint: newline-per-chained-call no-whitespace-before-property

// bad
$('#items').find('.selected').highlight().end().find('.open').updateCount();

// bad
$('#items').
  find('.selected').
    highlight().
    end().
  find('.open').
    updateCount();

// good
$('#items')
  .find('.selected')
    .highlight()
    .end()
  .find('.open')
    .updateCount();

// bad
const leds = stage.selectAll('.led').data(data).enter().append('svg:svg').classed('led', true)
    .attr('width', (radius + margin) * 2).append('svg:g')
    .attr('transform', `translate(${radius + margin},${radius + margin})`)
    .call(tron.led);

// good
const leds = stage.selectAll('.led')
    .data(data)
  .enter().append('svg:svg')
    .classed('led', true)
    .attr('width', (radius + margin) * 2)
  .append('svg:g')
    .attr('transform', `translate(${radius + margin},${radius + margin})`)
    .call(tron.led);

// good
const leds = stage.selectAll('.led').data(data);

19.7 Leave a blank line after blocks and before the next statement.

// bad
if (foo) {
  return bar;
}
return baz;

// good
if (foo) {
  return bar;
}

return baz;

// bad
const obj = {
  foo() {
  },
  bar() {
  },
};
return obj;

// good
const obj = {
  foo() {
  },

  bar() {
  },
};

return obj;

// bad
const arr = [
  function foo() {
  },
  function bar() {
  },
];
return arr;

// good
const arr = [
  function foo() {
  },

  function bar() {
  },
];

return arr;

19.8 Do not pad your blocks with blank lines. eslint: padded-blocks

// bad
function bar() {

  console.log(foo);

}

// bad
if (baz) {

  console.log(qux);
} else {
  console.log(foo);

}

// bad
class Foo {

  constructor(bar) {
    this.bar = bar;
  }
}

// good
function bar() {
  console.log(foo);
}

// good
if (baz) {
  console.log(qux);
} else {
  console.log(foo);
}

19.9 Do not use multiple blank lines to pad your code. eslint: no-multiple-empty-lines

// bad
class Person {
  constructor(fullName, email, birthday) {
    this.fullName = fullName;


    this.email = email;


    this.setAge(birthday);
  }


  setAge(birthday) {
    const today = new Date();


    const age = this.getAge(today, birthday);


    this.age = age;
  }


  getAge(today, birthday) {
    // ..
  }
}

// good
class Person {
  constructor(fullName, email, birthday) {
    this.fullName = fullName;
    this.email = email;
    this.setAge(birthday);
  }

  setAge(birthday) {
    const today = new Date();
    const age = getAge(today, birthday);
    this.age = age;
  }

  getAge(today, birthday) {
    // ..
  }
}

19.10 Do not add spaces inside parentheses. eslint: space-in-parens

// bad
function bar( foo ) {
  return foo;
}

// good
function bar(foo) {
  return foo;
}

// bad
if ( foo ) {
  console.log(foo);
}

// good
if (foo) {
  console.log(foo);
}

19.11 Do not add spaces inside brackets. eslint: array-bracket-spacing

// bad
const foo = [ 1, 2, 3 ];
console.log(foo[ 0 ]);

// good
const foo = [1, 2, 3];
console.log(foo[0]);

19.12 Add spaces inside curly braces. eslint: object-curly-spacing

// bad
const foo = {clark: 'kent'};

// good
const foo = { clark: 'kent' };

19.13 Avoid having lines of code that are longer than 100 characters (including whitespace). Note: per above, long strings are exempt from this rule, and should not be broken up. eslint: max-len

Why? This ensures readability and maintainability.

