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Modular standard library for JavaScript. Includes polyfills for ECMAScript up to 2025: 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.

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Example of usage:

1import 'core-js/actual';
2
3Promise.try(() => 42).then(it => console.log(it)); // => 42
4
5Array.from(new Set([1, 2, 3]).union(new Set([3, 4, 5]))); // => [1, 2, 3, 4, 5]
6
7[1, 2].flatMap(it => [it, it]); // => [1, 1, 2, 2]
8
9Iterator.concat([1, 2], function * (i) { while (true) yield i++; }(3))
10  .drop(1).take(5)
11  .filter(it => it % 2)
12  .map(it => it ** 2)
13  .toArray(); // => [9, 25]
14
15structuredClone(new Set([1, 2, 3])); // => new Set([1, 2, 3])

You can load only required features:

1import 'core-js/actual/promise';
2import 'core-js/actual/set';
3import 'core-js/actual/iterator';
4import 'core-js/actual/array/from';
5import 'core-js/actual/array/flat-map';
6import 'core-js/actual/structured-clone';
7
8Promise.try(() => 42).then(it => console.log(it)); // => 42
9
10Array.from(new Set([1, 2, 3]).union(new Set([3, 4, 5]))); // => [1, 2, 3, 4, 5]
11
12[1, 2].flatMap(it => [it, it]); // => [1, 1, 2, 2]
13
14Iterator.concat([1, 2], function * (i) { while (true) yield i++; }(3))
15  .drop(1).take(5)
16  .filter(it => it % 2)
17  .map(it => it ** 2)
18  .toArray(); // => [9, 25]
19
20structuredClone(new Set([1, 2, 3])); // => new Set([1, 2, 3])

Or use it without global namespace pollution:

1import Promise from 'core-js-pure/actual/promise';
2import Set from 'core-js-pure/actual/set';
3import Iterator from 'core-js-pure/actual/iterator';
4import from from 'core-js-pure/actual/array/from';
5import flatMap from 'core-js-pure/actual/array/flat-map';
6import structuredClone from 'core-js-pure/actual/structured-clone';
7
8Promise.try(() => 42).then(it => console.log(it)); // => 42
9
10from(new Set([1, 2, 3]).union(new Set([3, 4, 5]))); // => [1, 2, 3, 4, 5]
11
12flatMap([1, 2], it => [it, it]); // => [1, 1, 2, 2]
13
14Iterator.concat([1, 2], function * (i) { while (true) yield i++; }(3))
15  .drop(1).take(5)
16  .filter(it => it % 2)
17  .map(it => it ** 2)
18  .toArray(); // => [9, 25]
19
20structuredClone(new Set([1, 2, 3])); // => new Set([1, 2, 3])

It's a version without global namespace pollution (the third example), for more info see core-js documentation.