README.md

# core-js

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## As advertising: the author is looking for a good job :)

## [core-js@3, babel and a look into the future](https://github.com/zloirock/core-js/tree/master/docs/2019-03-19-core-js-3-babel-and-a-look-into-the-future.md)

## Raising funds

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---

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---

**It's documentation for obsolete `core-js@2`. If you looking documentation for actual `core-js` version, please, check [this branch](https://github.com/zloirock/core-js/tree/master).**

Modular standard library for JavaScript. Includes polyfills for [ECMAScript 5](#ecmascript-5), [ECMAScript 6](#ecmascript-6): [promises](#ecmascript-6-promise), [symbols](#ecmascript-6-symbol), [collections](#ecmascript-6-collections), iterators, [typed arrays](#ecmascript-6-typed-arrays), [ECMAScript 7+ proposals](#ecmascript-7-proposals), [setImmediate](#setimmediate), etc. Some additional features such as [dictionaries](#dict) or [extended partial application](#partial-application). You can require only needed features or use it without global namespace pollution.

[*Example*](http://goo.gl/a2xexl):
```js
Array.from(new Set([1, 2, 3, 2, 1]));          // => [1, 2, 3]
'*'.repeat(10);                                // => '**********'
Promise.resolve(32).then(x => console.log(x)); // => 32
setImmediate(x => console.log(x), 42);         // => 42
```

[*Without global namespace pollution*](http://goo.gl/paOHb0):
```js
var core = require('core-js/library'); // With a modular system, otherwise use global `core`
core.Array.from(new core.Set([1, 2, 3, 2, 1]));     // => [1, 2, 3]
core.String.repeat('*', 10);                        // => '**********'
core.Promise.resolve(32).then(x => console.log(x)); // => 32
core.setImmediate(x => console.log(x), 42);         // => 42
```

### Index
- [Usage](#usage)
  - [Basic](#basic)
  - [CommonJS](#commonjs)
  - [Custom build](#custom-build-from-the-command-line)
- [Supported engines](#supported-engines)
- [Features](#features)
  - [ECMAScript 5](#ecmascript-5)
  - [ECMAScript 6](#ecmascript-6)
    - [ECMAScript 6: Object](#ecmascript-6-object)
    - [ECMAScript 6: Function](#ecmascript-6-function)
    - [ECMAScript 6: Array](#ecmascript-6-array)
    - [ECMAScript 6: String](#ecmascript-6-string)
    - [ECMAScript 6: RegExp](#ecmascript-6-regexp)
    - [ECMAScript 6: Number](#ecmascript-6-number)
    - [ECMAScript 6: Math](#ecmascript-6-math)
    - [ECMAScript 6: Date](#ecmascript-6-date)
    - [ECMAScript 6: Promise](#ecmascript-6-promise)
    - [ECMAScript 6: Symbol](#ecmascript-6-symbol)
    - [ECMAScript 6: Collections](#ecmascript-6-collections)
    - [ECMAScript 6: Typed Arrays](#ecmascript-6-typed-arrays)
    - [ECMAScript 6: Reflect](#ecmascript-6-reflect)
  - [ECMAScript 7+ proposals](#ecmascript-7-proposals)
    - [stage 4 proposals](#stage-4-proposals)
    - [stage 3 proposals](#stage-3-proposals)
    - [stage 2 proposals](#stage-2-proposals)
    - [stage 1 proposals](#stage-1-proposals)
    - [stage 0 proposals](#stage-0-proposals)
    - [pre-stage 0 proposals](#pre-stage-0-proposals)
  - [Web standards](#web-standards)
    - [setTimeout / setInterval](#settimeout--setinterval)
    - [setImmediate](#setimmediate)
    - [iterable DOM collections](#iterable-dom-collections)
  - [Non-standard](#non-standard)
    - [Object](#object)
    - [Dict](#dict)
    - [partial application](#partial-application)
    - [Number Iterator](#number-iterator)
    - [escaping strings](#escaping-strings)
    - [delay](#delay)
    - [helpers for iterators](#helpers-for-iterators)
- [Missing polyfills](#missing-polyfills)
- [Changelog](./CHANGELOG.md)

## Usage
### Basic
```
npm i core-js
bower install core.js
```

```js
// Default
require('core-js');
// Without global namespace pollution
var core = require('core-js/library');
// Shim only
require('core-js/shim');
```
If you need complete build for browser, use builds from `core-js/client` path:  

* [default](https://raw.githack.com/zloirock/core-js/v2.6.12/client/core.min.js): Includes all features, standard and non-standard.
* [as a library](https://raw.githack.com/zloirock/core-js/v2.6.12/client/library.min.js): Like "default", but does not pollute the global namespace (see [2nd example at the top](#core-js)).
* [shim only](https://raw.githack.com/zloirock/core-js/v2.6.12/client/shim.min.js): Only includes the standard methods.

Warning: if you use `core-js` with the extension of native objects, require all needed `core-js` modules at the beginning of entry point of your application, otherwise, conflicts may occur.

### CommonJS
You can require only needed modules.

```js
require('core-js/fn/set');
require('core-js/fn/array/from');
require('core-js/fn/array/find-index');
Array.from(new Set([1, 2, 3, 2, 1])); // => [1, 2, 3]
[1, 2, NaN, 3, 4].findIndex(isNaN);   // => 2

// or, w/o global namespace pollution:

var Set       = require('core-js/library/fn/set');
var from      = require('core-js/library/fn/array/from');
var findIndex = require('core-js/library/fn/array/find-index');
from(new Set([1, 2, 3, 2, 1]));      // => [1, 2, 3]
findIndex([1, 2, NaN, 3, 4], isNaN); // => 2
```
Available entry points for methods / constructors, as above examples, and namespaces: for example, `core-js/es6/array` (`core-js/library/es6/array`) contains all [ES6 `Array` features](#ecmascript-6-array), `core-js/es6` (`core-js/library/es6`) contains all ES6 features.

