I highly recommend reading this: So, what's next?

Modular standard library for JavaScript. Includes polyfills for ECMAScript up to 2024: 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 are looking for documentation for obsolete core-js@2, please, check this branch.

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 if you are interested in core-js: Open Collective, Patreon, Boosty, Bitcoin ( bc1qlea7544qtsmj2rayg0lthvza9fau63ux0fstcz ), Alipay.

Example of usage:

import 'core-js/actual';

Promise.resolve(42).then(it => console.log(it)); // => 42

Array.from(new Set([1, 2, 3]).union(new Set([3, 4, 5]))); // => [1, 2, 3, 4, 5]

[1, 2].flatMap(it => [it, it]); // => [1, 1, 2, 2]

(function * (i) { while (true) yield i++; })(1)
  .drop(1).take(5)
  .filter(it => it % 2)
  .map(it => it ** 2)
  .toArray(); // => [9, 25]

structuredClone(new Set([1, 2, 3])); // => new Set([1, 2, 3])

You can load only required features:

import 'core-js/actual/promise';
import 'core-js/actual/set';
import 'core-js/actual/iterator';
import 'core-js/actual/array/from';
import 'core-js/actual/array/flat-map';
import 'core-js/actual/structured-clone';

Promise.resolve(42).then(it => console.log(it)); // => 42

Array.from(new Set([1, 2, 3]).union(new Set([3, 4, 5]))); // => [1, 2, 3, 4, 5]

[1, 2].flatMap(it => [it, it]); // => [1, 1, 2, 2]

(function * (i) { while (true) yield i++; })(1)
  .drop(1).take(5)
  .filter(it => it % 2)
  .map(it => it ** 2)
  .toArray(); // => [9, 25]

structuredClone(new Set([1, 2, 3])); // => new Set([1, 2, 3])

Or use it without global namespace pollution:

import Promise from 'core-js-pure/actual/promise';
import Set from 'core-js-pure/actual/set';
import Iterator from 'core-js-pure/actual/iterator';
import from from 'core-js-pure/actual/array/from';
import flatMap from 'core-js-pure/actual/array/flat-map';
import structuredClone from 'core-js-pure/actual/structured-clone';

Promise.resolve(42).then(it => console.log(it)); // => 42

from(new Set([1, 2, 3]).union(new Set([3, 4, 5]))); // => [1, 2, 3, 4, 5]

flatMap([1, 2], it => [it, it]); // => [1, 1, 2, 2]

Iterator.from(function * (i) { while (true) yield i++; }(1))
  .drop(1).take(5)
  .filter(it => it % 2)
  .map(it => it ** 2)
  .toArray(); // => [9, 25]

structuredClone(new Set([1, 2, 3])); // => new Set([1, 2, 3])

Index

• Usage
  • Installation
  • postinstall message
  • CommonJS API
  • Babel
    • @babel/polyfill
    • @babel/preset-env
    • @babel/runtime
  • swc
  • Configurable level of aggressiveness
  • Custom build
• Supported engines and compatibility data
• Features
  • ECMAScript
    • ECMAScript: Object
    • ECMAScript: Function
    • ECMAScript: Error
    • ECMAScript: Array
    • ECMAScript: String and RegExp
    • ECMAScript: Number
    • ECMAScript: Math
    • ECMAScript: Date
    • ECMAScript: Promise
    • ECMAScript: Symbol
    • ECMAScript: Collections
    • ECMAScript: Typed Arrays
    • ECMAScript: Reflect
    • ECMAScript: JSON
    • ECMAScript: globalThis
  • ECMAScript proposals
    • Finished proposals
      • globalThis
      • Relative indexing method
      • Array.prototype.includes
      • Array.prototype.flat / Array.prototype.flatMap
      • Array find from last
      • Change Array by copy
      • Array grouping
      • ArrayBuffer.prototype.transfer and friends
      • Object.values / Object.entries
      • Object.fromEntries
      • Object.getOwnPropertyDescriptors
      • Accessible Object.prototype.hasOwnProperty
      • String padding
      • String.prototype.matchAll
      • String.prototype.replaceAll
      • String.prototype.trimStart / String.prototype.trimEnd
      • RegExp s (dotAll) flag
      • RegExp named capture groups
      • Promise.allSettled
      • Promise.any
      • Promise.prototype.finally
      • Promise.withResolvers
      • Symbol.asyncIterator for asynchronous iteration
      • Symbol.prototype.description
      • Well-formed JSON.stringify
      • Well-formed unicode strings
      • New Set methods
    • Stage 3 proposals
      • Iterator helpers
      • Array.fromAsync
      • JSON.parse source text access
      • Float16 methods
      • Uint8Array to / from base64 and hex
      • Explicit resource management
      • Symbol.metadata for decorators metadata proposal
    • Stage 2.7 proposals
      • Promise.try
      • Math.sumPrecise
    • Stage 2 proposals
      • AsyncIterator helpers
      • Iterator.range
      • Map.prototype.emplace
      • Array.isTemplateObject
      • String.dedent
      • RegExp escaping
      • Symbol predicates
    • Stage 1 proposals
      • Observable
      • New collections methods
      • .of and .from methods on collection constructors
      • compositeKey and compositeSymbol
      • Array filtering
      • Array deduplication
      • DataView get / set Uint8Clamped methods
      • Number.fromString
      • String.cooked
      • String.prototype.codePoints
      • Symbol.customMatcher for pattern matching
      • Symbol.customMatcher for extractors
    • Stage 0 proposals
      • Function.prototype.demethodize
      • Function.{ isCallable, isConstructor }
    • Pre-stage 0 proposals
      • Reflect metadata
  • Web standards
    • self
    • structuredClone
    • Base64 utility methods
    • setTimeout and setInterval
    • setImmediate
    • queueMicrotask
    • URL and URLSearchParams
    • DOMException
    • iterable DOM collections
  • Iteration helpers
• Missing polyfills
• Contributing
• Security policy
• Changelog

