typed-function

Move type checking logic and type conversions outside of your function in a flexible, organized way. Automatically throw informative errors in case of wrong input arguments.

Features

typed-function has the following features:

• Runtime type-checking of input arguments.
• Automatic type conversion of arguments.
• Compose typed functions with multiple signatures.
• Supports union types, any type, and variable arguments.
• Detailed error messaging.

Supported environments: node.js, Chrome, Firefox, Safari, Opera, IE9+.

Why?

In JavaScript, functions can be called with any number and any type of arguments. When writing a function, the easiest way is to just assume that the function will be called with the correct input. This leaves the function's behavior on invalid input undefined. The function may throw some error, or worse, it may silently fail or return wrong results. Typical errors are TypeError: undefined is not a function or TypeError: Cannot call method 'request' of undefined. These error messages are not very helpful. It can be hard to debug them, as they can be the result of a series of nested function calls manipulating and propagating invalid or incomplete data.

Often, JavaScript developers add some basic type checking where it is important, using checks like typeof fn === 'function', date instanceof Date, and Array.isArray(arr). For functions supporting multiple signatures, the type checking logic can grow quite a bit, and distract from the actual logic of the function.

For functions dealing with a considerable amount of type checking and conversion logic, or functions facing a public API, it can be very useful to use the typed-function module to handle the type-checking logic. This way:

• Users of the function get useful and consistent error messages when using the function wrongly.
• The function cannot silently fail or silently give wrong results due to invalid input.
• Correct type of input is assured inside the function. The function's code becomes easier to understand as it only contains the actual function logic. Lower level utility functions called by the type-checked function can possibly be kept simpler as they don't need to do additional type checking.

It's important however not to overuse type checking:

• Locking down the type of input that a function accepts can unnecessary limit it's flexibility. Keep functions as flexible and forgiving as possible, follow the robustness principle here: "be liberal in what you accept and conservative in what you send" (Postel's law).
• There is no need to apply type checking to all functions. It may be enough to apply type checking to one tier of public facing functions.
• There is a performance penalty involved for all type checking, so applying it everywhere can unnecessarily worsen the performance.

Load

Install via npm:

npm install typed-function

Usage

Here some usage examples. More examples are available in the /examples folder.

var typed = require('typed-function');

// create a typed function
var fn1 = typed('number, string', function (a, b) {
  return 'a is a number, b is a string';
});

// create a typed function with multiple types per argument (type union)
var fn2 = typed('string, number | boolean', function (a, b) {
  return 'a is a string, b is a number or a boolean';
});

// create a typed function with any type argument
var fn3 = typed('string, any', function (a, b) {
  return 'a is a string, b can be anything';
});

// create a typed function with multiple signatures
var fn4 = typed({
  'number': function (a) {
    return 'a is a number';
  },
  'number, boolean': function (a, b) {
    return 'a is a number, b is a boolean';
  },
  'number, number': function (a, b) {
    return 'a is a number, b is a number';
  }
});

// use the functions
console.log(fn1(2, 'foo'));      // outputs 'a is a number, b is a string'
console.log(fn4(2));             // outputs 'a is a number'

// calling the function with a non-supported type signature will throw an error
try {
  fn2('hello', 'world');
}
catch (err) {
  console.log(err.toString());
  // outputs:  TypeError: Unexpected type of argument.
  //           Expected: number or boolean, actual: string, index: 1.
}

Types

typed-function has the following built-in types:

• null
• boolean
• number
• string
• function
• Array
• Date
• RegExp
• Object

The following type expressions are supported:

• Multiple arguments: string, number, function
• Union types: number | string
• Variable arguments: ...number
• Any type: any

API

Construction

A typed function can be constructed in three ways:

• With a single signature:

  typed(signature: string, fn: function) : function
  typed(name: string, signature: string, fn: function) : function
  

• With multiple signatures:

  typed(signatures: Object.<string, function>) : function
  typed(name: string, signatures: Object.<string, function>) : function
  

• Merge multiple typed functions into a new typed function

  typed(functions: ...function) : function
  

Methods

• typed.create() : function

  Create a new, isolated instance of typed-function.

Properties

• typed.types: Object

  A map with the object types as key and a type checking test as value. Custom types can be added like:

  function Person(...) {
    ...
  }
  
  typed.types['Person'] = function (x) {
    return x instanceof Person;
  };

-   `typed.conversions: Array`

  An Array with built-in conversions. Empty by default. Can be used for example
  to defined conversions from `boolean` to `number`. For example:

  ```js
  typed.conversions.push({
    from: 'boolean',
    to: 'number',
    convert: function (x) {
      return +x;
  });
  ```

### Output

The functions generated with `typed({...})` have:

- A function `toString`. Returns well readable code which can be used to see
what the function exactly does. Mostly for debugging purposes.
- A property `signatures`, which holds a map with the (normalized)
signatures as key and the original sub-functions as value.
- A property `name` containing name of the typed function or an empty string.


## Roadmap

### Version 1

- Be able to turn off exception throwing.
- Extend function signatures:
- Optional arguments like `'[number], array'` or like `number=, array`
- Nullable arguments like `'?Object'`
- Create a good benchmark, to get insight in the overhead.
- Allow conversions to fail (for example string to number is not always
possible). Call this `fallible` or `optional`?

### Version 2

- Extend function signatures:
- Constants like `'"linear" | "cubic"'`, `'0..10'`, etc.
- Object definitions like `'{name: string, age: number}'`
- Object definitions like `'Object.<string, Person>'`
- Array definitions like `'Array.<Person>'`
- Improve performance of both generating a typed function as well as
the performance and memory footprint of a typed function.


## Test

To test the library, run:

  npm test


## Minify

To generate the minified version of the library, run:

  npm run minify