core-js compat


Keywords
es2015, es2019, es6, es7, esnext, javascript, js, polyfill, ponyfill, promise, proposal, shim, symbol, weakmap
License
MIT
Install
npm install core-js-compat@3.2.1

Documentation

core-js

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Modular standard library for JavaScript. Includes polyfills for ECMAScript up to 2019: promises, symbols, collections, iterators, typed arrays, many other features, ECMAScript proposals, some cross-platform WHATWG / W3C features and proposals like URL. You can load only required features or use it without global namespace pollution.

If you looking documentation for obsolete core-js@2, please, check this branch.

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

core-js@3, babel and a look into the future

Raising funds

core-js isn't backed by a company, so the future of this project depends on you. Become a sponsor or a backer on Open Collective, on Patreon or on Tidelift if you are interested in core-js.




Example:

import 'core-js'; // <- at the top of your entry point

Array.from(new Set([1, 2, 3, 2, 1]));          // => [1, 2, 3]
[1, [2, 3], [4, [5]]].flat(2);                 // => [1, 2, 3, 4, 5]
Promise.resolve(32).then(x => console.log(x)); // => 32

You can load only required features:

import 'core-js/features/array/from'; // <- at the top of your entry point
import 'core-js/features/array/flat'; // <- at the top of your entry point
import 'core-js/features/set';        // <- at the top of your entry point
import 'core-js/features/promise';    // <- at the top of your entry point

Array.from(new Set([1, 2, 3, 2, 1]));          // => [1, 2, 3]
[1, [2, 3], [4, [5]]].flat(2);                 // => [1, 2, 3, 4, 5]
Promise.resolve(32).then(x => console.log(x)); // => 32

Or use it without global namespace pollution:

import from from 'core-js-pure/features/array/from';
import flat from 'core-js-pure/features/array/flat';
import Set from 'core-js-pure/features/set';
import Promise from 'core-js-pure/features/promise';

from(new Set([1, 2, 3, 2, 1]));                // => [1, 2, 3]
flat([1, [2, 3], [4, [5]]], 2);                // => [1, 2, 3, 4, 5]
Promise.resolve(32).then(x => console.log(x)); // => 32

Index

Usage

Installation:

// global version
npm install --save core-js@3.2.1
// version without global namespace pollution
npm install --save core-js-pure@3.2.1
// bundled global version
npm install --save core-js-bundle@3.2.1

Already bundled version of core-js on CDN (minified version).

CommonJS API

You can import only-required-for-you polyfills, like in examples at the top of README.md. Available CommonJS entry points for all polyfilled methods / constructors and namespaces. Just some examples:

// polyfill all `core-js` features:
import "core-js";
// polyfill only stable `core-js` features - ES and web standards:
import "core-js/stable";
// polyfill only stable ES features:
import "core-js/es";

// if you want to polyfill `Set`:
// all `Set`-related features, with ES proposals:
import "core-js/features/set";
// stable required for `Set` ES features and features from web standards
// (DOM collections iterator in this case):
import "core-js/stable/set";
// only stable ES features required for `Set`:
import "core-js/es/set";
// the same without global namespace pollution:
import Set from "core-js-pure/features/set";
import Set from "core-js-pure/stable/set";
import Set from "core-js-pure/es/set";

// if you want to polyfill just required methods:
import "core-js/features/set/intersection";
import "core-js/stable/queue-microtask";
import "core-js/es/array/from";

// polyfill reflect metadata proposal:
import "core-js/proposals/reflect-metadata";
// polyfill all stage 2+ proposals:
import "core-js/stage/2";
Caveats when using CommonJS API:
  • modules path is an 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.
  • If you use core-js with the extension of native objects, recommended load all core-js modules at the top of the entry point of your application, otherwise, you can have conflicts.
  • 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 pure version, we can't pollute prototypes of native constructors. Because of that, prototype methods transformed into static methods like in examples above. But with transpilers, we can use one more trick - bind operator and virtual methods. Special for that, available /virtual/ entry points. Example:

import fill from 'core-js-pure/features/array/virtual/fill';
import findIndex from 'core-js-pure/features/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-pure/features/array/virtual';

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

Babel

core-js is integrated with babel and is the base for polyfilling-related babel features:

@babel/polyfill

@babel/polyfill IS just the import of stable core-js features and regenerator-runtime for generators and async functions, so if you load @babel/polyfill - you load the global version of core-js without ES proposals.

Now it's deprecated in favour of separate inclusion of required parts of core-js and regenerator-runtime and, for preventing breaking changes, left on core-js@2.

As a full equal of @babel/polyfill, you can use this:

import 'core-js/stable';
import 'regenerator-runtime/runtime';

@babel/preset-env

@babel/preset-env has useBuiltIns option, which optimizes working with global version of core-js. With useBuiltIns option, you should also set corejs option to used version of core-js, like corejs: 3 or corejs: '3.2'.

  • useBuiltIns: 'entry' replaces imports of core-js to import only required for a target environment modules. So, for example,
import 'core-js/stable';

with chrome 71 target will be replaced just to:

import "core-js/modules/es.array.unscopables.flat";
import "core-js/modules/es.array.unscopables.flat-map";
import "core-js/modules/es.object.from-entries";
import "core-js/modules/web.immediate";

It works for all entry points of global version of core-js and their combinations, for example for

import 'core-js/es';
import 'core-js/proposals/set-methods';
import 'core-js/features/set/map';

with chrome 71 target you will have as a result:

import "core-js/modules/es.array.unscopables.flat";
import "core-js/modules/es.array.unscopables.flat-map";
import "core-js/modules/es.object.from-entries";
import "core-js/modules/esnext.set.difference";
import "core-js/modules/esnext.set.intersection";
import "core-js/modules/esnext.set.is-disjoint-from";
import "core-js/modules/esnext.set.is-subset-of";
import "core-js/modules/esnext.set.is-superset-of";
import "core-js/modules/esnext.set.map";
import "core-js/modules/esnext.set.symmetric-difference";
import "core-js/modules/esnext.set.union";
  • useBuiltIns: 'usage' adds to the top of each file import of polyfills for features used in this file and not supported by target environments, so for:
// first file:
var set = new Set([1, 2, 3]);

// second file:
var array = Array.of(1, 2, 3);

if target contains an old environment like IE 11 we will have something like:

// first file:
import 'core-js/modules/es.array.iterator';
import 'core-js/modules/es.object.to-string';
import 'core-js/modules/es.set';
var set = new Set([1, 2, 3]);

// second file:
import 'core-js/modules/es.array.of';
var array = Array.of(1, 2, 3);

By default, @babel/preset-env with useBuiltIns: 'usage' option only polyfills stable features, but you can enable polyfilling of proposals by proposals option, as corejs: { version: '3.2', proposals: true }.

@babel/runtime

@babel/runtime with corejs: 3 option simplifies work with core-js-pure. It automatically replaces usage of modern features from JS standard library to imports from the version of core-js without global namespace pollution, so instead of:

import from from 'core-js-pure/stable/array/from';
import flat from 'core-js-pure/stable/array/flat';
import Set from 'core-js-pure/stable/set';
import Promise from 'core-js-pure/stable/promise';

from(new Set([1, 2, 3, 2, 1]));
flat([1, [2, 3], [4, [5]]], 2);
Promise.resolve(32).then(x => console.log(x));

you can write just:

Array.from(new Set([1, 2, 3, 2, 1]));
[1, [2, 3], [4, [5]]].flat(2);
Promise.resolve(32).then(x => console.log(x));

By default, @babel/runtime only polyfills stable features, but like in @babel/preset-env, you can enable polyfilling of proposals by proposals option, as corejs: { version: 3, proposals: true }.

Configurable level of aggressiveness

By default, core-js sets polyfills only when they are required. That means that core-js checks if a feature is available and works correctly or not and if it has no problems, core-js use native implementation.

But sometimes core-js feature detection could be too strict for your case. For example, Promise constructor requires the support of unhandled rejection tracking and @@species.

Sometimes we could have inverse problem - knowingly broken environment with problems not covered by core-js feature detection.

For those cases, we could redefine this behaviour for certain polyfills:

const configurator = require('core-js/configurator');

configurator({
  useNative: ['Promise'],                                 // polyfills will be used only if natives completely unavailable
  usePolyfill: ['Array.from', 'String.prototype.padEnd'], // polyfills will be used anyway
  useFeatureDetection: ['Map', 'Set'],                    // default behaviour
});

require('core-js');

It does not work with some features. Also, if you change the default behaviour, even core-js internals may not work correctly.

Custom build

For some cases could be useful adding a blacklist of features or generation a polyfill for target engines. You could use core-js-builder package for that.

Compatibility data

core-js-compat package contains data about the necessity of core-js modules and API for getting a list of required core-js modules by browserslist query.

Supported engines

Tested in:

  • Chrome 26+
  • Firefox 4+
  • Safari 5+
  • Opera 12+
  • Internet Explorer 8+ (sure, IE8 with ES3 limitations; IE7- also should work, but no longer tested)
  • Edge
  • Android Browser 2.3+
  • iOS Safari 5.1+
  • PhantomJS 1.9+
  • 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:

core-js(-pure)

ECMAScript

CommonJS entry points:

core-js(-pure)/es

ECMAScript: Object

Modules es.object.assign, es.object.is, es.object.set-prototype-of, es.object.to-string, es.object.freeze, es.object.seal, es.object.prevent-extensions, es.object.is-frozen, es.object.is-sealed, es.object.is-extensible, es.object.get-own-property-descriptor, es.object.get-own-property-descriptors, es.object.get-prototype-of, es.object.keys, es.object.values, es.object.entries, es.object.get-own-property-names and es.object.from-entries.

Just ES5 features: es.object.create, es.object.define-property and es.object.define-properties.