// bad
const foo = someValue1 && someValue2 && someValue3 && someValue4 && someValue5 && someValue6 && someValue7;

// bad
$.ajax({ method: 'POST', url: 'https://airbnb.com/', data: { name: 'John' } }).done(() => console.log('Congratulations!')).fail(() => console.log('You have failed this city.'));

// good
const foo = someValue1
    && someValue2
    && someValue3
    && someValue4
    && someValue5
    && someValue6
    && someValue7;

// good
$.ajax({
  method: 'POST',
  url: 'https://airbnb.com/',
  data: { name: 'John' },
})
  .done(() => console.log('Congratulations!'))
  .fail(() => console.log('You have failed this city.'));

19.14 Require consistent spacing inside an open block token and the next token on the same line. This rule also enforces consistent spacing inside a close block token and previous token on the same line. eslint: block-spacing
```
// bad
function foo() {return true;}
if (foo) { bar = 0;}

// good
function foo() { return true; }
if (foo) { bar = 0; }
```

19.15 Avoid spaces before commas and require a space after commas. eslint: comma-spacing

// bad
var foo = 1,bar = 2;
var arr = [1 , 2];

// good
var foo = 1, bar = 2;
var arr = [1, 2];

19.16 Enforce spacing inside of computed property brackets. eslint: computed-property-spacing

// bad
obj[foo ]
obj[ 'foo']
var x = {[ b ]: a}
obj[foo[ bar ]]

// good
obj[foo]
obj['foo']
var x = { [b]: a }
obj[foo[bar]]

19.17 Avoid spaces between functions and their invocations. eslint: func-call-spacing
```
// bad
func ();

func
();

// good
func();
```

19.18 Enforce spacing between keys and values in object literal properties. eslint: key-spacing

// bad
var obj = { foo : 42 };
var obj2 = { foo:42 };

// good
var obj = { foo: 42 };

19.19 Avoid trailing spaces at the end of lines. eslint: no-trailing-spaces

19.20 Avoid multiple empty lines, only allow one newline at the end of files, and avoid a newline at the beginning of files. eslint: no-multiple-empty-lines

// bad - multiple empty lines
var x = 1;


var y = 2;

// bad - 2+ newlines at end of file
var x = 1;
var y = 2;


// bad - 1+ newline(s) at beginning of file

var x = 1;
var y = 2;

// good
var x = 1;
var y = 2;

Commas

20.1 Leading commas: Nope. eslint: comma-style

// bad
const story = [
    once
  , upon
  , aTime
];

// good
const story = [
  once,
  upon,
  aTime,
];

// bad
const hero = {
    firstName: 'Ada'
  , lastName: 'Lovelace'
  , birthYear: 1815
  , superPower: 'computers'
};

// good
const hero = {
  firstName: 'Ada',
  lastName: 'Lovelace',
  birthYear: 1815,
  superPower: 'computers',
};

20.2 Additional trailing comma: Yup. eslint: comma-dangle

Why? This leads to cleaner git diffs. Also, transpilers like Babel will remove the additional trailing comma in the transpiled code which means you don’t have to worry about the trailing comma problem in legacy browsers.

// bad - git diff without trailing comma
const hero = {
     firstName: 'Florence',
-    lastName: 'Nightingale'
+    lastName: 'Nightingale',
+    inventorOf: ['coxcomb chart', 'modern nursing']
};

// good - git diff with trailing comma
const hero = {
     firstName: 'Florence',
     lastName: 'Nightingale',
+    inventorOf: ['coxcomb chart', 'modern nursing'],
};

// bad
const hero = {
  firstName: 'Dana',
  lastName: 'Scully'
};

const heroes = [
  'Batman',
  'Superman'
];

// good
const hero = {
  firstName: 'Dana',
  lastName: 'Scully',
};

const heroes = [
  'Batman',
  'Superman',
];

// bad
function createHero(
  firstName,
  lastName,
  inventorOf
) {
  // does nothing
}

// good
function createHero(
  firstName,
  lastName,
  inventorOf,
) {
  // does nothing
}

// good (note that a comma must not appear after a "rest" element)
function createHero(
  firstName,
  lastName,
  inventorOf,
  ...heroArgs
) {
  // does nothing
}

// bad
createHero(
  firstName,
  lastName,
  inventorOf
);

// good
createHero(
  firstName,
  lastName,
  inventorOf,
);

// good (note that a comma must not appear after a "rest" element)
createHero(
  firstName,
  lastName,
  inventorOf,
  ...heroArgs
);

Semicolons

21.1 Yup. eslint: semi

Why? When JavaScript encounters a line break without a semicolon, it uses a set of rules called Automatic Semicolon Insertion to determine whether or not it should regard that line break as the end of a statement, and (as the name implies) place a semicolon into your code before the line break if it thinks so. ASI contains a few eccentric behaviors, though, and your code will break if JavaScript misinterprets your line break. These rules will become more complicated as new features become a part of JavaScript. Explicitly terminating your statements and configuring your linter to catch missing semicolons will help prevent you from encountering issues.

