Modular standard library for JavaScript. Includes polyfills for ECMAScript up to 2023: 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.

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

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

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Example

Example of usage:

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

Array.from(new Set([1, 2, 3, 2, 1]));          // => [1, 2, 3]
[1, 2, 3, 4, 5].group(it => it % 2);           // => { 1: [1, 3, 5], 0: [2, 4] }
Promise.resolve(42).then(x => console.log(x)); // => 42
structuredClone(new Set([1, 2, 3]));           // => new Set([1, 2, 3])
queueMicrotask(() => console.log('called as microtask'));

You can load only required features:

import 'core-js/actual/array/from';       // <- at the top of your entry point
import 'core-js/actual/array/group';      // <- at the top of your entry point
import 'core-js/actual/set';              // <- at the top of your entry point
import 'core-js/actual/promise';          // <- at the top of your entry point
import 'core-js/actual/structured-clone'; // <- at the top of your entry point
import 'core-js/actual/queue-microtask';  // <- at the top of your entry point

Array.from(new Set([1, 2, 3, 2, 1]));          // => [1, 2, 3]
[1, 2, 3, 4, 5].group(it => it % 2);           // => { 1: [1, 3, 5], 0: [2, 4] }
Promise.resolve(42).then(x => console.log(x)); // => 42
structuredClone(new Set([1, 2, 3]));           // => new Set([1, 2, 3])
queueMicrotask(() => console.log('called as microtask'));

Or use it without global namespace pollution:

import from from 'core-js-pure/actual/array/from';
import group from 'core-js-pure/actual/array/group';
import Set from 'core-js-pure/actual/set';
import Promise from 'core-js-pure/actual/promise';
import structuredClone from 'core-js-pure/actual/structured-clone';
import queueMicrotask from 'core-js-pure/actual/queue-microtask';

from(new Set([1, 2, 3, 2, 1]));                // => [1, 2, 3]
group([1, 2, 3, 4, 5], it => it % 2);          // => { 1: [1, 3, 5], 0: [2, 4] }
Promise.resolve(42).then(x => console.log(x)); // => 42
structuredClone(new Set([1, 2, 3]));           // => new Set([1, 2, 3])
queueMicrotask(() => console.log('called as microtask'));

Index

• Usage
  • Installation
  • postinstall message
  • CommonJS API
  • Babel
    • @babel/polyfill
    • @babel/preset-env
    • @babel/runtime
  • swc
  • Configurable level of aggressiveness
  • Custom build
• Compatibility data
• Supported engines
• 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
      • 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
      • Symbol.asyncIterator for asynchronous iteration
      • Symbol.prototype.description
      • Well-formed JSON.stringify
    • Stage 3 proposals
      • Array grouping
      • Change Array by copy
    • Stage 2 proposals
      • Iterator helpers
      • New Set methods
      • Map.prototype.emplace
      • Array.fromAsync
      • Array.isTemplateObject
      • Symbol.{ asyncDispose, dispose } for using statement
      • Symbol.metadataKey for decorators metadata proposal
    • Stage 1 proposals
      • Observable
      • New collections methods
      • .of and .from methods on collection constructors
      • compositeKey and compositeSymbol
      • Array filtering
      • Array deduplication
      • Getting last item from Array
      • Number.range
      • Number.fromString
      • Math extensions
      • Math.signbit
      • String.cooked
      • String.prototype.codePoints
      • Symbol.matcher for pattern matching
    • Stage 0 proposals
      • Function.prototype.unThis
      • Function.{ isCallable, isConstructor }
      • URL
    • Pre-stage 0 proposals
      • Reflect metadata
  • Web standards
    • 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:

postinstall message

CommonJS API

Caveats when using CommonJS API

CommonJS and prototype methods without global namespace pollution

Babel

@babel/polyfill

@babel/preset-env

@babel/runtime

swc

Configurable level of aggressiveness

Custom build

Compatibility data

Supported engines

Features:

ECMAScript

ECMAScript: Object

ECMAScript: Function

ECMAScript: Error

ECMAScript: Array

ECMAScript: String and RegExp

ECMAScript: Number

ECMAScript: Math

ECMAScript: Date

ECMAScript: Promise

Unhandled rejection tracking

ECMAScript: Symbol

Caveats when using Symbol polyfill

ECMAScript: Collections

Map

Set

WeakMap

WeakSet

Caveats when using collections polyfill

ECMAScript: Typed Arrays

Caveats when using typed arrays polyfills

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

Object.values / Object.entries

Object.fromEntries

Object.getOwnPropertyDescriptors

Accessible Object.prototype.hasOwnProperty

String padding

String#matchAll

String#replaceAll

String.prototype.trimStart / String.prototype.trimEnd

RegExp s (dotAll) flag

RegExp named capture groups

Promise.allSettled

Promise.any

Promise.prototype.finally

Symbol.asyncIterator for asynchronous iteration

Symbol.prototype.description

Well-formed JSON.stringify

Stage 3 proposals

Array grouping

Change Array by copy

Stage 2 proposals

Iterator helpers

• Caveats

New Set methods

Map.prototype.emplace

Array.fromAsync

Array.isTemplateObject

Symbol.{ asyncDispose, dispose } for using statement

Symbol.metadataKey for decorators metadata proposal

Stage 1 proposals

Observable

New collections methods

.of and .from methods on collection constructors

compositeKey and compositeSymbol

Array filtering

Array deduplication

Getting last item from Array

Number.range

Number.fromString

Math extensions

Math.signbit

String.cooked

String.prototype.codePoints

Symbol.matcher for pattern matching

Seeded pseudo-random numbers

Object iteration

Promise.try

Stage 0 proposals

Function.prototype.unThis

Function.{ isCallable, isConstructor }

URL

String#at

Efficient 64 bit arithmetic

Pre-stage 0 proposals

Reflect metadata

Web standards

structuredClone

Caveats when using structuredClone polyfill

Base64 utility methods

setTimeout and setInterval

setImmediate

QueueMicrotask

URL and URLSearchParams

Caveats when using URL and URLSearchParams

DOMException

Iterable DOM collections

Iteration helpers

Missing polyfills