ES2017 Annex B - modules es.object.define-setter, es.object.define-getter, es.object.lookup-setter and es.object.lookup-getter

class Object {
  toString(): string; // ES2015+ fix: @@toStringTag support
  __defineGetter__(property: PropertyKey, getter: Function): void;
  __defineSetter__(property: PropertyKey, setter: Function): void;
  __lookupGetter__(property: PropertyKey): Function | void;
  __lookupSetter__(property: PropertyKey): Function | void;
  static assign(target: Object, ...sources: Array<Object>): Object;
  static create(prototype: Object | null, properties?: { [property: PropertyKey]: PropertyDescriptor }): Object;
  static defineProperties(object: Object, properties: { [property: PropertyKey]: PropertyDescriptor })): Object;
  static defineProperty(object: Object, property: PropertyKey, attributes: PropertyDescriptor): Object;
  static entries(object: Object): Array<[string, mixed]>;
  static freeze(object: any): any;
  static fromEntries(iterable: Iterable<[key, value]>): Object;
  static getOwnPropertyDescriptor(object: any, property: PropertyKey): PropertyDescriptor | void;
  static getOwnPropertyDescriptors(object: any): { [property: PropertyKey]: PropertyDescriptor };
  static getOwnPropertyNames(object: any): Array<string>;
  static getPrototypeOf(object: any): Object | null;
  static is(value1: any, value2: any): boolean;
  static isExtensible(object: any): boolean;
  static isFrozen(object: any): boolean;
  static isSealed(object: any): boolean;
  static keys(object: any): Array<string>;
  static preventExtensions(object: any): any;
  static seal(object: any): any;
  static setPrototypeOf(target: any, prototype: Object | null): any; // required __proto__ - IE11+
  static values(object: any): Array<mixed>;
}

CommonJS entry points:

core-js(-pure)/es|stable|features/object
core-js(-pure)/es|stable|features/object/assign
core-js(-pure)/es|stable|features/object/is
core-js(-pure)/es|stable|features/object/set-prototype-of
core-js(-pure)/es|stable|features/object/get-prototype-of
core-js(-pure)/es|stable|features/object/create
core-js(-pure)/es|stable|features/object/define-property
core-js(-pure)/es|stable|features/object/define-properties
core-js(-pure)/es|stable|features/object/get-own-property-descriptor
core-js(-pure)/es|stable|features/object/get-own-property-descriptors
core-js(-pure)/es|stable|features/object/keys
core-js(-pure)/es|stable|features/object/values
core-js(-pure)/es|stable|features/object/entries
core-js(-pure)/es|stable|features/object/get-own-property-names
core-js(-pure)/es|stable|features/object/freeze
core-js(-pure)/es|stable|features/object/from-entries
core-js(-pure)/es|stable|features/object/seal
core-js(-pure)/es|stable|features/object/prevent-extensions
core-js(-pure)/es|stable|features/object/is-frozen
core-js(-pure)/es|stable|features/object/is-sealed
core-js(-pure)/es|stable|features/object/is-extensible
core-js/es|stable|features/object/to-string
core-js(-pure)/es|stable|features/object/define-getter
core-js(-pure)/es|stable|features/object/define-setter
core-js(-pure)/es|stable|features/object/lookup-getter
core-js(-pure)/es|stable|features/object/lookup-setter

Examples:

let foo = { q: 1, w: 2 };
let bar = { e: 3, r: 4 };
let 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

let object = {
  [Symbol.toStringTag]: 'Foo'
};

'' + object; // => '[object Foo]'

Object.keys('qwe'); // => ['0', '1', '2']
Object.getPrototypeOf('qwe') === String.prototype; // => true

Object.values({ a: 1, b: 2, c: 3 });  // => [1, 2, 3]
Object.entries({ a: 1, b: 2, c: 3 }); // => [['a', 1], ['b', 2], ['c', 3]]

for (let [key, value] of Object.entries({ a: 1, b: 2, c: 3 })) {
  console.log(key);   // => 'a', 'b', 'c'
  console.log(value); // => 1, 2, 3
}

// Shallow object cloning with prototype and descriptors:
let copy = Object.create(Object.getPrototypeOf(object), Object.getOwnPropertyDescriptors(object));
// Mixin:
Object.defineProperties(target, Object.getOwnPropertyDescriptors(source));

const map = new Map([['a', 1], ['b', 2]]);
Object.fromEntries(map); // => { a: 1, b: 2 }

class Unit {
  constructor(id) {
    this.id = id;
  }
  toString() {
    return `unit${ this.id }`;
  }
}

const units = new Set([new Unit(101), new Unit(102)]);

Object.fromEntries(units.entries()); // => { unit101: Unit { id: 101 }, unit102: Unit { id: 102 } }

ECMAScript: Function

Modules es.function.name, es.function.has-instance. Just ES5: es.function.bind.

class Function {
  name: string;
  bind(thisArg: any, ...args: Array<mixed>): Function;
  @@hasInstance(value: any): boolean;
}

CommonJS entry points:

core-js/es|stable|features/function
core-js/es|stable|features/function/name
core-js/es|stable|features/function/has-instance
core-js/es|stable|features/function/bind
core-js/es|stable|features/function/virtual/bind

Example:

(function foo() {}).name // => 'foo'

console.log.bind(console, 42)(43); // => 42 43

ECMAScript: Array

Modules es.array.from, es.array.is-array, es.array.of, es.array.copy-within, es.array.fill, es.array.find, es.array.find-index, es.array.iterator, es.array.includes, es.array.slice, es.array.join, es.array.index-of, es.array.last-index-of, es.array.every, es.array.some, es.array.for-each, es.array.map, es.array.filter, es.array.reduce, es.array.reduce-right, es.array.reverse, es.array.sort, es.array.flat, es.array.flat-map, es.array.unscopables.flat and es.array.unscopables.flat-map

class Array {
  concat(...args: Array<mixed>): Array<mixed>; // with adding support of @@isConcatSpreadable and @@species
  copyWithin(target: number, start: number, end?: number): this;
  entries(): Iterator<[index, value]>;
  every(callbackfn: (value: any, index: number, target: any) => boolean, thisArg?: any): boolean;
  fill(value: any, start?: number, end?: number): this;
  filter(callbackfn: (value: any, index: number, target: any) => boolean, thisArg?: any): Array<mixed>; // with adding support of @@species
  find(callbackfn: (value: any, index: number, target: any) => boolean), thisArg?: any): any;
  findIndex(callbackfn: (value: any, index: number, target: any) => boolean, thisArg?: any): number;
  flat(depthArg?: number = 1): Array<mixed>;
  flatMap(mapFn: (value: any, index: number, target: any) => any, thisArg: any): Array<mixed>;
  forEach(callbackfn: (value: any, index: number, target: any) => void, thisArg?: any): void;
  includes(searchElement: any, from?: number): boolean;
  indexOf(searchElement: any, from?: number): number;
  join(separator: string = ','): string;
  keys(): Iterator<index>;
  lastIndexOf(searchElement: any, from?: number): number;
  map(mapFn: (value: any, index: number, target: any) => any, thisArg?: any): Array<mixed>; // with adding support of @@species
  reduce(callbackfn: (memo: any, value: any, index: number, target: any) => any, initialValue?: any): any;
  reduceRight(callbackfn: (memo: any, value: any, index: number, target: any) => any, initialValue?: any): any;
  reverse(): this; // Safari 12.0 bug fix
  slice(start?: number, end?: number): Array<mixed>; // with adding support of @@species
  splice(start?: number, deleteCount?: number, ...items: Array<mixed>): Array<mixed>; // with adding support of @@species
  some(callbackfn: (value: any, index: number, target: any) => boolean, thisArg?: any): boolean;
  sort(comparefn?: (a: any, b: any) => number): this;
  values(): Iterator<value>;
  @@iterator(): Iterator<value>;
  @@unscopables: { [newMethodNames: string]: true };
  static from(items: Iterable | ArrayLike, mapFn?: (value: any, index: number) => any, thisArg?: any): Array<mixed>;
  static isArray(value: any): boolean;
  static of(...args: Array<mixed>): Array<mixed>;
}

class Arguments {
  @@iterator(): Iterator<value>; // available only in core-js methods
}

CommonJS entry points:

core-js(-pure)/es|stable|features/array
core-js(-pure)/es|stable|features/array/from
core-js(-pure)/es|stable|features/array/of
core-js(-pure)/es|stable|features/array/is-array
core-js(-pure)/es|stable|features/array/concat
core-js(-pure)/es|stable|features/array/entries
core-js(-pure)/es|stable|features/array/every
core-js(-pure)/es|stable|features/array/copy-within
core-js(-pure)/es|stable|features/array/fill
core-js(-pure)/es|stable|features/array/filter
core-js(-pure)/es|stable|features/array/find
core-js(-pure)/es|stable|features/array/find-index
core-js(-pure)/es|stable|features/array/flat
core-js(-pure)/es|stable|features/array/flat-map
core-js(-pure)/es|stable|features/array/for-each
core-js(-pure)/es|stable|features/array/includes
core-js(-pure)/es|stable|features/array/index-of
core-js(-pure)/es|stable|features/array/iterator
core-js(-pure)/es|stable|features/array/join
core-js(-pure)/es|stable|features/array/keys
core-js(-pure)/es|stable|features/array/last-index-of
core-js(-pure)/es|stable|features/array/map
core-js(-pure)/es|stable|features/array/reduce
core-js(-pure)/es|stable|features/array/reduce-right
core-js(-pure)/es|stable|features/array/reverse
core-js(-pure)/es|stable|features/array/slice
core-js(-pure)/es|stable|features/array/splice
core-js(-pure)/es|stable|features/array/some
core-js(-pure)/es|stable|features/array/sort
core-js(-pure)/es|stable|features/array/values
core-js(-pure)/es|stable|features/array/virtual/concat
core-js(-pure)/es|stable|features/array/virtual/copy-within
core-js(-pure)/es|stable|features/array/virtual/entries
core-js(-pure)/es|stable|features/array/virtual/every
core-js(-pure)/es|stable|features/array/virtual/fill
core-js(-pure)/es|stable|features/array/virtual/filter
core-js(-pure)/es|stable|features/array/virtual/find
core-js(-pure)/es|stable|features/array/virtual/find-index
core-js(-pure)/es|stable|features/array/virtual/flat
core-js(-pure)/es|stable|features/array/virtual/flat-map
core-js(-pure)/es|stable|features/array/virtual/for-each
core-js(-pure)/es|stable|features/array/virtual/includes
core-js(-pure)/es|stable|features/array/virtual/index-of
core-js(-pure)/es|stable|features/array/virtual/iterator
core-js(-pure)/es|stable|features/array/virtual/join
core-js(-pure)/es|stable|features/array/virtual/keys
core-js(-pure)/es|stable|features/array/virtual/last-index-of
core-js(-pure)/es|stable|features/array/virtual/map
core-js(-pure)/es|stable|features/array/virtual/reduce
core-js(-pure)/es|stable|features/array/virtual/reduce-right
core-js(-pure)/es|stable|features/array/virtual/reverse
core-js(-pure)/es|stable|features/array/virtual/slice
core-js(-pure)/es|stable|features/array/virtual/some
core-js(-pure)/es|stable|features/array/virtual/sort
core-js(-pure)/es|stable|features/array/virtual/splice
core-js(-pure)/es|stable|features/array/virtual/values

Examples:

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', it => it * it);            // => [1, 4, 9]

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

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

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

function isOdd(value) {
  return value % 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]


[1, 2, 3].includes(2);        // => true
[1, 2, 3].includes(4);        // => false
[1, 2, 3].includes(2, 2);     // => false

[NaN].indexOf(NaN);           // => -1
[NaN].includes(NaN);          // => true
Array(1).indexOf(undefined);  // => -1
Array(1).includes(undefined); // => true

[1, [2, 3], [4, 5]].flat();    // => [1, 2, 3, 4, 5]
[1, [2, [3, [4]]], 5].flat();  // => [1, 2, [3, [4]], 5]
[1, [2, [3, [4]]], 5].flat(3); // => [1, 2, 3, 4, 5]

[{ a: 1, b: 2 }, { a: 3, b: 4 }, { a: 5, b: 6 }].flatMap(it => [it.a, it.b]); // => [1, 2, 3, 4, 5, 6]

ECMAScript: String and RegExp

The main part of String features: modules es.string.from-code-point, es.string.raw, es.string.iterator, es.string.split, es.string.code-point-at, es.string.ends-with, es.string.includes, es.string.repeat, es.string.pad-start, es.string.pad-end, es.string.starts-with, es.string.trim, es.string.trim-start, es.string.trim-end, es.string.match-all.