// bad - raises exception
const luke = {}
const leia = {}
[luke, leia].forEach((jedi) => jedi.father = 'vader')

// bad - raises exception
const reaction = "No! That’s impossible!"
(async function meanwhileOnTheFalcon() {
  // handle `leia`, `lando`, `chewie`, `r2`, `c3p0`
  // ...
}())

// bad - returns `undefined` instead of the value on the next line - always happens when `return` is on a line by itself because of ASI!
function foo() {
  return
    'search your feelings, you know it to be foo'
}

// good
const luke = {};
const leia = {};
[luke, leia].forEach((jedi) => {
  jedi.father = 'vader';
});

// good
const reaction = "No! That’s impossible!";
(async function meanwhileOnTheFalcon() {
  // handle `leia`, `lando`, `chewie`, `r2`, `c3p0`
  // ...
}());

// good
function foo() {
  return 'search your feelings, you know it to be foo';
}

Type Casting & Coercion

22.1 Perform type coercion at the beginning of the statement.

22.2 Strings: eslint: no-new-wrappers

// => this.reviewScore = 9;

// bad
const totalScore = new String(this.reviewScore); // typeof totalScore is "object" not "string"

// bad
const totalScore = this.reviewScore + ''; // invokes this.reviewScore.valueOf()

// bad
const totalScore = this.reviewScore.toString(); // isn’t guaranteed to return a string

// good
const totalScore = String(this.reviewScore);

22.3 Numbers: Use Number for type casting and parseInt always with a radix for parsing strings. eslint: radix no-new-wrappers

Why? The parseInt function produces an integer value dictated by interpretation of the contents of the string argument according to the specified radix. Leading whitespace 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. This differs from ECMAScript 3, which merely discouraged (but allowed) octal interpretation. Many implementations have not adopted this behavior as of 2013. And, because older browsers must be supported, always specify a radix.
```
const inputValue = '4';

// bad
const val = new Number(inputValue);

// bad
const val = +inputValue;

// bad
const val = inputValue >> 0;

// bad
const val = parseInt(inputValue);

// good
const val = Number(inputValue);

// good
const val = parseInt(inputValue, 10);
```

22.4 If for whatever reason you are doing something wild and parseInt is your bottleneck and need to use Bitshift for performance reasons, leave a comment explaining why and what you’re doing.
```
// good
/**
 * parseInt was the reason my code was slow.
 * Bitshifting the String to coerce it to a
 * Number made it a lot faster.
 */
const val = inputValue >> 0;
```

22.5 Note: Be careful when using bitshift operations. Numbers are represented as 64-bit values, but bitshift operations always return a 32-bit integer (source). Bitshift can lead to unexpected behavior for integer values larger than 32 bits. Discussion. Largest signed 32-bit Int is 2,147,483,647:
```
2147483647 >> 0; // => 2147483647
2147483648 >> 0; // => -2147483648
2147483649 >> 0; // => -2147483647
```

22.6 Booleans: eslint: no-new-wrappers

const age = 0;

// bad
const hasAge = new Boolean(age);

// good
const hasAge = Boolean(age);

// best
const hasAge = !!age;

Naming Conventions

23.1 Avoid single letter names. Be descriptive with your naming. eslint: id-length

// bad
function q() {
  // ...
}

// good
function query() {
  // ...
}

23.2 Use camelCase when naming objects, functions, and instances. eslint: camelcase

// bad
const OBJEcttsssss = {};
const this_is_my_object = {};
function c() {}

// good
const thisIsMyObject = {};
function thisIsMyFunction() {}

23.3 Use PascalCase only when naming constructors or classes. eslint: new-cap

// bad
function user(options) {
  this.name = options.name;
}

const bad = new user({
  name: 'nope',
});