Usage⬆

Installation:⬆

// global version
npm install --save [email protected]
// version without global namespace pollution
npm install --save [email protected]
// bundled global version
npm install --save [email protected]

Or you can use core-js from CDN.

postinstall message⬆

The core-js project needs your help, so the package shows a message about it after installation. If it causes problems for you, you can disable it:

ADBLOCK=true npm install
// or
DISABLE_OPENCOLLECTIVE=true npm install
// or
npm install --loglevel silent

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, including early-stage proposals:
import "core-js";
// or:
import "core-js/full";
// polyfill all actual features - stable ES, web standards and stage 3 ES proposals:
import "core-js/actual";
// polyfill only stable 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 early-stage ES proposals:
import "core-js/full/set";
// stable required for `Set` ES features, features from web standards and stage 3 ES proposals:
import "core-js/actual/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/full/set";
import Set from "core-js-pure/actual/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/full/set/intersection";
import "core-js/actual/array/find-last";
import "core-js/stable/queue-microtask";
import "core-js/es/array/from";

// polyfill iterator helpers proposal:
import "core-js/proposals/iterator-helpers";
// polyfill all stage 2+ proposals:
import "core-js/stage/2";

Note: The usage of the /actual/ namespace is recommended since it includes all actual JavaScript features and does not include unstable early-stage proposals that are available mainly for experiments.

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.
  • For example, Google Maps use their own Symbol.iterator, conflicting with Array.from, URLSearchParams and/or something else from core-js, see related issues.
  • Such conflicts also resolvable by discovering and manual adding each conflicting entry from core-js.
• 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/actual/array/virtual/fill';
import findIndex from 'core-js-pure/actual/array/virtual/find-index';

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

Warning! The bind operator is an early-stage ECMAScript proposal and usage of this syntax can be dangerous.

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: '3.37'.

Warning! Recommended to specify used minor core-js version, like corejs: '3.37', instead of corejs: 3, since with corejs: 3 will not be injected modules which were added in minor core-js releases.

• 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/full/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: '3.37', proposals: true }.

Warning! In the case of useBuiltIns: 'usage', you should not add core-js imports by yourself, they will be added automatically.

@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 }.

Warning! If you use @babel/preset-env and @babel/runtime together, use corejs option only in one place since it's duplicate functionality and will cause conflicts.

swc⬆

Fast JavaScript transpiler swc contains integration with core-js, that optimizes work with the global version of core-js. Like @babel/preset-env, it has 2 modes: usage and entry, but usage mode still works not so good like in babel. Example of configuration in .swcrc:

{
  "env": {
    "targets": "> 0.25%, not dead",
    "mode": "entry",
    "coreJs": "3.37"
  }
}

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/actual');

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 to exclude some core-js features or generate a polyfill for target engines. You could use core-js-builder package for that.

Supported engines and compatibility data⬆

core-js tries to support all possible JS engines and environments with ES3 support. Some features have a higher lower bar - for example, some accessors can properly work only from ES5, promises require a way to set a microtask or a task, etc.