Adding support of well-known symbols @@match, @@replace, @@search and @@split and direct .exec calls to related String methods, modules es.string.match, es.string.replace, es.string.search and es.string.split.

Annex B HTML methods. Ugly, but it's also the part of the spec. Modules es.string.anchor, es.string.big, es.string.blink, es.string.bold, es.string.fixed, es.string.fontcolor, es.string.fontsize, es.string.italics, es.string.link, es.string.small, es.string.strike, es.string.sub and es.string.sup.

RegExp features: modules es.regexp.constructor and es.regexp.flags.

class String {
  static fromCodePoint(...codePoints: Array<number>): string;
  static raw({ raw: Array<string> }, ...substitutions: Array<string>): string;
  split(separator: string, limit?: number): Array<string>;
  includes(searchString: string, position?: number): boolean;
  startsWith(searchString: string, position?: number): boolean;
  endsWith(searchString: string, position?: number): boolean;
  repeat(count: number): string;
  padStart(length: number, fillStr?: string = ' '): string;
  padEnd(length: number, fillStr?: string = ' '): string;
  codePointAt(pos: number): number | void;
  match(template: any): any; // ES2015+ fix for support @@match
  matchAll(regexp: RegExp): Iterator;
  replace(template: any, replacer: any): any; // ES2015+ fix for support @@replace
  search(template: any): any; // ES2015+ fix for support @@search
  split(template: any, limit: any): any; // ES2015+ fix for support @@split, some fixes for old engines
  trim(): string;
  trimLeft(): string;
  trimRight(): string;
  trimStart(): string;
  trimEnd(): string;
  anchor(name: string): string;
  big(): string;
  blink(): string;
  bold(): string;
  fixed(): string;
  fontcolor(color: string): string;
  fontsize(size: any): string;
  italics(): string;
  link(url: string): string;
  small(): string;
  strike(): string;
  sub(): string;
  sup(): string;
  @@iterator(): Iterator<characters>;
}

class RegExp {
  constructor(pattern: RegExp | string, flags?: string): RegExp; // ES2015+ fix - can alter flags (IE9+)
  exec(): Array<string | undefined> | null; // IE8 fixes
  toString(): string; // ES2015+ fix - generic
  @@match(string: string): Array | null;
  @@replace(string: string, replaceValue: Function | string): string;
  @@search(string: string): number;
  @@split(string: string, limit: number): Array<string>;
  readonly attribute flags: string; // IE9+
}

CommonJS entry points:

core-js(-pure)/es|stable|features/string
core-js(-pure)/es|stable|features/string/from-code-point
core-js(-pure)/es|stable|features/string/raw
core-js(-pure)/es|stable|features/string/code-point-at
core-js(-pure)/es|stable|features/string/ends-with
core-js(-pure)/es|stable|features/string/includes
core-js(-pure)/es|stable|features/string/starts-with
core-js/es|stable|features/string/match
core-js(-pure)/es|stable|features/string/match-all
core-js(-pure)/es|stable|features/string/repeat
core-js(-pure)/es|stable|features/string/pad-start
core-js(-pure)/es|stable|features/string/pad-end
core-js/es|stable|features/string/replace
core-js/es|stable|features/string/search
core-js/es|stable|features/string/split
core-js(-pure)/es|stable|features/string/trim
core-js(-pure)/es|stable|features/string/trim-start
core-js(-pure)/es|stable|features/string/trim-end
core-js(-pure)/es|stable|features/string/trim-left
core-js(-pure)/es|stable|features/string/trim-right
core-js(-pure)/es|stable|features/string/anchor
core-js(-pure)/es|stable|features/string/big
core-js(-pure)/es|stable|features/string/blink
core-js(-pure)/es|stable|features/string/bold
core-js(-pure)/es|stable|features/string/fixed
core-js(-pure)/es|stable|features/string/fontcolor
core-js(-pure)/es|stable|features/string/fontsize
core-js(-pure)/es|stable|features/string/italics
core-js(-pure)/es|stable|features/string/link
core-js(-pure)/es|stable|features/string/small
core-js(-pure)/es|stable|features/string/strike
core-js(-pure)/es|stable|features/string/sub
core-js(-pure)/es|stable|features/string/sup
core-js(-pure)/es|stable|features/string/iterator
core-js(-pure)/es|stable|features/string/virtual/includes
core-js(-pure)/es|stable|features/string/virtual/starts-with
core-js(-pure)/es|stable|features/string/virtual/ends-with
core-js(-pure)/es|stable|features/string/virtual/match-all
core-js(-pure)/es|stable|features/string/virtual/repeat
core-js(-pure)/es|stable|features/string/virtual/pad-start
core-js(-pure)/es|stable|features/string/virtual/pad-end
core-js(-pure)/es|stable|features/string/virtual/code-point-at
core-js(-pure)/es|stable|features/string/virtual/trim
core-js(-pure)/es|stable|features/string/virtual/trim-start
core-js(-pure)/es|stable|features/string/virtual/trim-end
core-js(-pure)/es|stable|features/string/virtual/trim-left
core-js(-pure)/es|stable|features/string/virtual/trim-right
core-js(-pure)/es|stable|features/string/virtual/anchor
core-js(-pure)/es|stable|features/string/virtual/big
core-js(-pure)/es|stable|features/string/virtual/blink
core-js(-pure)/es|stable|features/string/virtual/bold
core-js(-pure)/es|stable|features/string/virtual/fixed
core-js(-pure)/es|stable|features/string/virtual/fontcolor
core-js(-pure)/es|stable|features/string/virtual/fontsize
core-js(-pure)/es|stable|features/string/virtual/italics
core-js(-pure)/es|stable|features/string/virtual/link
core-js(-pure)/es|stable|features/string/virtual/small
core-js(-pure)/es|stable|features/string/virtual/strike
core-js(-pure)/es|stable|features/string/virtual/sub
core-js(-pure)/es|stable|features/string/virtual/sup
core-js(-pure)/es|stable|features/string/virtual/iterator
core-js/es|stable|features/regexp
core-js/es|stable|features/regexp/constructor
core-js(-pure)/es|stable|features/regexp/flags
core-js/es|stable|features/regexp/to-string

Examples:

for (let value of 'a𠮷b') {
  console.log(value); // => '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'

'hello'.padStart(10);         // => '     hello'
'hello'.padStart(10, '1234'); // => '12341hello'
'hello'.padEnd(10);           // => 'hello     '
'hello'.padEnd(10, '1234');   // => 'hello12341'

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

let name = 'Bob';
String.raw`Hi\n${name}!`;             // => 'Hi\\nBob!' (ES2015 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>'

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'

'   hello   '.trimLeft();  // => 'hello   '
'   hello   '.trimRight(); // => '   hello'
'   hello   '.trimStart(); // => 'hello   '
'   hello   '.trimEnd();   // => '   hello'

for (let [_, d, D] of '1111a2b3cccc'.matchAll(/(\d)(\D)/g)) {
  console.log(d, D); // => 1 a, 2 b, 3 c
}

ECMAScript: Number

Module es.number.constructor. Number constructor support binary and octal literals, example:

Number('0b1010101'); // => 85
Number('0o7654321'); // => 2054353

Modules es.number.epsilon, es.number.is-finite, es.number.is-integer, es.number.is-nan, es.number.is-safe-integer, es.number.max-safe-integer, es.number.min-safe-integer, es.number.parse-float, es.number.parse-int, es.number.to-fixed, es.number.to-precision, es.parse-int, es.parse-float.

class Number {
  constructor(value: any): number;
  toFixed(digits: number): string;
  toPrecision(precision: number): string;
  static isFinite(number: any): boolean;
  static isNaN(number: any): boolean;
  static isInteger(number: any): boolean;
  static isSafeInteger(number: any): boolean;
  static parseFloat(string: string): number;
  static parseInt(string: string, radix?: number = 10): number;
  static EPSILON: number;
  static MAX_SAFE_INTEGER: number;
  static MIN_SAFE_INTEGER: number;
}

function parseFloat(string: string): number;
function parseInt(string: string, radix?: number = 10): number;

CommonJS entry points:

core-js(-pure)/es|stable|features/number
core-js/es|stable|features/number/constructor
core-js(-pure)/es|stable|features/number/is-finite
core-js(-pure)/es|stable|features/number/is-nan
core-js(-pure)/es|stable|features/number/is-integer
core-js(-pure)/es|stable|features/number/is-safe-integer
core-js(-pure)/es|stable|features/number/parse-float
core-js(-pure)/es|stable|features/number/parse-int
core-js(-pure)/es|stable|features/number/epsilon
core-js(-pure)/es|stable|features/number/max-safe-integer
core-js(-pure)/es|stable|features/number/min-safe-integer
core-js(-pure)/es|stable|features/number/to-fixed
core-js(-pure)/es|stable|features/number/to-precision
core-js(-pure)/es|stable|features/parse-float
core-js(-pure)/es|stable|features/parse-int

ECMAScript: Math

Modules es.math.acosh, es.math.asinh, es.math.atanh, es.math.cbrt, es.math.clz32, es.math.cosh, es.math.expm1, es.math.fround, es.math.hypot, es.math.imul, es.math.log10, es.math.log1p, es.math.log2, es.math.sign, es.math.sinh, es.math.tanh, es.math.trunc.

