core-js

What is core-js?

The core-js npm package is a modular standard library for JavaScript, which includes polyfills for ECMAScript features. It provides reliable polyfills to ensure that code behaves consistently across different environments, including older browsers.

What are core-js's main functionalities?

Polyfilling ECMAScript features

This feature allows developers to use the latest ECMAScript features while ensuring backward compatibility with older environments that do not support these features natively.

require('core-js/stable');
// Now you can use ES6 features like Promise in environments that do not support them natively.

Polyfilling Web Standards

Core-js can also polyfill web standards, allowing developers to use modern web APIs in environments that have not implemented them.

require('core-js/web');
// This includes polyfills for web standards like DOM collections (e.g., NodeList), timers, and more.

Polyfilling Proposals

Developers can experiment with proposed ECMAScript features before they are finalized and adopted into the standard, ensuring forward compatibility.

require('core-js/proposals');
// This will include polyfills for ECMAScript proposals that are not yet part of the standard.

Other packages similar to core-js

babel-polyfill

Babel-polyfill is a package that provides polyfills necessary for a full ES2015+ environment. It is similar to core-js but is more tightly coupled with Babel's transpilation process.

es6-shim

The es6-shim package provides polyfills for ECMAScript 6 (aka ECMAScript 2015) features. It is similar to core-js but focuses specifically on ES6 features and does not cover proposals or web standards.

polyfill-service

The polyfill-service by Financial Times is an online service that provides polyfills based on the user's browser. It is different from core-js in that it is a service rather than a package you include in your project, but it serves a similar purpose in polyfilling features.

README

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

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.

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])

It's a global version (first 2 examples), for more info see core-js documentation.

Changelog

3.44.0 - 2025.07.07

• Changes v3.43.0...v3.44.0 (87 commits)
• Uint8Array to / from base64 and hex stage 3 proposal:
  • Fixed several V8 bugs in Uint8Array.fromHex and Uint8Array.prototype.{ setFromBase64, toBase64, toHex }, thanks @brc-dd
• Joint iteration stage 2.7 proposal:
  • Uses Get in Iterator.zipKeyed, following tc39/proposal-joint-iteration#43
• Iterator sequencing stage 2.7 proposal:
  • Iterator.concat no longer reuses IteratorResult object of concatenated iterators, following tc39/proposal-iterator-sequencing#26
• Iterator chunking stage 2 proposal:
  • Added built-ins:
    • Iterator.prototype.sliding
• Number.prototype.clamp stage 2 proposal:
  • clamp no longer throws an error on NaN as min or max, following tc39/proposal-math-clamp#d2387791c265edf66fbe2455eab919016717ce6f
• Fixed some cases of Set.prototype.{ symmetricDifference, union } detection
• Added missing dependencies to some entries of static Iterator methods
• Added missing /full/{ instance, number/virtual }/clamp entries
• Some minor stylistic changes
• Compat data improvements:
  • Added Electron 38 and 39 compat data mapping
  • Added Oculus Quest Browser 38 and 39 compat data mapping
  • Iterator helpers marked as fixed and updated following the latest spec changes in Safari 26.0
  • Set.prototype.{ difference, symmetricDifference, union } marked as fixed in Safari 26.0
  • SuppressedError marked as fixed in FF141
  • Error.isError marked as fixed in Node 24.3
  • setImmediate and clearImmediate marked as available from Deno 2.4
  • Math.sumPrecise marked as shipped in Bun 1.2.18
  • %TypedArray%.prototype.with marked as fixed in Bun 1.2.18