// good
class User {
  constructor(options) {
    this.name = options.name;
  }
}

const good = new User({
  name: 'yup',
});

23.4 Do not use trailing or leading underscores. eslint: no-underscore-dangle

Why? JavaScript does not have the concept of privacy in terms of properties or methods. Although a leading underscore is a common convention to mean “private”, in fact, these properties are fully public, and as such, are part of your public API contract. This convention might lead developers to wrongly think that a change won’t count as breaking, or that tests aren’t needed. tl;dr: if you want something to be “private”, it must not be observably present.
```
// bad
this.__firstName__ = 'Panda';
this.firstName_ = 'Panda';
this._firstName = 'Panda';

// good
this.firstName = 'Panda';

// good, in environments where WeakMaps are available
// see https://kangax.github.io/compat-table/es6/#test-WeakMap
const firstNames = new WeakMap();
firstNames.set(this, 'Panda');
```

23.5 Don’t save references to this. Use arrow functions or Function#bind.

// bad
function foo() {
  const self = this;
  return function () {
    console.log(self);
  };
}

// bad
function foo() {
  const that = this;
  return function () {
    console.log(that);
  };
}

// good
function foo() {
  return () => {
    console.log(this);
  };
}

23.6 Use PascalCase when you export a constructor / class / singleton / function library / bare object.
```
const AirbnbStyleGuide = {
  es6: {
  },
};

export { AirbnbStyleGuide };
```

23.9 You may optionally uppercase constants.

// bad
export let REASSIGNABLE_VARIABLE = 'do not use let with uppercase variables';

// good
export const DEFAULT_LANG = 'et';

Accessors

24.1 Accessor functions for properties are not required.

24.2 Do not use JavaScript getters/setters as they cause unexpected side effects and are harder to test, maintain, and reason about. Instead, if you do make accessor functions, use getVal() and setVal('hello').

// bad
class Dragon {
  get age() {
    // ...
  }

  set age(value) {
    // ...
  }
}

// good
class Dragon {
  getAge() {
    // ...
  }

  setAge(value) {
    // ...
  }
}

24.3 If the property/method is a boolean, use isVal() or hasVal().

// bad
if (!dragon.age()) {
  return false;
}

// good
if (!dragon.hasAge()) {
  return false;
}

24.4 It’s okay to create get() and set() functions, but be consistent.

class Jedi {
  constructor(options = {}) {
    const lightsaber = options.lightsaber || 'blue';
    this.set('lightsaber', lightsaber);
  }

  set(key, val) {
    this[key] = val;
  }

  get(key) {
    return this[key];
  }
}

Events

25.1 When attaching data payloads to events (whether DOM events or something more proprietary like Backbone events), pass an object literal (also known as a "hash") instead of a raw value. This allows a subsequent contributor to add more data to the event payload without finding and updating every handler for the event. For example, instead of:
```
// bad
$(this).trigger('listingUpdated', listing.id);

// ...

$(this).on('listingUpdated', (e, listingID) => {
  // do something with listingID
});
```
prefer:
```
// good
$(this).trigger('listingUpdated', { listingID: listing.id });

// ...

$(this).on('listingUpdated', (e, data) => {
  // do something with data.listingID
});
```

jQuery

You might not need jQuery. Think twice before using jQuery for something that is available for use via native DOM APIs.

26.1 Do not prefix jQuery object variables with a $.

// bad
const $sidebar = $('.sidebar');

// bad
const $sidebarBtn = $('.sidebar-btn');

// good
const sidebar = $('.sidebar');

26.2 Cache jQuery lookups.

// bad
function setSidebar() {
  $('.sidebar').hide();

  // ...