##### Caveats when using CommonJS API:

* `modules` path is internal API, does not inject all required dependencies and can be changed in minor or patch releases. Use it only for a custom build and / or if you know what are you doing.
* `core-js` is extremely modular and uses a lot of very tiny modules, because of that for usage in browsers bundle up `core-js` instead of usage loader for each file, otherwise, you will have hundreds of requests.

#### CommonJS and prototype methods without global namespace pollution
In the `library` version, we can't pollute prototypes of native constructors. Because of that, prototype methods transformed to static methods like in examples above. `babel` `runtime` transformer also can't transform them. But with transpilers we can use one more trick - [bind operator and virtual methods](https://github.com/zenparsing/es-function-bind). Special for that, available `/virtual/` entry points. Example:
```js
import fill from 'core-js/library/fn/array/virtual/fill';
import findIndex from 'core-js/library/fn/array/virtual/find-index';

Array(10)::fill(0).map((a, b) => b * b)::findIndex(it => it && !(it % 8)); // => 4

// or

import {fill, findIndex} from 'core-js/library/fn/array/virtual';

Array(10)::fill(0).map((a, b) => b * b)::findIndex(it => it && !(it % 8)); // => 4

```

### Custom build (from the command-line)
```
npm i core-js && cd node_modules/core-js && npm i
npm run grunt build:core.dict,es6 -- --blacklist=es6.promise,es6.math --library=on --path=custom uglify
```
Where `core.dict` and `es6` are modules (namespaces) names, which will be added to the build, `es6.promise` and `es6.math` are modules (namespaces) names, which will be excluded from the build, `--library=on` is flag for build without global namespace pollution and `custom` is target file name.

Available namespaces: for example, `es6.array` contains [ES6 `Array` features](#ecmascript-6-array), `es6` contains all modules whose names start with `es6`.

### Custom build (from external scripts)

[`core-js-builder`](https://www.npmjs.com/package/core-js-builder) package exports a function that takes the same parameters as the `build` target from the previous section. This will conditionally include or exclude certain parts of `core-js`:

```js
require('core-js-builder')({
  modules: ['es6', 'core.dict'], // modules / namespaces
  blacklist: ['es6.reflect'],    // blacklist of modules / namespaces, by default - empty list
  library: false,                // flag for build without global namespace pollution, by default - false
  umd: true                      // use UMD wrapper for export `core` object, by default - true
}).then(code => {
  // ...
}).catch(error => {
  // ...
});
```
## Supported engines
**Tested in:**
- Chrome 26+
- Firefox 4+
- Safari 5+
- Opera 12+
- Internet Explorer 6+ (sure, IE8- with ES3 limitations)
- Edge
- Android Browser 2.3+
- iOS Safari 5.1+
- PhantomJS 1.9 / 2.1
- NodeJS 0.8+

...and it doesn't mean `core-js` will not work in other engines, they just have not been tested.

## Features:
[*CommonJS entry points:*](#commonjs)
```
core-js(/library)       <- all features
core-js(/library)/shim  <- only polyfills
```
### ECMAScript 5
All features moved to the [`es6` namespace](#ecmascript-6), here just a list of features:
```js
Object
  .create(proto | null, descriptors?)    -> object
  .getPrototypeOf(object)                -> proto | null
  .defineProperty(target, key, desc)     -> target, cap for ie8-
  .defineProperties(target, descriptors) -> target, cap for ie8-
  .getOwnPropertyDescriptor(object, key) -> desc
  .getOwnPropertyNames(object)           -> array
  .keys(object)                          -> array
  .seal(object)                          -> object, cap for ie8-
  .freeze(object)                        -> object, cap for ie8-
  .preventExtensions(object)             -> object, cap for ie8-
  .isSealed(object)                      -> bool, cap for ie8-
  .isFrozen(object)                      -> bool, cap for ie8-
  .isExtensible(object)                  -> bool, cap for ie8-
Array
  .isArray(var)                                -> bool
  #slice(start?, end?)                         -> array, fix for ie7-
  #join(string = ',')                          -> string, fix for ie7-
  #indexOf(var, from?)                         -> int
  #lastIndexOf(var, from?)                     -> int
  #every(fn(val, index, @), that)              -> bool
  #some(fn(val, index, @), that)               -> bool
  #forEach(fn(val, index, @), that)            -> void
  #map(fn(val, index, @), that)                -> array
  #filter(fn(val, index, @), that)             -> array
  #reduce(fn(memo, val, index, @), memo?)      -> var
  #reduceRight(fn(memo, val, index, @), memo?) -> var
  #sort(fn?)                                   -> @, fixes for some engines
Function
  #bind(object, ...args) -> boundFn(...args)
String
  #split(separator, limit) -> array
  #trim()                  -> str
RegExp
  #toString() -> str
Number
  #toFixed(digits)        -> string
  #toPrecision(precision) -> string
parseInt(str, radix) -> int
parseFloat(str)      -> num
Date
  .now()         -> int
  #toISOString() -> string
  #toJSON()      -> string
```
[*CommonJS entry points:*](#commonjs)
```
core-js(/library)/es5
```

### ECMAScript 6
[*CommonJS entry points:*](#commonjs)
```
core-js(/library)/es6
```
#### ECMAScript 6: Object
Modules [`es6.object.assign`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.object.assign.js), [`es6.object.is`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.object.is.js), [`es6.object.set-prototype-of`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.object.set-prototype-of.js) and [`es6.object.to-string`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.object.to-string.js).