However, I have no possibility to test core-js absolutely everywhere - for example, testing in IE7- and some other ancient was stopped. The list of definitely supported engines you can see in the compatibility table by the link below. Write if you have issues or questions with the support of any engine.

core-js project provides (as core-js-compat package) all required data about the necessity of core-js modules, entry points, and tools for work with it - it's useful for integration with tools like babel or swc. If you wanna help, you could take a look at the related section of CONTRIBUTING.md. The visualization of compatibility data and the browser tests runner is available here, the example:

Features:⬆

CommonJS entry points:

core-js(-pure)

ECMAScript⬆

CommonJS entry points:

core-js(-pure)/es

ECMAScript: Object⬆

Modules es.object.assign, es.object.create, es.object.define-getter, es.object.define-property, es.object.define-properties, es.object.define-setter, es.object.entries, es.object.freeze, es.object.from-entries, es.object.get-own-property-descriptor, es.object.get-own-property-descriptors, es.object.get-own-property-names, es.object.get-prototype-of, es.object.group-by, es.object.has-own, es.object.is, es.object.is-extensible, es.object.is-frozen, es.object.is-sealed, es.object.keys, es.object.lookup-setter, es.object.lookup-getter, es.object.prevent-extensions, es.object.proto, es.object.to-string, es.object.seal, es.object.set-prototype-of, es.object.values.

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;
  __proto__: Object | null; // required a way setting of prototype - will not in IE10-, it's for modern engines like Deno
  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 groupBy(items: Iterable, callbackfn: (value: any, index: number) => key): { [key]: Array<mixed> };
  static hasOwn(object: object, key: PropertyKey): boolean;
  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|actual|full/object
core-js(-pure)/es|stable|actual|full/object/assign
core-js(-pure)/es|stable|actual|full/object/is
core-js(-pure)/es|stable|actual|full/object/set-prototype-of
core-js(-pure)/es|stable|actual|full/object/get-prototype-of
core-js(-pure)/es|stable|actual|full/object/create
core-js(-pure)/es|stable|actual|full/object/define-property
core-js(-pure)/es|stable|actual|full/object/define-properties
core-js(-pure)/es|stable|actual|full/object/get-own-property-descriptor
core-js(-pure)/es|stable|actual|full/object/get-own-property-descriptors
core-js(-pure)/es|stable|actual|full/object/group-by
core-js(-pure)/es|stable|actual|full/object/has-own
core-js(-pure)/es|stable|actual|full/object/keys
core-js(-pure)/es|stable|actual|full/object/values
core-js(-pure)/es|stable|actual|full/object/entries
core-js(-pure)/es|stable|actual|full/object/get-own-property-names
core-js(-pure)/es|stable|actual|full/object/freeze
core-js(-pure)/es|stable|actual|full/object/from-entries
core-js(-pure)/es|stable|actual|full/object/seal
core-js(-pure)/es|stable|actual|full/object/prevent-extensions
core-js/es|stable|actual|full/object/proto
core-js(-pure)/es|stable|actual|full/object/is-frozen
core-js(-pure)/es|stable|actual|full/object/is-sealed
core-js(-pure)/es|stable|actual|full/object/is-extensible
core-js/es|stable|actual|full/object/to-string
core-js(-pure)/es|stable|actual|full/object/define-getter
core-js(-pure)/es|stable|actual|full/object/define-setter
core-js(-pure)/es|stable|actual|full/object/lookup-getter
core-js(-pure)/es|stable|actual|full/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 } }

Object.hasOwn({ foo: 42 }, 'foo'); // => true
Object.hasOwn({ foo: 42 }, 'bar'); // => false
Object.hasOwn({}, 'toString');     // => false

Object.groupBy([1, 2, 3, 4, 5], it => it % 2); // => { 1: [1, 3, 5], 0: [2, 4] }

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|actual|full/function
core-js/es|stable|actual|full/function/name
core-js/es|stable|actual|full/function/has-instance
core-js(-pure)/es|stable|actual|full/function/bind
core-js(-pure)/es|stable|actual|full/function/virtual/bind

Example:

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

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

ECMAScript: Error⬆

Modules es.aggregate-error, es.aggregate-error.cause, es.error.cause, es.error.to-string.