namespace Math {
  acosh(number: number): number;
  asinh(number: number): number;
  atanh(number: number): number;
  cbrt(number: number): number;
  clz32(number: number): number;
  cosh(number: number): number;
  expm1(number: number): number;
  fround(number: number): number;
  hypot(...args: Array<number>): number;
  imul(number1: number, number2: number): number;
  log1p(number: number): number;
  log10(number: number): number;
  log2(number: number): number;
  sign(number: number): 1 | -1 | 0 | -0 | NaN;
  sinh(number: number): number;
  tanh(number: number): number;
  trunc(number: number): number;
}

CommonJS entry points:

core-js(-pure)/es|stable|features/math
core-js(-pure)/es|stable|features/math/acosh
core-js(-pure)/es|stable|features/math/asinh
core-js(-pure)/es|stable|features/math/atanh
core-js(-pure)/es|stable|features/math/cbrt
core-js(-pure)/es|stable|features/math/clz32
core-js(-pure)/es|stable|features/math/cosh
core-js(-pure)/es|stable|features/math/expm1
core-js(-pure)/es|stable|features/math/fround
core-js(-pure)/es|stable|features/math/hypot
core-js(-pure)/es|stable|features/math/imul
core-js(-pure)/es|stable|features/math/log1p
core-js(-pure)/es|stable|features/math/log10
core-js(-pure)/es|stable|features/math/log2
core-js(-pure)/es|stable|features/math/sign
core-js(-pure)/es|stable|features/math/sinh
core-js(-pure)/es|stable|features/math/tanh
core-js(-pure)/es|stable|features/math/trunc

ECMAScript: Date

Modules es.date.to-string, ES5 features with fixes: es.date.now, es.date.to-iso-string, es.date.to-json and es.date.to-primitive.

class Date {
  toISOString(): string;
  toJSON(): string;
  toString(): string;
  @@toPrimitive(hint: 'default' | 'number' | 'string'): string | number;
  static now(): number;
}

CommonJS entry points:

core-js/es|stable|features/date
core-js/es|stable|features/date/to-string
core-js(-pure)/es|stable|features/date/now
core-js(-pure)/es|stable|features/date/to-iso-string
core-js(-pure)/es|stable|features/date/to-json
core-js(-pure)/es|stable|features/date/to-primitive

Example:

new Date(NaN).toString(); // => 'Invalid Date'

ECMAScript: Promise

Modules es.promise, es.promise.all-settled and es.promise.finally.

class Promise {
  constructor(executor: (resolve: Function, reject: Function) => void): Promise;
  then(onFulfilled: Function, onRejected: Function): Promise;
  catch(onRejected: Function): Promise;
  finally(onFinally: Function): Promise;
  static resolve(x: any): Promise;
  static reject(r: any): Promise;
  static all(iterable: Iterable): Promise;
  static allSettled(iterable: Iterable): Promise;
  static race(iterable: Iterable): Promise;
}

CommonJS entry points:

core-js(-pure)/es|stable|features/promise
core-js(-pure)/es|stable|features/promise/all-settled
core-js(-pure)/es|stable|features/promise/finally

Basic example:

function sleepRandom(time) {
  return new Promise((resolve, reject) => {
    setTimeout(resolve, time * 1e3, 0 | Math.random() * 1e3);
  });
}

console.log('Run');                    // => Run
sleepRandom(5).then(result => {
  console.log(result);                 // => 869, after 5 sec.
  return sleepRandom(10);
}).then(result => {
  console.log(result);                 // => 202, after 10 sec.
}).then(() => {
  console.log('immediately after');    // => immediately after
  throw Error('Irror!');
}).then(() => {
  console.log('will not be displayed');
}).catch(x => console.log(x));         // => => Error: Irror!

Promise.resolve and Promise.reject example:

Promise.resolve(42).then(x => console.log(x)); // => 42
Promise.reject(42).catch(x => console.log(x)); // => 42

Promise.resolve($.getJSON('/data.json')); // => ES promise

Promise#finally example:

Promise.resolve(42).finally(() => console.log('You will see it anyway'));

Promise.reject(42).finally(() => console.log('You will see it anyway'));

Promise.all example:

Promise.all([
  'foo',
  sleepRandom(5),
  sleepRandom(15),
  sleepRandom(10)             // after 15 sec:
]).then(x => console.log(x)); // => ['foo', 956, 85, 382]

Promise.race example:

function timeLimit(promise, time) {
  return Promise.race([promise, new Promise((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

Promise.allSettled example:

Promise.allSettled([
  Promise.resolve(1),
  Promise.reject(2),
  Promise.resolve(3),
]).then(console.log); // => [{ value: 1, status: 'fulfilled' }, { reason: 2, status: 'rejected' }, { value: 3, status: 'fulfilled' }]

Example with async functions:

let 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.
    let [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, 'Error!');
    console.log('Will not be displayed');
  } catch (e) {
    console.log(e);                    // => Error: 'Error!', after 5 sec.
  }
})();
Unhandled rejection tracking

In Node.js, like in native implementation, available events unhandledRejection and rejectionHandled:

process.on('unhandledRejection', (reason, promise) => console.log('unhandled', reason, promise));
process.on('rejectionHandled', (promise) => console.log('handled', promise));

let promise = Promise.reject(42);
// unhandled 42 [object Promise]

setTimeout(() => promise.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:

window.addEventListener('unhandledrejection', e => console.log('unhandled', e.reason, e.promise));
window.addEventListener('rejectionhandled', e => console.log('handled', e.reason, e.promise));
// or
window.onunhandledrejection = e => console.log('unhandled', e.reason, e.promise);
window.onrejectionhandled = e => console.log('handled', e.reason, e.promise);

let promise = Promise.reject(42);
// => unhandled 42 [object Promise]

setTimeout(() => promise.catch(() => {}), 1e3);
// => handled 42 [object Promise]

ECMAScript: Symbol

Modules es.symbol, es.symbol.async-iterator, es.symbol.description, es.symbol.has-instance, es.symbol.is-concat-spreadable, es.symbol.iterator, es.symbol.match, es.symbol.replace, es.symbol.search, es.symbol.species, es.symbol.split, es.symbol.to-primitive, es.symbol.to-string-tag, es.symbol.unscopables, es.math.to-string-tag, es.json.to-string-tag.

class Symbol {
  constructor(description?): symbol;
  readonly attribute description: string | void;
  static asyncIterator: @@asyncIterator;
  static hasInstance: @@hasInstance;
  static isConcatSpreadable: @@isConcatSpreadable;
  static iterator: @@iterator;
  static match: @@match;
  static replace: @@replace;
  static search: @@search;
  static species: @@species;
  static split: @@split;
  static toPrimitive: @@toPrimitive;
  static toStringTag: @@toStringTag;
  static unscopables: @@unscopables;
  static for(key: string): symbol;
  static keyFor(sym: symbol): string;
  static useSimple(): void;
  static useSetter(): void;
}

class Object {
  static getOwnPropertySymbols(object: any): Array<symbol>;
}

Also wrapped some methods for correct work with Symbol polyfill.

class Object {
  static create(prototype: Object | null, properties?: { [property: PropertyKey]: PropertyDescriptor }): Object;
  static defineProperties(object: Object, properties: { [property: PropertyKey]: PropertyDescriptor })): Object;
  static defineProperty(object: Object, property: PropertyKey, attributes: PropertyDescriptor): Object;
  static getOwnPropertyDescriptor(object: any, property: PropertyKey): PropertyDescriptor | void;
  static getOwnPropertyNames(object: any): Array<string>;
  propertyIsEnumerable(key: PropertyKey): boolean;
}

namespace JSON {
  stringify(target: any, replacer?: Function | Array, space?: string | number): string | void;
}

CommonJS entry points:

core-js(-pure)/es|stable|features/symbol
core-js(-pure)/es|stable|features/symbol/async-iterator
core-js/es|stable|features/symbol/description
core-js(-pure)/es|stable|features/symbol/has-instance
core-js(-pure)/es|stable|features/symbol/is-concat-spreadable
core-js(-pure)/es|stable|features/symbol/iterator
core-js(-pure)/es|stable|features/symbol/match
core-js(-pure)/es|stable|features/symbol/replace
core-js(-pure)/es|stable|features/symbol/search
core-js(-pure)/es|stable|features/symbol/species
core-js(-pure)/es|stable|features/symbol/split
core-js(-pure)/es|stable|features/symbol/to-primitive
core-js(-pure)/es|stable|features/symbol/to-string-tag
core-js(-pure)/es|stable|features/symbol/unscopables
core-js(-pure)/es|stable|features/symbol/for
core-js(-pure)/es|stable|features/symbol/key-for
core-js(-pure)/es|stable|features/object/get-own-property-symbols
core-js(-pure)/es|stable|features/math/to-string-tag
core-js(-pure)/es|stable|features/json/to-string-tag

Basic example:

let Person = (() => {
  let NAME = Symbol('name');
  return class {
    constructor(name) {
      this[NAME] = name;
    }
    getName() {
      return this[NAME];
    }
  }
})();

let person = new Person('Vasya');
console.log(person.getName());            // => 'Vasya'
console.log(person['name']);              // => undefined
console.log(person[Symbol('name')]);      // => undefined, symbols are uniq
for (let key in person) console.log(key); // => nothing, symbols are not enumerable

Symbol.for & Symbol.keyFor example:

let symbol = Symbol.for('key');
symbol === Symbol.for('key'); // true
Symbol.keyFor(symbol);        // 'key'

Example with methods for getting own object keys:

let object = { a: 1 };
Object.defineProperty(object, 'b', { value: 2 });
object[Symbol('c')] = 3;
Object.keys(object);                  // => ['a']
Object.getOwnPropertyNames(object);   // => ['a', 'b']
Object.getOwnPropertySymbols(object); // => [Symbol(c)]
Reflect.ownKeys(object);              // => ['a', 'b', Symbol(c)]

Symbol#description getter:

Symbol('foo').description; // => 'foo'
Symbol().description;      // => undefined
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:

Symbol.useSimple();
let symbol1 = Symbol('symbol1');
let object1 = {};
object1[symbol1] = true;
for (let key in object1) console.log(key); // => 'Symbol(symbol1)_t.qamkg9f3q', w/o native Symbol

Symbol.useSetter();
let symbol2 = Symbol('symbol2');
let object2 = {};
object2[symbol2] = true;
for (let key in object2) 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: 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 es.map.