In ES6 most `Object` static methods should work with primitives. Modules [`es6.object.freeze`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.object.freeze.js), [`es6.object.seal`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.object.seal.js), [`es6.object.prevent-extensions`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.object.prevent-extensions.js), [`es6.object.is-frozen`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.object.is-frozen.js), [`es6.object.is-sealed`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.object.is-sealed.js), [`es6.object.is-extensible`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.object.is-extensible.js), [`es6.object.get-own-property-descriptor`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.object.get-own-property-descriptor.js), [`es6.object.get-prototype-of`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.object.get-prototype-of.js), [`es6.object.keys`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.object.keys.js) and [`es6.object.get-own-property-names`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.object.get-own-property-names.js).

Just ES5 features: [`es6.object.create`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.object.create.js), [`es6.object.define-property`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.object.define-property.js) and [`es6.object.define-properties`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.object.es6.object.define-properties.js).
```js
Object
  .assign(target, ...src)                -> target
  .is(a, b)                              -> bool
  .setPrototypeOf(target, proto | null)  -> target (required __proto__ - IE11+)
  .create(object | null, descriptors?)   -> object
  .getPrototypeOf(var)                   -> object | null
  .defineProperty(object, key, desc)     -> target
  .defineProperties(object, descriptors) -> target
  .getOwnPropertyDescriptor(var, key)    -> desc | undefined
  .keys(var)                             -> array
  .getOwnPropertyNames(var)              -> array
  .freeze(var)                           -> var
  .seal(var)                             -> var
  .preventExtensions(var)                -> var
  .isFrozen(var)                         -> bool
  .isSealed(var)                         -> bool
  .isExtensible(var)                     -> bool
  #toString()                            -> string, ES6 fix: @@toStringTag support
```
[*CommonJS entry points:*](#commonjs)
```
core-js(/library)/es6/object
core-js(/library)/fn/object/assign
core-js(/library)/fn/object/is
core-js(/library)/fn/object/set-prototype-of
core-js(/library)/fn/object/get-prototype-of
core-js(/library)/fn/object/create
core-js(/library)/fn/object/define-property
core-js(/library)/fn/object/define-properties
core-js(/library)/fn/object/get-own-property-descriptor
core-js(/library)/fn/object/keys
core-js(/library)/fn/object/get-own-property-names
core-js(/library)/fn/object/freeze
core-js(/library)/fn/object/seal
core-js(/library)/fn/object/prevent-extensions
core-js(/library)/fn/object/is-frozen
core-js(/library)/fn/object/is-sealed
core-js(/library)/fn/object/is-extensible
core-js/fn/object/to-string
```
[*Examples*](http://goo.gl/ywdwPz):
```js
var foo = {q: 1, w: 2}
  , bar = {e: 3, r: 4}
  , baz = {t: 5, y: 6};
Object.assign(foo, bar, baz); // => foo = {q: 1, w: 2, e: 3, r: 4, t: 5, y: 6}

Object.is(NaN, NaN); // => true
Object.is(0, -0);    // => false
Object.is(42, 42);   // => true
Object.is(42, '42'); // => false

function Parent(){}
function Child(){}
Object.setPrototypeOf(Child.prototype, Parent.prototype);
new Child instanceof Child;  // => true
new Child instanceof Parent; // => true

var O = {};
O[Symbol.toStringTag] = 'Foo';
'' + O; // => '[object Foo]'

Object.keys('qwe'); // => ['0', '1', '2']
Object.getPrototypeOf('qwe') === String.prototype; // => true
```
#### ECMAScript 6: Function
Modules [`es6.function.name`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.function.name.js), [`es6.function.has-instance`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.function.has-instance.js). Just ES5: [`es6.function.bind`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.function.bind.js).
```js
Function
  #bind(object, ...args) -> boundFn(...args)
  #name                  -> string (IE9+)
  #@@hasInstance(var)    -> bool
```
[*CommonJS entry points:*](#commonjs)
```
core-js/es6/function
core-js/fn/function/name
core-js/fn/function/has-instance
core-js/fn/function/bind
core-js/fn/function/virtual/bind
```
[*Example*](http://goo.gl/zqu3Wp):
```js
(function foo(){}).name // => 'foo'

console.log.bind(console, 42)(43); // => 42 43
```
#### ECMAScript 6: Array
Modules [`es6.array.from`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.array.from.js), [`es6.array.of`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.array.of.js), [`es6.array.copy-within`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.array.copy-within.js), [`es6.array.fill`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.array.fill.js), [`es6.array.find`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.array.find.js), [`es6.array.find-index`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.array.find-index.js), [`es6.array.iterator`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.array.iterator.js). ES5 features with fixes: [`es6.array.is-array`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.array.is-array.js), [`es6.array.slice`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.array.slice.js), [`es6.array.join`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.array.join.js), [`es6.array.index-of`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.array.index-of.js), [`es6.array.last-index-of`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.array.last-index-of.js), [`es6.array.every`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.array.every.js), [`es6.array.some`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.array.some.js), [`es6.array.for-each`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.array.for-each.js), [`es6.array.map`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.array.map.js), [`es6.array.filter`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.array.filter.js), [`es6.array.reduce`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.array.reduce.js), [`es6.array.reduce-right`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.array.reduce-right.js), [`es6.array.sort`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.array.sort.js).
