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.

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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

3.40.0 - 2025.01.08

• Changes v3.39.0...v3.40.0 (130 commits)
• Added Error.isError stage 3 proposal:
  • Added built-ins:
    • Error.isError
  • We have no bulletproof way to polyfill this method / check if the object is an error, so it's an enough naive implementation that is marked as .sham
• Explicit Resource Management stage 3 proposal:
  • Updated the way async disposing of only sync disposable resources, tc39/proposal-explicit-resource-management/218
• Iterator sequencing stage 2.7 proposal:
  • Reuse IteratorResult objects when possible, tc39/proposal-iterator-sequencing/17, tc39/proposal-iterator-sequencing/18, December 2024 TC39 meeting
• Added a fix of V8 < 12.8 / NodeJS < 22.10 bug with handling infinite length of set-like objects in Set methods
• Optimized DataView.prototype.{ getFloat16, setFloat16 } performance, #1379, thanks @LeviPesin
• Dropped unneeded feature detection of non-standard %TypedArray%.prototype.toSpliced
• Dropped possible re-usage of some non-standard / early stage features (like Math.scale) available on global
• Some other minor improvements
• Compat data improvements:
  • RegExp.escape marked as shipped from Safari 18.2
  • Promise.try marked as shipped from Safari 18.2
  • Math.f16round and DataView.prototype.{ getFloat16, setFloat16 } marked as shipped from Safari 18.2
  • Uint8Array to / from base64 and hex proposal methods marked as shipped from Safari 18.2
  • JSON.parse source text access proposal features marked as shipped from FF135
  • RegExp.escape marked as shipped from FF134
  • Promise.try marked as shipped from FF134
  • Symbol.dispose, Symbol.asyncDispose and Iterator.prototype[@@dispose] marked as shipped from FF135
  • JSON.parse source text access proposal features marked as shipped from Bun 1.1.43
  • Fixed NodeJS version where URL.parse was added - 22.1 instead of 22.0
  • Added Deno 2.1 compat data mapping
  • Added Rhino 1.8.0 compat data with significant number of modern features
  • Added Electron 35 compat data mapping
  • Updated Opera 115+ compat data mapping
  • Added Opera Android 86 and 87 compat data mapping