class Map {
  constructor(iterable?: Iterable<[key, value]>): Map;
  clear(): void;
  delete(key: any): boolean;
  forEach(callbackfn: (value: any, key: any, target: any) => void, thisArg: any): void;
  get(key: any): any;
  has(key: any): boolean;
  set(key: any, val: any): this;
  values(): Iterator<value>;
  keys(): Iterator<key>;
  entries(): Iterator<[key, value]>;
  @@iterator(): Iterator<[key, value]>;
  readonly attribute size: number;
}

CommonJS entry points:

core-js(-pure)/es|stable|features/map

Examples:

let array = [1];

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

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

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

for (let [key, value] of map) {
  console.log(key);                                 // => 'a', 'b', 'c'
  console.log(value);                               // => 1, 2, 3
}
for (let value of map.values()) console.log(value); // => 1, 2, 3
for (let key of map.keys()) console.log(key);       // => 'a', 'b', 'c'
for (let [key, value] of map.entries()) {
  console.log(key);                                 // => 'a', 'b', 'c'
  console.log(value);                               // => 1, 2, 3
}

Set

Module es.set.

class Set {
  constructor(iterable?: Iterable<value>): Set;
  add(key: any): this;
  clear(): void;
  delete(key: any): boolean;
  forEach((value: any, key: any, target: any) => void, thisArg: any): void;
  has(key: any): boolean;
  values(): Iterator<value>;
  keys(): Iterator<value>;
  entries(): Iterator<[value, value]>;
  @@iterator(): Iterator<value>;
  readonly attribute size: number;
}

CommonJS entry points:

core-js(-pure)/es|stable|features/set

Examples:

let set = new Set(['a', 'b', 'a', 'c']);
set.add('d').add('b').add('e');
console.log(set.size);        // => 5
console.log(set.has('b'));    // => true
set.forEach(it => {
  console.log(it);            // => 'a', 'b', 'c', 'd', 'e'
});
set.delete('b');
console.log(set.size);        // => 4
console.log(set.has('b'));    // => false
console.log(Array.from(set)); // => ['a', 'c', 'd', 'e']

let set = new Set([1, 2, 3, 2, 1]);

for (let value of set) console.log(value);          // => 1, 2, 3
for (let value of set.values()) console.log(value); // => 1, 2, 3
for (let key of set.keys()) console.log(key);       // => 1, 2, 3
for (let [key, value] of set.entries()) {
  console.log(key);                                 // => 1, 2, 3
  console.log(value);                               // => 1, 2, 3
}

WeakMap

Module es.weak-map.

class WeakMap {
  constructor(iterable?: Iterable<[key, value]>): WeakMap;
  delete(key: Object): boolean;
  get(key: Object): any;
  has(key: Object): boolean;
  set(key: Object, val: any): this;
}

CommonJS entry points:

core-js(-pure)/es|stable|features/weak-map

Examples:

let a = [1];
let b = [2];
let c = [3];

let weakmap = new WeakMap([[a, 1], [b, 2]]);
weakmap.set(c, 3).set(b, 4);
console.log(weakmap.has(a));   // => true
console.log(weakmap.has([1])); // => false
console.log(weakmap.get(a));   // => 1
weakmap.delete(a);
console.log(weakmap.get(a));   // => undefined

// Private properties store:
let Person = (() => {
  let names = new WeakMap;
  return class {
    constructor(name) {
      names.set(this, name);
    }
    getName() {
      return names.get(this);
    }
  }
})();

let person = new Person('Vasya');
console.log(person.getName());            // => 'Vasya'
for (let key in person) console.log(key); // => only 'getName'

WeakSet

Module es.weak-set.

class WeakSet {
  constructor(iterable?: Iterable<value>): WeakSet;
  add(key: Object): this;
  delete(key: Object): boolean;
  has(key: Object): boolean;
}

CommonJS entry points:

core-js(-pure)/es|stable|features/weak-set

Examples:

let a = [1];
let b = [2];
let c = [3];

let weakset = new WeakSet([a, b, a]);
weakset.add(c).add(b).add(c);
console.log(weakset.has(b));   // => true
console.log(weakset.has([2])); // => false
weakset.delete(b);
console.log(weakset.has(b));   // => false
Caveats when using collections polyfill:
  • Weak-collections polyfill stores values as hidden properties of keys. It works correct and not leak in most cases. However, it is desirable to store a collection longer than its keys.

ECMAScript: Typed Arrays

Implementations and fixes for ArrayBuffer, DataView, Typed Arrays constructors, static and prototype methods. Typed arrays work only in environments with support descriptors (IE9+), ArrayBuffer and DataView should work anywhere.

Modules es.array-buffer.constructor, es.array-buffer.is-view, es.array-buffer.slice, es.data-view, es.typed-array.int8-array, es.typed-array.uint8-array, es.typed-array.uint8-clamped-array, es.typed-array.int16-array, es.typed-array.uint16-array, es.typed-array.int32-array, es.typed-array.uint32-array, es.typed-array.float32-array, es.typed-array.float64-array, es.typed-array.copy-within, es.typed-array.every, es.typed-array.fill, es.typed-array.filter, es.typed-array.find, es.typed-array.find-index, es.typed-array.for-each, es.typed-array.from, es.typed-array.includes, es.typed-array.index-of, es.typed-array.iterator, es.typed-array.last-index-of, es.typed-array.map, es.typed-array.of, es.typed-array.reduce, es.typed-array.reduce-right, es.typed-array.reverse, es.typed-array.set, es.typed-array.slice, es.typed-array.some, es.typed-array.sort, es.typed-array.subarray, es.typed-array.to-locale-string and es.typed-array.to-string.

class ArrayBuffer {
  constructor(length: any): ArrayBuffer;
  slice(start: any, end: any): ArrayBuffer;
  readonly attribute byteLength: number;
  static isView(arg: any): boolean;
}

class DataView {
  constructor(buffer: ArrayBuffer, byteOffset?: number, byteLength?: number): DataView;
  getInt8(offset: any): int8;
  getUint8(offset: any): uint8
  getInt16(offset: any, littleEndian?: boolean = false): int16;
  getUint16(offset: any, littleEndian?: boolean = false): uint16;
  getInt32(offset: any, littleEndian?: boolean = false): int32;
  getUint32(offset: any, littleEndian?: boolean = false): uint32;
  getFloat32(offset: any, littleEndian?: boolean = false): float32;
  getFloat64(offset: any, littleEndian?: boolean = false): float64;
  setInt8(offset: any, value: any): void;
  setUint8(offset: any, value: any): void;
  setInt16(offset: any, value: any, littleEndian?: boolean = false): void;
  setUint16(offset: any, value: any, littleEndian?: boolean = false): void;
  setInt32(offset: any, value: any, littleEndian?: boolean = false): void;
  setUint32(offset: any, value: any, littleEndian?: boolean = false): void;
  setFloat32(offset: any, value: any, littleEndian?: boolean = false): void;
  setFloat64(offset: any, value: any, littleEndian?: boolean = false): void;
  readonly attribute buffer: ArrayBuffer;
  readonly attribute byteLength: number;
  readonly attribute byteOffset: number;
}

class [
  Int8Array,
  Uint8Array,
  Uint8ClampedArray,
  Int16Array,
  Uint16Array,
  Int32Array,
  Uint32Array,
  Float32Array,
  Float64Array,
] {
  constructor(length: number): %TypedArray%;
  constructor(object: %TypedArray% | Iterable | ArrayLike): %TypedArray%;
  constructor(buffer: ArrayBuffer, byteOffset?: number, length?: number): %TypedArray%;
  copyWithin(target: number, start: number, end?: number): this;
  every(callbackfn: (value: number, index: number, target: any) => boolean, thisArg?: any): boolean;
  fill(value: number, start?: number, end?: number): this;
  filter(callbackfn: (value: number, index: number, target: any) => boolean, thisArg?: any): %TypedArray%;
  find(callbackfn: (value: number, index: number, target: any) => boolean), thisArg?: any): any;
  findIndex(callbackfn: (value: number, index: number, target: any) => boolean, thisArg?: any): number;
  forEach(callbackfn: (value: number, index: number, target: any) => void, thisArg?: any): void;
  includes(searchElement: any, from?: number): boolean;
  indexOf(searchElement: any, from?: number): number;
  join(separator: string = ','): string;
  lastIndexOf(searchElement: any, from?: number): number;
  map(mapFn: (value: number, index: number, target: any) => number, thisArg?: any): %TypedArray%;
  reduce(callbackfn: (memo: any, value: number, index: number, target: any) => any, initialValue?: any): any;
  reduceRight(callbackfn: (memo: any, value: number, index: number, target: any) => any, initialValue?: any): any;
  reverse(): this;
  set(array: ArrayLike, offset?: number): void;
  slice(start?: number, end?: number): %TypedArray%;
  some(callbackfn: (value: number, index: number, target: any) => boolean, thisArg?: any): boolean;
  sort(comparefn?: (a: number, b: number) => number): this;
  subarray(begin?: number, end?: number): %TypedArray%;
  toString(): string;
  toLocaleString(): string;
  values(): Iterator<value>;
  keys(): Iterator<index>;
  entries(): Iterator<[index, value]>;
  @@iterator(): Iterator<value>;
  readonly attribute buffer: ArrayBuffer;
  readonly attribute byteLength: number;
  readonly attribute byteOffset: number;
  readonly attribute length: number;
  BYTES_PER_ELEMENT: number;
  static from(items: Iterable | ArrayLike, mapFn?: (value: any, index: number) => any, thisArg?: any): %TypedArray%;
  static of(...args: Array<mixed>): %TypedArray%;
  static BYTES_PER_ELEMENT: number;
}