```js
Array
  .from(iterable | array-like, mapFn(val, index)?, that) -> array
  .of(...args)                                           -> array
  .isArray(var)                                          -> bool
  #copyWithin(target = 0, start = 0, end = @length)      -> @
  #fill(val, start = 0, end = @length)                   -> @
  #find(fn(val, index, @), that)                         -> val
  #findIndex(fn(val, index, @), that)                    -> index | -1
  #values()                                              -> iterator
  #keys()                                                -> iterator
  #entries()                                             -> iterator
  #join(string = ',')                                    -> string, fix for ie7-
  #slice(start?, end?)                                   -> array, fix for ie7-
  #indexOf(var, from?)                                   -> index | -1
  #lastIndexOf(var, from?)                               -> index | -1
  #every(fn(val, index, @), that)                        -> bool
  #some(fn(val, index, @), that)                         -> bool
  #forEach(fn(val, index, @), that)                      -> void
  #map(fn(val, index, @), that)                          -> array
  #filter(fn(val, index, @), that)                       -> array
  #reduce(fn(memo, val, index, @), memo?)                -> var
  #reduceRight(fn(memo, val, index, @), memo?)           -> var
  #sort(fn?)                                             -> @, invalid arguments fix
  #@@iterator()                                          -> iterator (values)
  #@@unscopables                                         -> object (cap)
Arguments
  #@@iterator() -> iterator (values, available only in core-js methods)
```
[*CommonJS entry points:*](#commonjs)
```
core-js(/library)/es6/array
core-js(/library)/fn/array/from
core-js(/library)/fn/array/of
core-js(/library)/fn/array/is-array
core-js(/library)/fn/array/iterator
core-js(/library)/fn/array/copy-within
core-js(/library)/fn/array/fill
core-js(/library)/fn/array/find
core-js(/library)/fn/array/find-index
core-js(/library)/fn/array/values
core-js(/library)/fn/array/keys
core-js(/library)/fn/array/entries
core-js(/library)/fn/array/slice
core-js(/library)/fn/array/join
core-js(/library)/fn/array/index-of
core-js(/library)/fn/array/last-index-of
core-js(/library)/fn/array/every
core-js(/library)/fn/array/some
core-js(/library)/fn/array/for-each
core-js(/library)/fn/array/map
core-js(/library)/fn/array/filter
core-js(/library)/fn/array/reduce
core-js(/library)/fn/array/reduce-right
core-js(/library)/fn/array/sort
core-js(/library)/fn/array/virtual/iterator
core-js(/library)/fn/array/virtual/copy-within
core-js(/library)/fn/array/virtual/fill
core-js(/library)/fn/array/virtual/find
core-js(/library)/fn/array/virtual/find-index
core-js(/library)/fn/array/virtual/values
core-js(/library)/fn/array/virtual/keys
core-js(/library)/fn/array/virtual/entries
core-js(/library)/fn/array/virtual/slice
core-js(/library)/fn/array/virtual/join
core-js(/library)/fn/array/virtual/index-of
core-js(/library)/fn/array/virtual/last-index-of
core-js(/library)/fn/array/virtual/every
core-js(/library)/fn/array/virtual/some
core-js(/library)/fn/array/virtual/for-each
core-js(/library)/fn/array/virtual/map
core-js(/library)/fn/array/virtual/filter
core-js(/library)/fn/array/virtual/reduce
core-js(/library)/fn/array/virtual/reduce-right
core-js(/library)/fn/array/virtual/sort
```
[*Examples*](http://goo.gl/oaUFUf):
```js
Array.from(new Set([1, 2, 3, 2, 1]));      // => [1, 2, 3]
Array.from({0: 1, 1: 2, 2: 3, length: 3}); // => [1, 2, 3]
Array.from('123', Number);                 // => [1, 2, 3]
Array.from('123', function(it){
  return it * it;
});                                        // => [1, 4, 9]

Array.of(1);       // => [1]
Array.of(1, 2, 3); // => [1, 2, 3]

var array = ['a', 'b', 'c'];

for(var val of array)console.log(val);          // => 'a', 'b', 'c'
for(var val of array.values())console.log(val); // => 'a', 'b', 'c'
for(var key of array.keys())console.log(key);   // => 0, 1, 2
for(var [key, val] of array.entries()){
  console.log(key);                             // => 0, 1, 2
  console.log(val);                             // => 'a', 'b', 'c'
}

function isOdd(val){
  return val % 2;
}
[4, 8, 15, 16, 23, 42].find(isOdd);      // => 15
[4, 8, 15, 16, 23, 42].findIndex(isOdd); // => 2
[4, 8, 15, 16, 23, 42].find(isNaN);      // => undefined
[4, 8, 15, 16, 23, 42].findIndex(isNaN); // => -1

Array(5).fill(42); // => [42, 42, 42, 42, 42]

[1, 2, 3, 4, 5].copyWithin(0, 3); // => [4, 5, 3, 4, 5]
```
#### ECMAScript 6: String
Modules [`es6.string.from-code-point`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.string.from-code-point.js), [`es6.string.raw`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.string.raw.js), [`es6.string.iterator`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.string.iterator.js), [`es6.string.code-point-at`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.string.code-point-at.js), [`es6.string.ends-with`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.string.ends-with.js), [`es6.string.includes`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.string.includes.js), [`es6.string.repeat`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.string.repeat.js), [`es6.string.starts-with`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.string.starts-with.js) and [`es6.string.trim`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.string.trim.js).

