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core-js is the most widely used JavaScript polyfill library. It provides modular polyfills for ECMAScript features from ES5 through ES2025, TC39 stage proposals, and select Web/WHATWG APIs — letting you use modern JavaScript everywhere, without waiting for runtime support.

Installation

Install core-js and its companion packages via npm, yarn, or pnpm.

Quick Start

Start polyfilling in minutes with a working code example.

Entry Points

Learn how to import only the features you need.

Babel Integration

Automate polyfill injection with @babel/preset-env and @babel/runtime.

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What is core-js?

core-js is a modular standard library for JavaScript. It includes polyfills for:
  • ECMAScript up to ES2025 — Promise, Symbol, Map, Set, Iterator, TypedArrays, Array methods, String methods, Object methods, and hundreds more
  • TC39 proposals — from finished proposals (already in the language) down through Stage 0 experiments
  • Web standards — URL, URLSearchParams, structuredClone, queueMicrotask, setImmediate, DOMException, and iterable DOM collections
You can load the entire library, or import only the specific features your code uses — keeping bundle sizes small and avoiding unnecessary overhead.
1

Install the package

npm install --save core-js@3
2

Import features you need

// All stable ES + Web + Stage 3 proposals:
import 'core-js/actual';

// Or import just what you use:
import 'core-js/actual/promise';
import 'core-js/actual/array/flat-map';
import 'core-js/actual/set';
3

Write modern JavaScript everywhere

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

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

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

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Choose your package

core-js

The global polyfill — patches built-in prototypes and constructors directly. Best for application code.

core-js-pure

Pollution-free variant — exports standalone functions without touching globals. Best for library authors.

core-js-compat

Compatibility data — get a list of required core-js modules for any browserslist target.

core-js-builder

Custom build — generate a targeted bundle with only the polyfills your environments need.

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Explore the API

ECMAScript builtins

Object, Array, String, Number, Math, Date, Function, Error

Collections

Map, Set, WeakMap, WeakSet, WeakRef

Iterators

Iterator helpers, AsyncIterator, for-of support

Typed Arrays

Int8Array, Uint8Array, Float32Array, and all TypedArray methods

Promise

Promise, Promise.all, Promise.allSettled, Promise.any, Promise.try

Web Standards

URL, structuredClone, queueMicrotask, setImmediate, DOMException