class [
  Error,
  EvalError,
  RangeError,
  ReferenceError,
  SyntaxError,
  TypeError,
  URIError,
  WebAssembly.CompileError,
  WebAssembly.LinkError,
  WebAssembly.RuntimeError,
] {
  constructor(message: string, { cause: any }): %Error%;
}

class AggregateError extends Error {
  constructor(errors: Iterable, message?: string, { cause: any }?): AggregateError;
  errors: Array<any>;
  message: string;
  cause: any;
}

class Error {
  toString(): string; // different fixes
}

CommonJS entry points:

core-js(-pure)/es|stable|actual|full/aggregate-error
core-js/es|stable|actual|full/error
core-js/es|stable|actual|full/error/constructor
core-js/es|stable|actual|full/error/to-string

Example:

const error1 = new TypeError('Error 1');
const error2 = new TypeError('Error 2');
const aggregate = new AggregateError([error1, error2], 'Collected errors');
aggregate.errors[0] === error1; // => true
aggregate.errors[1] === error2; // => true

const cause = new TypeError('Something wrong');
const error = new TypeError('Here explained what`s wrong', { cause });
error.cause === cause; // => true

Error.prototype.toString.call({ message: 1, name: 2 }) === '2: 1'; // => true

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.find-last, es.array.find-last-index, es.array.iterator, es.array.includes, es.array.push, es.array.slice, es.array.join, es.array.unshift, 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, es.array.unscopables.flat-map, es.array.at, es.array.to-reversed, es.array.to-sorted, es.array.to-spliced, es.array.with.

class Array {
  at(index: int): any;
  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): uint;
  findLast(callbackfn: (value: any, index: number, target: any) => boolean, thisArg?: any): any;
  findLastIndex(callbackfn: (value: any, index: number, target: any) => boolean, thisArg?: any): uint;
  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
  push(...args: Array<mixed>): uint;
  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; // with modern behavior like stable sort
  toReversed(): Array<mixed>;
  toSpliced(start?: number, deleteCount?: number, ...items: Array<mixed>): Array<mixed>;
  toSorted(comparefn?: (a: any, b: any) => number): Array<mixed>;
  unshift(...args: Array<mixed>): uint;
  values(): Iterator<value>;
  with(index: includes, value: any): Array<mixed>;
  @@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|actual|full/array
core-js(-pure)/es|stable|actual|full/array/from
core-js(-pure)/es|stable|actual|full/array/of
core-js(-pure)/es|stable|actual|full/array/is-array
core-js(-pure)/es|stable|actual|full/array(/virtual)/at
core-js(-pure)/es|stable|actual|full/array(/virtual)/concat
core-js(-pure)/es|stable|actual|full/array(/virtual)/copy-within
core-js(-pure)/es|stable|actual|full/array(/virtual)/entries
core-js(-pure)/es|stable|actual|full/array(/virtual)/every
core-js(-pure)/es|stable|actual|full/array(/virtual)/fill
core-js(-pure)/es|stable|actual|full/array(/virtual)/filter
core-js(-pure)/es|stable|actual|full/array(/virtual)/find
core-js(-pure)/es|stable|actual|full/array(/virtual)/find-index
core-js(-pure)/es|stable|actual|full/array(/virtual)/find-last
core-js(-pure)/es|stable|actual|full/array(/virtual)/find-last-index
core-js(-pure)/es|stable|actual|full/array(/virtual)/flat
core-js(-pure)/es|stable|actual|full/array(/virtual)/flat-map
core-js(-pure)/es|stable|actual|full/array(/virtual)/for-each
core-js(-pure)/es|stable|actual|full/array(/virtual)/includes
core-js(-pure)/es|stable|actual|full/array(/virtual)/index-of
core-js(-pure)/es|stable|actual|full/array(/virtual)/iterator
core-js(-pure)/es|stable|actual|full/array(/virtual)/join
core-js(-pure)/es|stable|actual|full/array(/virtual)/keys
core-js(-pure)/es|stable|actual|full/array(/virtual)/last-index-of
core-js(-pure)/es|stable|actual|full/array(/virtual)/map
core-js(-pure)/es|stable|actual|full/array(/virtual)/push
core-js(-pure)/es|stable|actual|full/array(/virtual)/reduce
core-js(-pure)/es|stable|actual|full/array(/virtual)/reduce-right
core-js(-pure)/es|stable|actual|full/array(/virtual)/reverse
core-js(-pure)/es|stable|actual|full/array(/virtual)/slice
core-js(-pure)/es|stable|actual|full/array(/virtual)/some
core-js(-pure)/es|stable|actual|full/array(/virtual)/sort
core-js(-pure)/es|stable|actual|full/array(/virtual)/splice
core-js(-pure)/es|stable|actual|full/array(/virtual)/to-reversed
core-js(-pure)/es|stable|actual|full/array(/virtual)/to-sorted
core-js(-pure)/es|stable|actual|full/array(/virtual)/to-spliced
core-js(-pure)/es|stable|actual|full/array(/virtual)/unshift
core-js(-pure)/es|stable|actual|full/array(/virtual)/values
core-js(-pure)/es|stable|actual|full/array(/virtual)/with