Annex B HTML methods. Ugly, but it's also the part of the spec. Modules [`es6.string.anchor`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.string.anchor.js), [`es6.string.big`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.string.big.js), [`es6.string.blink`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.string.blink.js), [`es6.string.bold`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.string.bold.js), [`es6.string.fixed`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.string.fixed.js), [`es6.string.fontcolor`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.string.fontcolor.js), [`es6.string.fontsize`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.string.fontsize.js), [`es6.string.italics`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.string.italics.js), [`es6.string.link`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.string.link.js), [`es6.string.small`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.string.small.js), [`es6.string.strike`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.string.strike.js), [`es6.string.sub`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.string.sub.js) and [`es6.string.sup`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.string.sup.js).
```js
String
  .fromCodePoint(...codePoints) -> str
  .raw({raw}, ...substitutions) -> str
  #includes(str, from?) -> bool
  #startsWith(str, from?) -> bool
  #endsWith(str, from?) -> bool
  #repeat(num) -> str
  #codePointAt(pos) -> uint
  #trim() -> str, ES6 fix
  #anchor(name)     -> str
  #big()            -> str
  #blink()          -> str
  #bold()           -> str
  #fixed()          -> str
  #fontcolor(color) -> str
  #fontsize(size)   -> str
  #italics()        -> str
  #link(url)        -> str
  #small()          -> str
  #strike()         -> str
  #sub()            -> str
  #sup()            -> str
  #@@iterator() -> iterator (code points)
```
[*CommonJS entry points:*](#commonjs)
```
core-js(/library)/es6/string
core-js(/library)/fn/string/from-code-point
core-js(/library)/fn/string/raw
core-js(/library)/fn/string/includes
core-js(/library)/fn/string/starts-with
core-js(/library)/fn/string/ends-with
core-js(/library)/fn/string/repeat
core-js(/library)/fn/string/code-point-at
core-js(/library)/fn/string/trim
core-js(/library)/fn/string/anchor
core-js(/library)/fn/string/big
core-js(/library)/fn/string/blink
core-js(/library)/fn/string/bold
core-js(/library)/fn/string/fixed
core-js(/library)/fn/string/fontcolor
core-js(/library)/fn/string/fontsize
core-js(/library)/fn/string/italics
core-js(/library)/fn/string/link
core-js(/library)/fn/string/small
core-js(/library)/fn/string/strike
core-js(/library)/fn/string/sub
core-js(/library)/fn/string/sup
core-js(/library)/fn/string/iterator
core-js(/library)/fn/string/virtual/includes
core-js(/library)/fn/string/virtual/starts-with
core-js(/library)/fn/string/virtual/ends-with
core-js(/library)/fn/string/virtual/repeat
core-js(/library)/fn/string/virtual/code-point-at
core-js(/library)/fn/string/virtual/trim
core-js(/library)/fn/string/virtual/anchor
core-js(/library)/fn/string/virtual/big
core-js(/library)/fn/string/virtual/blink
core-js(/library)/fn/string/virtual/bold
core-js(/library)/fn/string/virtual/fixed
core-js(/library)/fn/string/virtual/fontcolor
core-js(/library)/fn/string/virtual/fontsize
core-js(/library)/fn/string/virtual/italics
core-js(/library)/fn/string/virtual/link
core-js(/library)/fn/string/virtual/small
core-js(/library)/fn/string/virtual/strike
core-js(/library)/fn/string/virtual/sub
core-js(/library)/fn/string/virtual/sup
core-js(/library)/fn/string/virtual/iterator
```
[*Examples*](http://goo.gl/3UaQ93):
```js
for(var val of 'a𠮷b'){
  console.log(val); // => 'a', '𠮷', 'b'
}

'foobarbaz'.includes('bar');      // => true
'foobarbaz'.includes('bar', 4);   // => false
'foobarbaz'.startsWith('foo');    // => true
'foobarbaz'.startsWith('bar', 3); // => true
'foobarbaz'.endsWith('baz');      // => true
'foobarbaz'.endsWith('bar', 6);   // => true

'string'.repeat(3); // => 'stringstringstring'

'𠮷'.codePointAt(0); // => 134071
String.fromCodePoint(97, 134071, 98); // => 'a𠮷b'

var name = 'Bob';
String.raw`Hi\n${name}!`;           // => 'Hi\\nBob!' (ES6 template string syntax)
String.raw({raw: 'test'}, 0, 1, 2); // => 't0e1s2t'

'foo'.bold();                     // => '<b>foo</b>'
'bar'.anchor('a"b');              // => '<a name="a&quot;b">bar</a>'
'baz'.link('http://example.com'); // => '<a href="http://example.com">baz</a>'
```
#### ECMAScript 6: RegExp
Modules [`es6.regexp.constructor`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.regexp.constructor.js) and [`es6.regexp.flags`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.regexp.flags.js).