  $('.sidebar').css({
    'background-color': 'pink',
  });
}

// good
function setSidebar() {
  const sidebar = $('.sidebar');
  sidebar.hide();

  // ...

  sidebar.css({
    'background-color': 'pink',
  });
}

26.3 For DOM queries use Cascading $('.sidebar ul') or parent > child $('.sidebar > ul'). jsPerf

26.4 Use find with scoped jQuery object queries.

// bad
$('ul', '.sidebar').hide();

// bad
$('.sidebar').find('ul').hide();

// good
$('.sidebar ul').hide();

// good
$('.sidebar > ul').hide();

// good
const sidebar = $('.sidebar');
sidebar.find('ul').hide();

26.5 Name queried element objects inside each appropriately.

  // good
  $('.sidebar').each((element: HTMLElement) => {
    const sidebar = $(element);
  });

ECMAScript 5 Compatibility

27.1 Refer to Kangax’s ES5 compatibility table.

ECMAScript 6+ (ES 2015+) Styles

28.1 This is a collection of links to the various ES6+ features.

Arrow Functions
Classes
Object Shorthand
Object Concise
Object Computed Properties
Template Strings
Destructuring
Default Parameters
Rest
Array Spreads
Let and Const
Exponentiation Operator
Iterators and Generators
Modules

28.2 Do not use TC39 proposals that have not reached stage 3.

Why? They are not finalized, and they are subject to change or to be withdrawn entirely. We want to use JavaScript, and proposals are not JavaScript yet.

Standard Library

The Standard Library contains utilities that are functionally broken but remain for legacy reasons.

29.1 Use Number.isNaN instead of global isNaN. eslint: no-restricted-globals

Why? The global isNaN coerces non-numbers to numbers, returning true for anything that coerces to NaN. If this behavior is desired, make it explicit.
```
// bad
isNaN('1.2'); // false
isNaN('1.2.3'); // true

// good
Number.isNaN('1.2.3'); // false
Number.isNaN(Number('1.2.3')); // true
```

29.2 Use Number.isFinite instead of global isFinite. eslint: no-restricted-globals

Why? The global isFinite coerces non-numbers to numbers, returning true for anything that coerces to a finite number. If this behavior is desired, make it explicit.
```
// bad
isFinite('2e3'); // true

// good
Number.isFinite('2e3'); // false
Number.isFinite(parseInt('2e3', 10)); // true
```

TypeScript

TypeScript saves you time catching errors and providing fixes before you run code.

30.1 Use strict mode in TypeScript.

Set "strict": true, in your tsconfig.json to catch subtle bugs and enforce best practices.

30.2 Specify return types for functions that return object literals. eslint: require-return-types-for-object-literals

This helps to catch bugs when you accidentally include extraneous properties in a directly returned object literal.

For example, in this code, TypeScript won't help you catch the slightly different property name:

interface User {
    name: string
    isDisabled?: boolean
}

const createUser = () => ({ name: 'Steve', disabled: true })
const user: User = createUser()

Also, searching for all usages of the name property in an IDE would not list this function. Neither would you be able to navigate to the definition from the property in the literal.

But if you specify the return type - const createUser = (): User => ... - the error is caught and code navigation works as expected.

// bad
const fn = () => {
    return { foo: 'bar' };
};

// good
const fn = (): Foo => {
    return { foo: 'bar' };
};

// bad
const fn = () => ({ foo: 'bar' });

// good
const fn = (): Foo => ({ foo: 'bar' });

// ok - we're returning a typed variable instead of a literal
const fn = () => {
    const foo: Foo = { foo: 'bar' };
    return foo;
};

// ok - we're not returning an object literal
const fn1 = () => 'foo'; // string literal
const fn2 = () => foo; // variable - may hold an object
const fn3 = () => foo(); // expression - may evaluate to an object

30.3 Avoid excessive type annotations.

There are cases where annotating code elements with explicit types is highly useful or even necessary, for example:
- Function parameters
- Return types of some functions, such as:
  - Module boundaries (exported functions)
  - The scenario described above
- Where TypeScript is unable to infer the correct type
- Where it improves code readability and/or type checking performance
However, too much of a good thing can be a bad thing.

Use of the ESLint typedef rule is discouraged, even by its own documentation:

Note: requiring type annotations unnecessarily can be cumbersome to maintain and generally reduces code readability. TypeScript is often better at inferring types than easily written type annotations would allow.