CommonJS entry points:

core-js/es|stable|features/array-buffer
core-js/es|stable|features/array-buffer/constructor
core-js/es|stable|features/array-buffer/is-view
core-js/es|stable|features/array-buffer/slice
core-js/es|stable|features/data-view
core-js/es|stable|features/typed-array
core-js/es|stable|features/typed-array/int8-array
core-js/es|stable|features/typed-array/uint8-array
core-js/es|stable|features/typed-array/uint8-clamped-array
core-js/es|stable|features/typed-array/int16-array
core-js/es|stable|features/typed-array/uint16-array
core-js/es|stable|features/typed-array/int32-array
core-js/es|stable|features/typed-array/uint32-array
core-js/es|stable|features/typed-array/float32-array
core-js/es|stable|features/typed-array/float64-array
core-js/es|stable|features/typed-array/copy-within
core-js/es|stable|features/typed-array/entries
core-js/es|stable|features/typed-array/every
core-js/es|stable|features/typed-array/fill
core-js/es|stable|features/typed-array/filter
core-js/es|stable|features/typed-array/find
core-js/es|stable|features/typed-array/find-index
core-js/es|stable|features/typed-array/for-each
core-js/es|stable|features/typed-array/from
core-js/es|stable|features/typed-array/includes
core-js/es|stable|features/typed-array/index-of
core-js/es|stable|features/typed-array/iterator
core-js/es|stable|features/typed-array/join
core-js/es|stable|features/typed-array/keys
core-js/es|stable|features/typed-array/last-index-of
core-js/es|stable|features/typed-array/map
core-js/es|stable|features/typed-array/of
core-js/es|stable|features/typed-array/reduce
core-js/es|stable|features/typed-array/reduce-right
core-js/es|stable|features/typed-array/reverse
core-js/es|stable|features/typed-array/set
core-js/es|stable|features/typed-array/slice
core-js/es|stable|features/typed-array/some
core-js/es|stable|features/typed-array/sort
core-js/es|stable|features/typed-array/subarray
core-js/es|stable|features/typed-array/to-locale-string
core-js/es|stable|features/typed-array/to-string
core-js/es|stable|features/typed-array/values

Examples:

new Int32Array(4);                          // => [0, 0, 0, 0]
new Uint8ClampedArray([1, 2, 3, 666]);      // => [1, 2, 3, 255]
new Float32Array(new Set([1, 2, 3, 2, 1])); // => [1, 2, 3]

let buffer = new ArrayBuffer(8);
let view   = new DataView(buffer);
view.setFloat64(0, 123.456, true);
new Uint8Array(buffer.slice(4)); // => [47, 221, 94, 64]

Int8Array.of(1, 1.5, 5.7, 745);      // => [1, 1, 5, -23]
Uint8Array.from([1, 1.5, 5.7, 745]); // => [1, 1, 5, 233]

let typed = new Uint8Array([1, 2, 3]);

let a = typed.slice(1);    // => [2, 3]
typed.buffer === a.buffer; // => false
let b = typed.subarray(1); // => [2, 3]
typed.buffer === b.buffer; // => true

typed.filter(it => it % 2); // => [1, 3]
typed.map(it => it * 1.5);  // => [1, 3, 4]

for (let value of typed) console.log(value);          // => 1, 2, 3
for (let value of typed.values()) console.log(value); // => 1, 2, 3
for (let key of typed.keys()) console.log(key);       // => 0, 1, 2
for (let [key, value] of typed.entries()) {
  console.log(key);                                   // => 0, 1, 2
  console.log(value);                                 // => 1, 2, 3
}
Caveats when using typed arrays polyfills:
  • Polyfills of Typed Arrays constructors work completely how should work by the spec, but because of internal usage of getters / setters on each instance, are slow and consumes significant memory. However, polyfills of Typed Arrays constructors required mainly for old IE, all modern engines have native Typed Arrays constructors and require only fixes of constructors and polyfills of methods.

ECMAScript: Reflect

Modules es.reflect.apply, es.reflect.construct, es.reflect.define-property, es.reflect.delete-property, es.reflect.get, es.reflect.get-own-property-descriptor, es.reflect.get-prototype-of, es.reflect.has, es.reflect.is-extensible, es.reflect.own-keys, es.reflect.prevent-extensions, es.reflect.set, es.reflect.set-prototype-of.

namespace Reflect {
  apply(target: Function, thisArgument: any, argumentsList: Array<mixed>): any;
  construct(target: Function, argumentsList: Array<mixed>, newTarget?: Function): Object;
  defineProperty(target: Object, propertyKey: PropertyKey, attributes: PropertyDescriptor): boolean;
  deleteProperty(target: Object, propertyKey: PropertyKey): boolean;
  get(target: Object, propertyKey: PropertyKey, receiver?: any): any;
  getOwnPropertyDescriptor(target: Object, propertyKey: PropertyKey): PropertyDescriptor | void;
  getPrototypeOf(target: Object): Object | null;
  has(target: Object, propertyKey: PropertyKey): boolean;
  isExtensible(target: Object): boolean;
  ownKeys(target: Object): Array<string | symbol>;
  preventExtensions(target: Object): boolean;
  set(target: Object, propertyKey: PropertyKey, V: any, receiver?: any): boolean;
  setPrototypeOf(target: Object, proto: Object | null): boolean; // required __proto__ - IE11+
}

CommonJS entry points:

core-js(-pure)/es|stable|features/reflect
core-js(-pure)/es|stable|features/reflect/apply
core-js(-pure)/es|stable|features/reflect/construct
core-js(-pure)/es|stable|features/reflect/define-property
core-js(-pure)/es|stable|features/reflect/delete-property
core-js(-pure)/es|stable|features/reflect/get
core-js(-pure)/es|stable|features/reflect/get-own-property-descriptor
core-js(-pure)/es|stable|features/reflect/get-prototype-of
core-js(-pure)/es|stable|features/reflect/has
core-js(-pure)/es|stable|features/reflect/is-extensible
core-js(-pure)/es|stable|features/reflect/own-keys
core-js(-pure)/es|stable|features/reflect/prevent-extensions
core-js(-pure)/es|stable|features/reflect/set
core-js(-pure)/es|stable|features/reflect/set-prototype-of

Examples:

let object = { a: 1 };
Object.defineProperty(object, 'b', { value: 2 });
object[Symbol('c')] = 3;
Reflect.ownKeys(object); // => ['a', 'b', Symbol(c)]

function C(a, b) {
  this.c = a + b;
}

let instance = Reflect.construct(C, [20, 22]);
instance.c; // => 42

ECMAScript proposals

The TC39 process. core-js/stage/4 entry point contains only stage 4 proposals, core-js/stage/3 - stage 3 and stage 4, etc.

Stage 4 proposals

Stage 4 proposals already marked in core-js as stable ECMAScript, they will be removed from proposals namespace in the next major core-js version. CommonJS entry points:

core-js(-pure)/stage/4
class String {
  matchAll(regexp: RegExp): Iterator;
}

CommonJS entry points:

core-js/proposals/string-match-all
class Promise {
  static allSettled(iterable: Iterable): Promise;
}

CommonJS entry points:

core-js/proposals/promise-all-settled

Stage 3 proposals

CommonJS entry points:

core-js(-pure)/stage/3
let globalThis: Object;

CommonJS entry points:

core-js/proposals/global-this
core-js(-pure)/features/global-this

Examples:

globalThis.Array === Array; // => true

Stage 2 proposals

CommonJS entry points:

core-js(-pure)/stage/2
class Set {
  difference(iterable: Iterable<mixed>): Set;
  intersection(iterable: Iterable<mixed>): Set;
  isDisjointFrom(iterable: Iterable<mixed>): boolean;
  isSubsetOf(iterable: Iterable<mixed>): boolean;
  isSupersetOf(iterable: Iterable<mixed>): boolean;
  symmetric-difference(iterable: Iterable<mixed>): Set;
  union(iterable: Iterable<mixed>): Set;
}

CommonJS entry points:

core-js/proposals/set-methods
core-js(-pure)/features/set/difference
core-js(-pure)/features/set/intersection
core-js(-pure)/features/set/is-disjoint-from
core-js(-pure)/features/set/is-subset-of
core-js(-pure)/features/set/is-superset-of
core-js(-pure)/features/set/symmetric-difference
core-js(-pure)/features/set/union

Examples:

new Set([1, 2, 3]).union([3, 4, 5]);               // => Set {1, 2, 3, 4, 5}
new Set([1, 2, 3]).intersection([3, 4, 5]);        // => Set {3}
new Set([1, 2, 3]).difference([3, 4, 5]);          // => Set {1, 2}
new Set([1, 2, 3]).symmetricDifference([3, 4, 5]); // => Set {1, 2, 4, 5}

new Set([1, 2, 3]).isDisjointFrom([4, 5, 6]);      // => true
new Set([1, 2, 3]).isSubsetOf([5, 4, 3, 2, 1]);    // => true
new Set([5, 4, 3, 2, 1]).isSupersetOf([1, 2, 3]);  // => true
class String {
  replaceAll(searchValue: string | RegExp, replaceString: string): string;
}

CommonJS entry points:

core-js/proposals/string-replace-all
core-js/features/string/replace-all

Examples:

'Test abc test test abc test.'.replaceAll('abc', 'foo'); // -> 'Test foo test test foo test.'
class AggregateError {
  constructor(errors: Iterable, message: string): AggregateError;
  errors: Array<any>;
  message: string;
}

class Promise {
  static any(promises: Iterable): Promise<any>;
}

CommonJS entry points:

core-js/proposals/promise-any
core-js(-pure)/features/promise/any
core-js(-pure)/features/aggregate-error

Examples:

Promise.any([
  Promise.resolve(1),
  Promise.reject(2),
  Promise.resolve(3),
]).then(console.log); // => 1

Promise.any([
  Promise.reject(1),
  Promise.reject(2),
  Promise.reject(3),
]).catch(({ errors }) => console.log(errors)); // => [1, 2, 3]
class Symbol {
  static asyncDispose: @@asyncDispose;
  static dispose: @@dispose;
}

CommonJS entry points:

core-js/proposals/using-statement
core-js(-pure)/features/symbol/async-dispose
core-js(-pure)/features/symbol/dispose
class Array {
  static isTemplateObject(value: any): boolean
}

CommonJS entry points:

core-js/proposals/array-is-template-object
core-js(-pure)/features/array/is-template-object

Example:

console.log(Array.isTemplateObject((it => it)`qwe${ 123 }asd`)); // => true

Stage 1 proposals

CommonJS entry points:

core-js(-pure)/stage/1
class Array {
  attribute lastItem: any;
  readonly attribute lastIndex: uint;
}

CommonJS entry points:

core-js/proposals/array-last
core-js/features/array/last-item
core-js/features/array/last-index

Examples:

[1, 2, 3].lastItem;  // => 3
[1, 2, 3].lastIndex; // => 2

const array = [1, 2, 3];
array.lastItem = 4;

array; // => [1, 2, 4]
class Promise {
  static try(callbackfn: Function): promise;
}

CommonJS entry points:

core-js/proposals/promise-try
core-js(-pure)/features/promise/try

Examples:

Promise.try(() => 42).then(it => console.log(`Promise, resolved as ${it}`));