Support well-known [symbols](#ecmascript-6-symbol) `@@match`, `@@replace`, `@@search` and `@@split`, modules [`es6.regexp.match`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.regexp.match.js), [`es6.regexp.replace`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.regexp.replace.js), [`es6.regexp.search`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.regexp.search.js) and [`es6.regexp.split`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.regexp.split.js).
```
[new] RegExp(pattern, flags?) -> regexp, ES6 fix: can alter flags (IE9+)
  #flags -> str (IE9+)
  #toString() -> str, ES6 fixes
  #@@match(str)             -> array | null
  #@@replace(str, replacer) -> string
  #@@search(str)            -> index
  #@@split(str, limit)      -> array
String
  #match(tpl)             -> var, ES6 fix for support @@match
  #replace(tpl, replacer) -> var, ES6 fix for support @@replace
  #search(tpl)            -> var, ES6 fix for support @@search
  #split(tpl, limit)      -> var, ES6 fix for support @@split, some fixes for old engines
```
[*CommonJS entry points:*](#commonjs)
```
core-js/es6/regexp
core-js/fn/regexp/constructor
core-js(/library)/fn/regexp/flags
core-js/fn/regexp/to-string
core-js/fn/regexp/match
core-js/fn/regexp/replace
core-js/fn/regexp/search
core-js/fn/regexp/split
```
[*Examples*](http://goo.gl/PiJxBD):
```js
RegExp(/./g, 'm'); // => /./m

/foo/.flags;    // => ''
/foo/gim.flags; // => 'gim'

'foo'.match({[Symbol.match]: _ => 1});     // => 1
'foo'.replace({[Symbol.replace]: _ => 2}); // => 2
'foo'.search({[Symbol.search]: _ => 3});   // => 3
'foo'.split({[Symbol.split]: _ => 4});     // => 4

RegExp.prototype.toString.call({source: 'foo', flags: 'bar'}); // => '/foo/bar'
```
#### ECMAScript 6: Number
Module [`es6.number.constructor`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.number.constructor.js). `Number` constructor support binary and octal literals, [*example*](http://goo.gl/jRd6b3):
```js
Number('0b1010101'); // => 85
Number('0o7654321'); // => 2054353
```
Modules [`es6.number.epsilon`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.number.epsilon.js), [`es6.number.is-finite`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.number.is-finite.js), [`es6.number.is-integer`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.number.is-integer.js), [`es6.number.is-nan`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.number.is-nan.js), [`es6.number.is-safe-integer`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.number.is-safe-integer.js), [`es6.number.max-safe-integer`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.number.max-safe-integer.js), [`es6.number.min-safe-integer`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.number.min-safe-integer.js), [`es6.number.parse-float`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.number.parse-float.js), [`es6.number.parse-int`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.number.parse-int.js), [`es6.number.to-fixed`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.number.to-fixed.js), [`es6.number.to-precision`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.number.to-precision.js), [`es6.parse-int`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.parse-int.js), [`es6.parse-float`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.parse-float.js).
```js
[new] Number(var)         -> number | number object
  .isFinite(num)          -> bool
  .isNaN(num)             -> bool
  .isInteger(num)         -> bool
  .isSafeInteger(num)     -> bool
  .parseFloat(str)        -> num
  .parseInt(str)          -> int
  .EPSILON                -> num
  .MAX_SAFE_INTEGER       -> int
  .MIN_SAFE_INTEGER       -> int
  #toFixed(digits)        -> string, fixes
  #toPrecision(precision) -> string, fixes
parseFloat(str)           -> num, fixes
parseInt(str)             -> int, fixes
```
[*CommonJS entry points:*](#commonjs)
```
core-js(/library)/es6/number
core-js/es6/number/constructor
core-js(/library)/fn/number/is-finite
core-js(/library)/fn/number/is-nan
core-js(/library)/fn/number/is-integer
core-js(/library)/fn/number/is-safe-integer
core-js(/library)/fn/number/parse-float
core-js(/library)/fn/number/parse-int
core-js(/library)/fn/number/epsilon
core-js(/library)/fn/number/max-safe-integer
core-js(/library)/fn/number/min-safe-integer
core-js(/library)/fn/number/to-fixed
core-js(/library)/fn/number/to-precision
core-js(/library)/fn/parse-float
core-js(/library)/fn/parse-int
```
#### ECMAScript 6: Math
Modules [`es6.math.acosh`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.math.acosh.js), [`es6.math.asinh`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.math.asinh.js), [`es6.math.atanh`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.math.atanh.js), [`es6.math.cbrt`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.math.cbrt.js), [`es6.math.clz32`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.math.clz32.js), [`es6.math.cosh`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.math.cosh.js), [`es6.math.expm1`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.math.expm1.js), [`es6.math.fround`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.math.fround.js), [`es6.math.hypot`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.math.hypot.js), [`es6.math.imul`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.math.imul.js), [`es6.math.log10`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.math.log10.js), [`es6.math.log1p`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.math.log1p.js), [`es6.math.log2`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.math.log2.js), [`es6.math.sign`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.math.sign.js), [`es6.math.sinh`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.math.sinh.js), [`es6.math.tanh`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.math.tanh.js), [`es6.math.trunc`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.math.trunc.js).
```js
Math
  .acosh(num)     -> num
  .asinh(num)     -> num
  .atanh(num)     -> num
  .cbrt(num)      -> num
  .clz32(num)     -> uint
  .cosh(num)      -> num
  .expm1(num)     -> num
  .fround(num)    -> num
  .hypot(...args) -> num
  .imul(num, num) -> int
  .log1p(num)     -> num
  .log10(num)     -> num
  .log2(num)      -> num
  .sign(num)      -> 1 | -1 | 0 | -0 | NaN
  .sinh(num)      -> num
  .tanh(num)      -> num
  .trunc(num)     -> num
```
[*CommonJS entry points:*](#commonjs)
```
core-js(/library)/es6/math
core-js(/library)/fn/math/acosh
core-js(/library)/fn/math/asinh
core-js(/library)/fn/math/atanh
core-js(/library)/fn/math/cbrt
core-js(/library)/fn/math/clz32
core-js(/library)/fn/math/cosh
core-js(/library)/fn/math/expm1
core-js(/library)/fn/math/fround
core-js(/library)/fn/math/hypot
core-js(/library)/fn/math/imul
core-js(/library)/fn/math/log1p
core-js(/library)/fn/math/log10
core-js(/library)/fn/math/log2
core-js(/library)/fn/math/sign
core-js(/library)/fn/math/sinh
core-js(/library)/fn/math/tanh
core-js(/library)/fn/math/trunc
```
#### ECMAScript 6: Date
Modules [`es6.date.to-string`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.date.to-string.js), ES5 features with fixes: [`es6.date.now`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.date.now.js), [`es6.date.to-iso-string`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.date.to-iso-string.js), [`es6.date.to-json`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.date.to-json.js) and [`es6.date.to-primitive`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.date.to-primitive.js).
```js
Date
  .now()               -> int
  #toISOString()       -> string
  #toJSON()            -> string
  #toString()          -> string
  #@@toPrimitive(hint) -> primitive
```
[*CommonJS entry points:*](#commonjs)
```
core-js/es6/date
core-js/fn/date/to-string
core-js(/library)/fn/date/now
core-js(/library)/fn/date/to-iso-string
core-js(/library)/fn/date/to-json
core-js(/library)/fn/date/to-primitive
```
[*Example*](http://goo.gl/haeHLR):
```js
new Date(NaN).toString(); // => 'Invalid Date'
```