Instead of enabling typedef, it is generally recommended to use the --noImplicitAny and --strictPropertyInitialization compiler options to enforce type annotations only when useful.

Both of the recommended compiler options are enabled in strict mode.

When tools like TypeScript and ESLint allow either specifying or omitting the type, use your best judgement and these examples to decide:
```
// good
const text = 'text';
const value = 42;
const [first, second] = array;
const { name, role } = user;

// unnecessary in most cases
const text: string = 'text';
const value: number = 42;
const [first, second]: [string, string] = array;
const { name, role }: { name: string, role: string } = user;

// bad (and an error in TypeScript strict mode)
const logSize = (size) => console.log(size);

// good
const logSize = (size: number) => console.log(size);

// good
const logSize = (size: number): void => console.log(size);

// bad
const logSize: (size: number) => void = (size: number): void => console.log(size);

// good
strings.map((str) => str.length);

// unnecessary in most cases
strings.map((str: string) => str.length);
```

30.4 Never use any type.

Use unknown if you truly don't know the type. unknown enforces using type guards to check the actual type of variable, while any does not.

  // bad
  const foo = (bar: any) => {
    return bar.test; // possible runtime error, no compile error
  }

  const foo = (bar: unknown) => {
    return bar.test; // compile error
  }

  // good
  interface TestData {
      test: string;
  }

  const barIsTest = (obj: unknown): obj is TestData => {
      return typeof obj === "object" && !! obj && "test" in obj;
  }

  const fooUnknown = (bar: unknown) => {
      if (barIsTest(bar)) {
          return bar.test; // no compile error
      }
      return;
  }

30.5 null vs undefined.

Use null type to mark a variable explicitly empty. undefined is filled from default values, while null is not.

  interface ButtonData {
    value?: string | null
  }

  const renderButton = (data?: ButtonData): string | null => {
    const {
      value = 'defaultValue',
    } = data

    return value;
  }

  renderButton() // -> "defaultValue"
  renderButton({ value: 'myValue' }) // -> "myValue"
  renderButton({ value: null }) // -> null
  renderButton({ value: undefined }) // -> "defaultValue"

30.6 Do not add prefixes to types and interfaces.

Types and interfaces should have descriptive, distinct names. It is not necessary to prefix types and interfaces when they are named appropriately.

  // worst
  interface IButton {
    text: string;
  }
  type TType = 'primary' | 'secondary';

  // bad
  interface IButtonProps {
    text: string;
  }
  type TButtonType = 'primary' | 'secondary';

  // good
  interface ButtonData {
    text: string;
  }
  type ButtonType = 'primary' | 'secondary';

30.7 type vs interface.

Use interface instead of type for object-like types, wherever possible. Interfaces perform better.

  // bad
  type Foo = {
    foo: string;
  }

  type Bar = Foo & {
    bar: string;
  }
  // good
  interface Foo {
    foo: string;
  }

  interface Bar extends Foo {
    bar: string;
  }

Using types is still OK if the base type is a union, as that cannot be extended as an interface:

  // OK
  interface FooA {
    fooA: string;
  }
  interface FooB {
    fooB: string;
  }
  type Foo = FooA | FooB;

  type Bar = Foo & {
    bar: string;
  }

30.8 Export types only when they are part of the public API

  // This should be exported because render function is exported.
  export interface RenderData {
    text: string;
  }

  export const render = (data: RenderData) => {
    return format({ text: data.text });
  });

  // This should not be exported because it is used in an internal function that is not exported.
  interface FormatData = {
    text: string;
  }

  const format = (data: FormatData) => data.text;

30.9 Using modifiers

Access modifiers is a tool to help you to prevent accidentally breaking encapsulation. Ask yourself if you intend the member to be something that's internal to the class, class hierarchy or public, and choose access level accordingly.