Promise.try(() => { throw 42; }).catch(it => console.log(`Promise, rejected as ${it}`));
  • New collections methods proposals:
class Set {
  static of(...args: Array<mixed>): Set;
  static from(iterable: Iterable<mixed>, mapFn?: (value: any, index: number) => any, thisArg?: any): Set;
  addAll(...args: Array<mixed>): this;
  deleteAll(...args: Array<mixed>): boolean;
  every(callbackfn: (value: any, key: any, target: any) => boolean, thisArg?: any): boolean;
  filter(callbackfn: (value: any, key: any, target: any) => boolean, thisArg?: any): Set;
  find(callbackfn: (value: any, key: any, target: any) => boolean), thisArg?: any): any;
  join(separator: string = ','): string;
  map(callbackfn: (value: any, key: any, target: any) => boolean, thisArg?: any): Set;
  reduce(callbackfn: (memo: any, value: any, key: any, target: any) => any, initialValue?: any): any;
  some(callbackfn: (value: any, key: any, target: any) => boolean, thisArg?: any): boolean;
}

class Map {
  static groupBy(iterable: Iterable<mixed>, callbackfn?: (value: any) => any): Map;
  static of(...args: Array<[key, value]>): Map;
  static from(iterable: Iterable<mixed>, mapFn?: (value: any, index: number) => [key: any, value: any], thisArg?: any): Map;
  static keyBy(iterable: Iterable<mixed>, callbackfn?: (value: any) => any): Map;
  deleteAll(...args: Array<mixed>): boolean;
  every(callbackfn: (value: any, key: any, target: any) => boolean, thisArg?: any): boolean;
  filter(callbackfn: (value: any, key: any, target: any) => boolean, thisArg?: any): Map;
  find(callbackfn: (value: any, key: any, target: any) => boolean), thisArg?: any): any;
  findKey(callbackfn: (value: any, key: any, target: any) => boolean), thisArg?: any): any;
  includes(searchElement: any): boolean;
  keyOf(searchElement: any): any;
  mapKeys(mapFn: (value: any, index: number, target: any) => any, thisArg?: any): Map;
  mapValues(mapFn: (value: any, index: number, target: any) => any, thisArg?: any): Map;
  merge(...iterables: Array<Iterable>): this;
  reduce(callbackfn: (memo: any, value: any, key: any, target: any) => any, initialValue?: any): any;
  some(callbackfn: (value: any, key: any, target: any) => boolean, thisArg?: any): boolean;
  update(key: any, callbackfn: (value: any, key: any, target: any) => any, thunk?: (key: any, target: any) => any): this;
  updateOrInsert(key: any, onUpdate: (value: any) => updated: any, onInsert: () => value: any): updated | value;
}

class WeakSet {
  static of(...args: Array<mixed>): WeakSet;
  static from(iterable: Iterable<mixed>, mapFn?: (value: any, index: number) => Object, thisArg?: any): WeakSet;
  addAll(...args: Array<mixed>): this;
  deleteAll(...args: Array<mixed>): boolean;
}

class WeakMap {
  static of(...args: Array<[key, value]>): WeakMap;
  static from(iterable: Iterable<mixed>, mapFn?: (value: any, index: number) => [key: Object, value: any], thisArg?: any): WeakMap;
  deleteAll(...args: Array<mixed>): boolean;
}

CommonJS entry points:

core-js/proposals/collection-methods
core-js/proposals/collection-of-from
core-js/proposals/map-update-or-insert
core-js(-pure)/features/set/add-all
core-js(-pure)/features/set/delete-all
core-js(-pure)/features/set/every
core-js(-pure)/features/set/filter
core-js(-pure)/features/set/find
core-js(-pure)/features/set/from
core-js(-pure)/features/set/join
core-js(-pure)/features/set/map
core-js(-pure)/features/set/of
core-js(-pure)/features/set/reduce
core-js(-pure)/features/set/some
core-js(-pure)/features/map/delete-all
core-js(-pure)/features/map/every
core-js(-pure)/features/map/filter
core-js(-pure)/features/map/find
core-js(-pure)/features/map/find-key
core-js(-pure)/features/map/from
core-js(-pure)/features/map/group-by
core-js(-pure)/features/map/includes
core-js(-pure)/features/map/key-by
core-js(-pure)/features/map/key-of
core-js(-pure)/features/map/map-keys
core-js(-pure)/features/map/map-values
core-js(-pure)/features/map/merge
core-js(-pure)/features/map/of
core-js(-pure)/features/map/reduce
core-js(-pure)/features/map/some
core-js(-pure)/features/map/update
core-js(-pure)/features/map/update-or-insert
core-js(-pure)/features/weak-set/add-all
core-js(-pure)/features/weak-set/delete-all
core-js(-pure)/features/weak-set/of
core-js(-pure)/features/weak-set/from
core-js(-pure)/features/weak-map/delete-all
core-js(-pure)/features/weak-map/of
core-js(-pure)/features/weak-map/from

Map#updateOrInsert examples:

const map = new Map([['a', 2]]);

map.updateOrInsert('a', it => it ** 2, () => 3); // => 4

map.updateOrInsert('b', it => it ** 2, () => 3); // => 3

console.log(map); // => Map { 'a': 4, 'b': 3 }

.of / .from examples:

Set.of(1, 2, 3, 2, 1); // => Set {1, 2, 3}

Map.from([[1, 2], [3, 4]], ([key, value]) => [key ** 2, value ** 2]); // => Map { 1: 4, 9: 16 }
function compositeKey(...args: Array<mixed>): object;
function compositeSymbol(...args: Array<mixed>): symbol;

CommonJS entry points:

core-js/proposals/keys-composition
core-js(-pure)/features/composite-key
core-js(-pure)/features/composite-symbol

Examples:

// returns a symbol
const symbol = compositeSymbol({});
console.log(typeof symbol); // => 'symbol'

// works the same, but returns a plain frozen object without a prototype
const key = compositeKey({});
console.log(typeof key); // => 'object'
console.log({}.toString.call(key)); // => '[object Object]'
console.log(Object.getPrototypeOf(key)); // => null
console.log(Object.isFrozen(key)); // => true

const a = ['a'];
const b = ['b'];
const c = ['c'];

console.log(compositeSymbol(a) === compositeSymbol(a)); // => true
console.log(compositeSymbol(a) !== compositeSymbol(['a'])); // => true
console.log(compositeSymbol(a, 1) === compositeSymbol(a, 1)); // => true
console.log(compositeSymbol(a, b) !== compositeSymbol(b, a)); // => true
console.log(compositeSymbol(a, b, c) === compositeSymbol(a, b, c)); // => true
console.log(compositeSymbol(1, a) === compositeSymbol(1, a)); // => true
console.log(compositeSymbol(1, a, 2, b) === compositeSymbol(1, a, 2, b)); // => true
console.log(compositeSymbol(a, a) === compositeSymbol(a, a)); // => true
class Observable {
  constructor(subscriber: Function): Observable;
  subscribe(observer: Function | { next?: Function, error?: Function, complete?: Function }): Subscription;
  @@observable(): this;
  static of(...items: Aray<mixed>): Observable;
  static from(x: Observable | Iterable): Observable;
  static readonly attribute @@species: this;
}

class Symbol {
  static observable: @@observable;
}

CommonJS entry points:

core-js/proposals/observable
core-js(-pure)/features/observable
core-js(-pure)/features/symbol/observable

Examples:

new Observable(observer => {
  observer.next('hello');
  observer.next('world');
  observer.complete();
}).subscribe({
  next(it) { console.log(it); },
  complete() { console.log('!'); }
});
namespace Math {
  DEG_PER_RAD: number;
  RAD_PER_DEG: number;
  clamp(x: number, lower: number, upper: number): number;
  degrees(radians: number): number;
  fscale(x: number, inLow: number, inHigh: number, outLow: number, outHigh: number): number;
  radians(degrees: number): number;
  scale(x: number, inLow: number, inHigh: number, outLow: number, outHigh: number): number;
}

CommonJS entry points:

core-js/proposals/math-extensions
core-js(-pure)/features/math/clamp
core-js(-pure)/features/math/deg-per-rad
core-js(-pure)/features/math/degrees
core-js(-pure)/features/math/fscale
core-js(-pure)/features/math/rad-per-deg
core-js(-pure)/features/math/radians
core-js(-pure)/features/math/scale
namespace Math {
  signbit(x: number): boolean;
}

CommonJS entry points:

core-js/proposals/math-signbit
core-js(-pure)/features/math/signbit

Examples:

Math.signbit(NaN); // => NaN
Math.signbit(1);   // => true
Math.signbit(-1);  // => false
Math.signbit(0);   // => true
Math.signbit(-0);  // => false
class Number {
  fromString(string: string, radix: number): number;
}

CommonJS entry points:

core-js/proposals/number-from-string
core-js(-pure)/features/number/from-string
class String {
  codePoints(): Iterator<codePoint, position>;
}

CommonJS entry points:

core-js/proposals/string-code-points
core-js(-pure)/features/string/code-points

Example:

for (let { codePoint, position } of 'qwe'.codePoints()) {
  console.log(codePoint); // => 113, 119, 101
  console.log(position);  // => 0, 1, 2
}
class Math {
  seededPRNG({ seed: number }): Iterator<number>;
}

CommonJS entry points:

core-js/proposals/seeded-random
core-js(-pure)/features/math/seeded-prng

Example:

for (let x of Math.seededPRNG({ seed: 42 })) {
  console.log(x); // => 0.16461519912315087, 0.2203933906000046, 0.8249682894209105
  if (x > .8) break;
}
class Symbol {
  static patternMatch: @@patternMatch;
}

CommonJS entry points:

core-js/proposals/pattern-matching
core-js(-pure)/features/symbol/pattern-match

Stage 0 proposals

CommonJS entry points:

core-js(-pure)/stage/0
class String {
  at(index: number): string;
}

CommonJS entry points:

core-js/proposals/string-at
core-js(-pure)/features/string/at
core-js(-pure)/features/string/virtual/at

Examples:

'a𠮷b'.at(1);        // => '𠮷'
'a𠮷b'.at(1).length; // => 2
namespace Math {
  iaddh(lo0: number, hi0: number, lo1: number, hi1: number): number;
  isubh(lo0: number, hi0: number, lo1: number, hi1: number): number;
  imulh(a: number, b: number): number;
  umulh(a: number, b: number): number;
}

CommonJS entry points:

core-js/proposals/efficient-64-bit-arithmetic
core-js(-pure)/features/math/iaddh
core-js(-pure)/features/math/isubh
core-js(-pure)/features/math/imulh
core-js(-pure)/features/math/umulh