#### ECMAScript 6: Promise
Module [`es6.promise`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.promise.js).
```js
new Promise(executor(resolve(var), reject(var))) -> promise
  #then(resolved(var), rejected(var))            -> promise
  #catch(rejected(var))                          -> promise
  .resolve(promise | var)                        -> promise
  .reject(var)                                   -> promise
  .all(iterable)                                 -> promise
  .race(iterable)                                -> promise
```
[*CommonJS entry points:*](#commonjs)
```
core-js(/library)/es6/promise
core-js(/library)/fn/promise
```
Basic [*example*](http://goo.gl/vGrtUC):
```js
function sleepRandom(time){
  return new Promise(function(resolve, reject){
    setTimeout(resolve, time * 1e3, 0 | Math.random() * 1e3);
  });
}

console.log('Run');                    // => Run
sleepRandom(5).then(function(result){
  console.log(result);                 // => 869, after 5 sec.
  return sleepRandom(10);
}).then(function(result){
  console.log(result);                 // => 202, after 10 sec.
}).then(function(){
  console.log('immediately after');    // => immediately after
  throw Error('Irror!');
}).then(function(){
  console.log('will not be displayed');
}).catch(x => console.log(x));         // => => Error: Irror!
```
`Promise.resolve` and `Promise.reject` [*example*](http://goo.gl/vr8TN3):
```js
Promise.resolve(42).then(x => console.log(x)); // => 42
Promise.reject(42).catch(x => console.log(x)); // => 42

Promise.resolve($.getJSON('/data.json')); // => ES6 promise
```
`Promise.all` [*example*](http://goo.gl/RdoDBZ):
```js
Promise.all([
  'foo',
  sleepRandom(5),
  sleepRandom(15),
  sleepRandom(10)             // after 15 sec:
]).then(x => console.log(x)); // => ['foo', 956, 85, 382]
```
`Promise.race` [*example*](http://goo.gl/L8ovkJ):
```js
function timeLimit(promise, time){
  return Promise.race([promise, new Promise(function(resolve, reject){
    setTimeout(reject, time * 1e3, Error('Await > ' + time + ' sec'));
  })]);
}

timeLimit(sleepRandom(5), 10).then(x => console.log(x));   // => 853, after 5 sec.
timeLimit(sleepRandom(15), 10).catch(x => console.log(x)); // Error: Await > 10 sec
```
ECMAScript 7 [async functions](https://tc39.github.io/ecmascript-asyncawait) [example](http://goo.gl/wnQS4j):
```js
var delay = time => new Promise(resolve => setTimeout(resolve, time))

async function sleepRandom(time){
  await delay(time * 1e3);
  return 0 | Math.random() * 1e3;
};
async function sleepError(time, msg){
  await delay(time * 1e3);
  throw Error(msg);
};

(async () => {
  try {
    console.log('Run');                // => Run
    console.log(await sleepRandom(5)); // => 936, after 5 sec.
    var [a, b, c] = await Promise.all([
      sleepRandom(5),
      sleepRandom(15),
      sleepRandom(10)
    ]);
    console.log(a, b, c);              // => 210 445 71, after 15 sec.
    await sleepError(5, 'Irror!');
    console.log('Will not be displayed');
  } catch(e){
    console.log(e);                    // => Error: 'Irror!', after 5 sec.
  }
})();
```

##### Unhandled rejection tracking

In Node.js, like in native implementation, available events [`unhandledRejection`](https://nodejs.org/api/process.html#process_event_unhandledrejection) and [`rejectionHandled`](https://nodejs.org/api/process.html#process_event_rejectionhandled):
```js
process.on('unhandledRejection', (reason, promise) => console.log('unhandled', reason, promise));
process.on('rejectionHandled', (promise) => console.log('handled', promise));

var p = Promise.reject(42);
// unhandled 42 [object Promise]

setTimeout(() => p.catch(_ => _), 1e3);
// handled [object Promise]
```
In a browser on rejection, by default, you will see notify in the console, or you can add a custom handler and a handler on handling unhandled, [*example*](http://goo.gl/Wozskl):
```js
window.onunhandledrejection = e => console.log('unhandled', e.reason, e.promise);
window.onrejectionhandled = e => console.log('handled', e.reason, e.promise);

var p = Promise.reject(42);
// unhandled 42 [object Promise]

setTimeout(() => p.catch(_ => _), 1e3);
// handled 42 [object Promise]
```