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
[1, 2, 3, 4].findLast(isOdd);            // => 3
[1, 2, 3, 4].findLastIndex(isOdd);       // => 2

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]

[1, 2, 3].at(1);  // => 2
[1, 2, 3].at(-1); // => 3

const sequence = [1, 2, 3];
sequence.toReversed(); // => [3, 2, 1]
sequence; // => [1, 2, 3]

const array = [1, 2, 3, 4];
array.toSpliced(1, 2, 5, 6, 7); // => [1, 5, 6, 7, 4]
array; // => [1, 2, 3, 4]

const outOfOrder = [3, 1, 2];
outOfOrder.toSorted(); // => [1, 2, 3]
outOfOrder; // => [3, 1, 2]

const correctionNeeded = [1, 1, 3];
correctionNeeded.with(1, 2); // => [1, 2, 3]
correctionNeeded; // => [1, 1, 3]

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, es.string.replace-all, es.string.at-alternative, es.string.is-well-formed, es.string.to-well-formed.

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 methods. 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, es.string.sup, es.string.substr, es.escape and es.unescape.

RegExp features: modules es.regexp.constructor, es.regexp.dot-all, es.regexp.flags, es.regexp.sticky and es.regexp.test.

class String {
  static fromCodePoint(...codePoints: Array<number>): string;
  static raw({ raw: Array<string> }, ...substitutions: Array<string>): string;
  at(index: int): 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
  replaceAll(searchValue: string | RegExp, replaceString: string | (searchValue, index, this) => string): string;
  search(template: any): any; // ES2015+ fix for support @@search
  split(template: any, limit?: int): Array<string>;; // ES2015+ fix for support @@split, some fixes for old engines
  trim(): string;
  trimLeft(): string;
  trimRight(): string;
  trimStart(): string;
  trimEnd(): string;
  isWellFormed(): boolean;
  toWellFormed(): 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;
  substr(start: int, length?: int): string;
  sup(): string;
  @@iterator(): Iterator<characters>;
}

class RegExp {
  // support of sticky (`y`) flag, dotAll (`s`) flag, named capture groups, can alter flags
  constructor(pattern: RegExp | string, flags?: string): RegExp;
  exec(): Array<string | undefined> | null; // IE8 fixes
  test(string: string): boolean; // delegation to `.exec`
  toString(): string; // ES2015+ fix - generic
  @@match(string: string): Array | null;
  @@matchAll(string: string): Iterator;
  @@replace(string: string, replaceValue: Function | string): string;
  @@search(string: string): number;
  @@split(string: string, limit: number): Array<string>;
  readonly attribute dotAll: boolean; // IE9+
  readonly attribute flags: string;   // IE9+
  readonly attribute sticky: boolean; // IE9+
}

function escape(string: string): string;
function unescape(string: string): string;

CommonJS entry points:

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

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('https://example.com'); // => '<a href="https://example.com">baz</a>'

RegExp('.', 's').test('\n'); // => true
RegExp('.', 's').dotAll;     // => true

RegExp('foo:(?<foo>\\w+),bar:(?<bar>\\w+)').exec('foo:abc,bar:def').groups.bar; // => 'def'

'foo:abc,bar:def'.replace(RegExp('foo:(?<foo>\\w+),bar:(?<bar>\\w+)'), '$<bar>,$<foo>'); // => 'def,abc'

RegExp(/./g, 'm'); // => /./m

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

RegExp('foo', 'y').sticky; // => true

const text = 'First line\nSecond line';
const regex = RegExp('(\\S+) line\\n?', 'y');

regex.exec(text)[1]; // => 'First'
regex.exec(text)[1]; // => 'Second'
regex.exec(text);    // => null

'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
}

'Test abc test test abc test.'.replaceAll('abc', 'foo'); // -> 'Test foo test test foo test.'

'abc'.at(1);  // => 'b'
'abc'.at(-1); // => 'c'

'a💩b'.isWellFormed();      // => true
'a\uD83Db'.isWellFormed();  // => false

'a💩b'.toWellFormed();      // => 'a💩b'
'a\uD83Db'.toWellFormed();  // => 'a�b'

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,