Pre-stage 0 proposals

CommonJS entry points:

core-js(-pure)/stage/pre
namespace Reflect {
  defineMetadata(metadataKey: any, metadataValue: any, target: Object, propertyKey?: PropertyKey): void;
  getMetadata(metadataKey: any, target: Object, propertyKey?: PropertyKey): any;
  getOwnMetadata(metadataKey: any, target: Object, propertyKey?: PropertyKey): any;
  hasMetadata(metadataKey: any, target: Object, propertyKey?: PropertyKey): boolean;
  hasOwnMetadata(metadataKey: any, target: Object, propertyKey?: PropertyKey): boolean;
  deleteMetadata(metadataKey: any, target: Object, propertyKey?: PropertyKey): boolean;
  getMetadataKeys(target: Object, propertyKey?: PropertyKey): Array<mixed>;
  getOwnMetadataKeys(target: Object, propertyKey?: PropertyKey): Array<mixed>;
  metadata(metadataKey: any, metadataValue: any): decorator(target: Object, targetKey?: PropertyKey) => void;
}

CommonJS entry points:

core-js/proposals/reflect-metadata
core-js(-pure)/features/reflect/define-metadata
core-js(-pure)/features/reflect/delete-metadata
core-js(-pure)/features/reflect/get-metadata
core-js(-pure)/features/reflect/get-metadata-keys
core-js(-pure)/features/reflect/get-own-metadata
core-js(-pure)/features/reflect/get-own-metadata-keys
core-js(-pure)/features/reflect/has-metadata
core-js(-pure)/features/reflect/has-own-metadata
core-js(-pure)/features/reflect/metadata

Examples:

let object = {};
Reflect.defineMetadata('foo', 'bar', object);
Reflect.ownKeys(object);               // => []
Reflect.getOwnMetadataKeys(object);    // => ['foo']
Reflect.getOwnMetadata('foo', object); // => 'bar'

Web standards

CommonJS entry points:

core-js(-pure)/web

setTimeout and setInterval

Module web.timers. Additional arguments fix for IE9-.

function setTimeout(callback: any, time: any, ...args: Array<mixed>): number;
function setInterval(callback: any, time: any, ...args: Array<mixed>): number;

CommonJS entry points:

core-js(-pure)/web/timers
core-js(-pure)/stable|features/set-timeout
core-js(-pure)/stable|features/set-interval
// Before:
setTimeout(log.bind(null, 42), 1000);
// After:
setTimeout(log, 1000, 42);

setImmediate

Module web.immediate. setImmediate polyfill.

function setImmediate(callback: any, ...args: Array<mixed>): number;
function clearImmediate(id: number): void;

CommonJS entry points:

core-js(-pure)/web/immediate
core-js(-pure)/stable|features/set-immediate
core-js(-pure)/stable|features/clear-immediate

Examples:

setImmediate((arg1, arg2) => {
  console.log(arg1, arg2); // => Message will be displayed with minimum delay
}, 'Message will be displayed', 'with minimum delay');

clearImmediate(setImmediate(() => {
  console.log('Message will not be displayed');
}));

queueMicrotask

Spec, module web.queue-microtask

function queueMicrotask(fn: Function): void;

CommonJS entry points:

core-js/web/queue-microtask
core-js(-pure)/stable|features/queue-microtask

Examples:

queueMicrotask(() => console.log('called as microtask'));

URL and URLSearchParams

URL standard implementation. Modules web.url, web.url.to-json, web.url-search-params.

class URL {
  constructor(url: string, base?: string);
  attribute href: string;
  readonly attribute origin: string;
  attribute protocol: string;
  attribute username: string;
  attribute password: string;
  attribute host: string;
  attribute hostname: string;
  attribute port: string;
  attribute pathname: string;
  attribute search: string;
  readonly attribute searchParams: URLSearchParams;
  attribute hash: string;
  toJSON(): string;
  toString(): string;
}

class URLSearchParams {
  constructor(params?: string | Iterable<[key, value]> | Object);
  append(name: string, value: string): void;
  delete(name: string): void;
  get(name: string): string | void;
  getAll(name: string): Array<string>;
  has(name: string): boolean;
  set(name: string, value: string): void;
  sort(): void;
  toString(): string;
  forEach(callbackfn: (value: any, index: number, target: any) => void, thisArg: any): void;
  entries(): Iterator<[key, value]>;
  keys(): Iterator<key>;
  values(): Iterator<value>;
  @@iterator(): Iterator<[key, value]>;
}

CommonJS entry points:

core-js/proposals/url
core-js(-pure)/web/url
core-js(-pure)/web/url-search-params
core-js(-pure)/stable|features/url
core-js/stable|features/url/to-json
core-js(-pure)/stable|features/url-search-params
core-js/stable|features/url-search-params/sort

Examples:

const url = new URL('http://login:password@example.com:8080/foo/bar?a=1&b=2&a=3#fragment');

console.log(url.href);       // => 'http://login:password@example.com:8080/foo/bar?a=1&b=2#fragment'
console.log(url.origin);     // => 'http://example.com:8080'
console.log(url.protocol);   // => 'http:'
console.log(url.username);   // => 'login'
console.log(url.password);   // => 'password'
console.log(url.host);       // => 'example.com:8080'
console.log(url.hostname);   // => 'example.com'
console.log(url.port);       // => '8080'
console.log(url.pathname);   // => '/foo/bar'
console.log(url.search);     // => '?a=1&b=2&a=3'
console.log(url.hash);       // => '#fragment'
console.log(url.toJSON());   // => 'http://login:password@example.com:8080/foo/bar?a=1&b=2&a=3#fragment'
console.log(url.toString()); // => 'http://login:password@example.com:8080/foo/bar?a=1&b=2&a=3#fragment'

for (let [key, value] of url.searchParams) {
  console.log(key);   // => 'a', 'b'
  console.log(value); // => '1', '2'
}

url.pathname = '';
url.searchParams.append('c', 3);

console.log(url.search); // => '?a=1&b=2&c=3'
console.log(url.href);   // => 'http://login:password@example.com:8080/?a=1&a=3&b=2&c=4#fragment'

const params = new URLSearchParams('?a=1&b=2&a=3');

params.append('c', 4);
params.append('a', 2);
params.sort();

for (let [key, value] of params) {
  console.log(key);   // => 'a', 'a', 'a', 'b', 'c'
  console.log(value); // => '1', '3', '2', '2', '4'
}

console.log(params.toString()); // => 'a=1&a=3&a=2&b=2&c=4'
Caveats when using URL and URLSearchParams:
  • IE8 does not support setters, so they do not work on URL instances. However, URL constructor can be used for basic URL parsing.
  • Legacy encodings in a search query are not supported. Also, core-js implementation has some other encoding-related issues.
  • URL implementations from all of the popular browsers have much more problems than core-js, however, replacing all of them does not looks like a good idea. You can customize the aggressiveness of polyfill by your requirements.

Iterable DOM collections

Some DOM collections should have iterable interface or should be inherited from Array. That means they should have forEach, keys, values, entries and @@iterator methods for iteration. So add them. Modules web.dom-collections.iterator and web.dom-collections.for-each.

class [
  CSSRuleList,
  CSSStyleDeclaration,
  CSSValueList,
  ClientRectList,
  DOMRectList,
  DOMStringList,
  DataTransferItemList,
  FileList,
  HTMLAllCollection,
  HTMLCollection,
  HTMLFormElement,
  HTMLSelectElement,
  MediaList,
  MimeTypeArray,
  NamedNodeMap,
  PaintRequestList,
  Plugin,
  PluginArray,
  SVGLengthList,
  SVGNumberList,
  SVGPathSegList,
  SVGPointList,
  SVGStringList,
  SVGTransformList,
  SourceBufferList,
  StyleSheetList,
  TextTrackCueList,
  TextTrackList,
  TouchList,
] {
  @@iterator(): Iterator<value>;
}

class [DOMTokenList, NodeList] {
  forEach(callbackfn: (value: any, index: number, target: any) => void, thisArg: any): void;
  entries(): Iterator<[key, value]>;
  keys(): Iterator<key>;
  values(): Iterator<value>;
  @@iterator(): Iterator<value>;
}

CommonJS entry points:

core-js(-pure)/web/dom-collections
core-js(-pure)/stable|features/dom-collections/iterator
core-js/stable|features/dom-collections/for-each

Examples:

for (let { id } of document.querySelectorAll('*')) {
  if (id) console.log(id);
}

for (let [index, { id }] of document.querySelectorAll('*').entries()) {
  if (id) console.log(index, id);
}

document.querySelectorAll('*').forEach(it => console.log(it.id));

Iteration helpers

Modules core.is-iterable, core.get-iterator, core.get-iterator-method - helpers for check iterability / get iterator in the pure version or, for example, for arguments object:

function isIterable(value: any): boolean;
function getIterator(value: any): Object;
function getIteratorMethod(value: any): Function | void;

CommonJS entry points:

core-js-pure/features/is-iterable
core-js-pure/features/get-iterator
core-js-pure/features/get-iterator-method

Examples:

import isIterable from 'core-js-pure/features/is-iterable';
import getIterator from 'core-js-pure/features/get-iterator';
import getIteratorMethod from 'core-js-pure/features/get-iterator-method';

let list = (function () {
  return arguments;
})(1, 2, 3);

console.log(isIterable(list)); // true;

let iterator = getIterator(list);
console.log(iterator.next().value); // 1
console.log(iterator.next().value); // 2
console.log(iterator.next().value); // 3
console.log(iterator.next().value); // undefined

getIterator({});   // TypeError: [object Object] is not iterable!

let method = getIteratorMethod(list);
console.log(typeof method);         // 'function'
let iterator = method.call(list);
console.log(iterator.next().value); // 1
console.log(iterator.next().value); // 2
console.log(iterator.next().value); // 3
console.log(iterator.next().value); // undefined

console.log(getIteratorMethod({})); // undefined

Missing polyfills

  • ES String#normalize is not a very useful feature, but this polyfill will be very large. If you need it, you can use unorm.
  • ES Proxy can't be polyfilled, but for Node.js / Chromium with additional flags, you can try harmony-reflect for adapt old style Proxy API to final ES2015 version.
  • window.fetch is not a cross-platform feature, in some environments, it makes no sense. For this reason, I don't think it should be in core-js. Looking at a large number of requests it might be added in the future. Now you can use, for example, this polyfill.
  • ECMA-402 Intl is missed because of the size. You can use this polyfill.