#### ECMAScript 6: Symbol
Module [`es6.symbol`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.symbol.js).
```js
Symbol(description?)  -> symbol
  .hasInstance        -> @@hasInstance
  .isConcatSpreadable -> @@isConcatSpreadable
  .iterator           -> @@iterator
  .match              -> @@match
  .replace            -> @@replace
  .search             -> @@search
  .species            -> @@species
  .split              -> @@split
  .toPrimitive        -> @@toPrimitive
  .toStringTag        -> @@toStringTag
  .unscopables        -> @@unscopables
  .for(key)           -> symbol
  .keyFor(symbol)     -> key
  .useSimple()        -> void
  .useSetter()        -> void
Object
  .getOwnPropertySymbols(object) -> array
```
Also wrapped some methods for correct work with `Symbol` polyfill.
```js
Object
  .create(proto | null, descriptors?)    -> object
  .defineProperty(target, key, desc)     -> target
  .defineProperties(target, descriptors) -> target
  .getOwnPropertyDescriptor(var, key)    -> desc | undefined
  .getOwnPropertyNames(var)              -> array
  #propertyIsEnumerable(key)             -> bool
JSON
  .stringify(target, replacer?, space?) -> string | undefined
```
[*CommonJS entry points:*](#commonjs)
```
core-js(/library)/es6/symbol
core-js(/library)/fn/symbol
core-js(/library)/fn/symbol/has-instance
core-js(/library)/fn/symbol/is-concat-spreadable
core-js(/library)/fn/symbol/iterator
core-js(/library)/fn/symbol/match
core-js(/library)/fn/symbol/replace
core-js(/library)/fn/symbol/search
core-js(/library)/fn/symbol/species
core-js(/library)/fn/symbol/split
core-js(/library)/fn/symbol/to-primitive
core-js(/library)/fn/symbol/to-string-tag
core-js(/library)/fn/symbol/unscopables
core-js(/library)/fn/symbol/for
core-js(/library)/fn/symbol/key-for
```
[*Basic example*](http://goo.gl/BbvWFc):
```js
var Person = (function(){
  var NAME = Symbol('name');
  function Person(name){
    this[NAME] = name;
  }
  Person.prototype.getName = function(){
    return this[NAME];
  };
  return Person;
})();

var person = new Person('Vasya');
console.log(person.getName());          // => 'Vasya'
console.log(person['name']);            // => undefined
console.log(person[Symbol('name')]);    // => undefined, symbols are uniq
for(var key in person)console.log(key); // => only 'getName', symbols are not enumerable
```
`Symbol.for` & `Symbol.keyFor` [*example*](http://goo.gl/0pdJjX):
```js
var symbol = Symbol.for('key');
symbol === Symbol.for('key'); // true
Symbol.keyFor(symbol);        // 'key'
```
[*Example*](http://goo.gl/mKVOQJ) with methods for getting own object keys:
```js
var O = {a: 1};
Object.defineProperty(O, 'b', {value: 2});
O[Symbol('c')] = 3;
Object.keys(O);                  // => ['a']
Object.getOwnPropertyNames(O);   // => ['a', 'b']
Object.getOwnPropertySymbols(O); // => [Symbol(c)]
Reflect.ownKeys(O);              // => ['a', 'b', Symbol(c)]
```
##### Caveats when using `Symbol` polyfill:

* We can't add new primitive type, `Symbol` returns object.
* `Symbol.for` and `Symbol.keyFor` can't be shimmed cross-realm.
* By default, to hide the keys, `Symbol` polyfill defines setter in `Object.prototype`. For this reason, uncontrolled creation of symbols can cause memory leak and the `in` operator is not working correctly with `Symbol` polyfill: `Symbol() in {} // => true`.

You can disable defining setters in `Object.prototype`. [Example](http://goo.gl/N5UD7J):
```js
Symbol.useSimple();
var s1 = Symbol('s1')
  , o1 = {};
o1[s1] = true;
for(var key in o1)console.log(key); // => 'Symbol(s1)_t.qamkg9f3q', w/o native Symbol

Symbol.useSetter();
var s2 = Symbol('s2')
  , o2 = {};
o2[s2] = true;
for(var key in o2)console.log(key); // nothing
```
* Currently, `core-js` not adds setters to `Object.prototype` for well-known symbols for correct work something like `Symbol.iterator in foo`. It can cause problems with their enumerability.
* Some problems possible with environment exotic objects (for example, IE `localStorage`).

#### ECMAScript 6: Collections
`core-js` uses native collections in most case, just fixes methods / constructor, if it's required, and in old environment uses fast polyfill (O(1) lookup).
#### Map
Module [`es6.map`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.map.js).
```js
new Map(iterable (entries) ?)     -> map
  #clear()                        -> void
  #delete(key)                    -> bool
  #forEach(fn(val, key, @), that) -> void
  #get(key)                       -> val
  #has(key)                       -> bool
  #set(key, val)                  -> @
  #size                           -> uint
  #values()                       -> iterator
  #keys()                         -> iterator
  #entries()                      -> iterator
  #@@iterator()                   -> iterator (entries)
```
[*CommonJS entry points:*](#commonjs)
```
core-js(/library)/es6/map
core-js(/library)/fn/map
```
[*Examples*](http://goo.gl/GWR7NI):
```js
var a = [1];

var map = new Map([['a', 1], [42, 2]]);
map.set(a, 3).set(true, 4);

console.log(map.size);        // => 4
console.log(map.has(a));      // => true
console.log(map.has([1]));    // => false
console.log(map.get(a));      // => 3
map.forEach(function(val, key){
  console.log(val);           // => 1, 2, 3, 4
  console.log(key);           // => 'a', 42, [1], true
});
map.delete(a);
console.log(map.size);        // => 3
console.log(map.get(a));      // => undefined
console.log(Array.from(map)); // => [['a', 1], [42, 2], [true, 4]]

var map = new Map([['a', 1], ['b', 2], ['c', 3]]);

for(var [key, val] of map){
  console.log(key);                           // => 'a', 'b', 'c'
  console.log(val);                           // => 1, 2, 3
}
for(var val of map.values())console.log(val); // => 1, 2, 3
for(var key of map.keys())console.log(key);   // => 'a', 'b', 'c'
for(var [key, val] of map.entries()){
  console.log(key);                           // => 'a', 'b', 'c'
  console.log(val);                           // => 1, 2, 3
}
```
#### Set
Module [`es6.set`](https://github.com/zloirock/core-js/blob/v2.6.12/modules/es6.set.js).