Examples:
- A field internalCounter should probably be private since it's mutable and an implementation detail.
- An internal beforeRender() method called right before rendering and used as a hook in subclasses should be protected.
- A render() method which is called from external code should be public.
```
  class Foo {
    private renderCounter: number = 0;

    public render(): void {
        this.beforeRender();
        // render
    }

    protected beforeRender(): void {
        this.renderCounter += 1;
    }
  }
```

30.10 Property order

In object literals, prefer the same property order as in the interface declaration.

interface TextField {
  id: string;
  value: string;
  onChange: (newValue: string) => void;
  attributes?: Attributes;
}

// bad
const textField: TextField = {
  attributes,
  value: 'foo',
  id: 'bar',
  onChange: handleChange,
};

// good
const textField: TextField = {
  id: 'bar',
  value: 'foo',
  onChange: handleChange,
  attributes,
};

Note: this may become less clear in certain situations, for example when one interface extends another.

Therefore it is not always a strictly enforcable rule - simply try to order the properties in a way that feels "natural".

Testing: coming soon!

Performance

Resources

Learning ES6+

Maybe Read This

Standard ECMA-262

Tools

ESlint - Airbnb Style .eslintrc

Other Style Guides

Other Styles

Naming this in nested functions - Christian Johansen
Conditional Callbacks - Ross Allen
Popular JavaScript Coding Conventions on GitHub - JeongHoon Byun
Multiple var statements in JavaScript, not superfluous - Ben Alman

Further Reading

Understanding JavaScript Closures - Angus Croll
Basic JavaScript for the impatient programmer - Dr. Axel Rauschmayer
You Might Not Need jQuery - Zack Bloom & Adam Schwartz
ES6 Features - Luke Hoban
Frontend Guidelines - Benjamin De Cock
clean-code-javascript

Books

JavaScript: The Good Parts - Douglas Crockford
JavaScript Patterns - Stoyan Stefanov
Pro JavaScript Design Patterns - Ross Harmes and Dustin Diaz
High Performance Web Sites: Essential Knowledge for Front-End Engineers - Steve Souders
Maintainable JavaScript - Nicholas C. Zakas
JavaScript Web Applications - Alex MacCaw
Pro JavaScript Techniques - John Resig
Smashing Node.js: JavaScript Everywhere - Guillermo Rauch
Secrets of the JavaScript Ninja - John Resig and Bear Bibeault
Human JavaScript - Henrik Joreteg
Superhero.js - Kim Joar Bekkelund, Mads Mobæk, & Olav Bjorkoy
JSBooks - Julien Bouquillon
Third Party JavaScript - Ben Vinegar and Anton Kovalyov
Effective JavaScript: 68 Specific Ways to Harness the Power of JavaScript - David Herman
Eloquent JavaScript - Marijn Haverbeke
You Don’t Know JS: ES6 & Beyond - Kyle Simpson

Blogs

Podcasts

Contributors

View Contributors

License

(The MIT License)

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the 'Software'), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED 'AS IS', WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

Amendments

We encourage you to fork this guide and change the rules to fit your team’s style guide. Below, you may list some amendments to the style guide. This allows you to periodically update your style guide without having to deal with merge conflicts.

Name		Name	Last commit message	Last commit date
Latest commit History 1,910 Commits
linters		linters
packages		packages
.editorconfig		.editorconfig
.gitignore		.gitignore
.npmrc		.npmrc
.travis.yml		.travis.yml
LICENSE.md		LICENSE.md
README.md		README.md
package.json		package.json

License

gtap-dev/javascript

Folders and files

Latest commit

History

Repository files navigation

gotoAndPlay JavaScript Style Guide() {

Types

References

Objects

Arrays

Destructuring

Strings

Functions

Arrow Functions

Classes & Constructors

Modules

Iterators and Generators

Properties

Variables

Hoisting

Comparison Operators & Equality

Blocks

Control Statements

Comments

Whitespace

Commas

Semicolons

Type Casting & Coercion

Naming Conventions

Accessors

Events

jQuery

ECMAScript 5 Compatibility

ECMAScript 6+ (ES 2015+) Styles

Standard Library

TypeScript

Testing: coming soon!

Performance

Resources

Contributors

License

Amendments

};

About

Resources

License

Stars

Watchers

Forks

Releases

Packages 0

Languages

Packages