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core-js includes polyfills for select Web/WHATWG platform APIs that are widely used but not always available in all environments. These are available under the stable, actual, and full namespaces. 
// Load all web standard polyfills
import 'core-js/actual';

// Or load specific ones
import 'core-js/stable/url';
import 'core-js/stable/structured-clone';
import 'core-js/stable/queue-microtask';

​
URL and URLSearchParams

Full implementation of the WHATWG URL standard. 
import 'core-js/stable/url';
import 'core-js/stable/url-search-params';

URL.canParse('https://example.com'); // => true
URL.canParse('not-a-url');           // => false

URL.parse('https://example.com');    // => URL object
URL.parse('not-a-url');              // => null

const url = new URL('https://login:password@example.com:8080/foo/bar?a=1&b=2#frag');
url.href;       // => 'https://login:password@example.com:8080/foo/bar?a=1&b=2#frag'
url.origin;     // => 'https://example.com:8080'
url.protocol;   // => 'https:'
url.username;   // => 'login'
url.password;   // => 'password'
url.host;       // => 'example.com:8080'
url.hostname;   // => 'example.com'
url.port;       // => '8080'
url.pathname;   // => '/foo/bar'
url.search;     // => '?a=1&b=2'
url.hash;       // => '#frag'

const params = new URLSearchParams('a=1&b=2&a=3');
params.getAll('a'); // => ['1', '3']
params.has('b');    // => true
params.get('b');    // => '2'
params.size;        // => 3
Entry points: 
core-js(-pure)/stable|actual|full/url
core-js(-pure)/stable|actual|full/url/can-parse
core-js/stable|actual|full/url/to-json
core-js(-pure)/stable|actual|full/url-search-params

​
structuredClone

Deep clones a value using the structured clone algorithm. 
import 'core-js/stable/structured-clone';

const obj = { a: [1, 2, 3] };
const clone = structuredClone(obj);
clone !== obj;       // => true
clone.a !== obj.a;  // => true

// Supports complex types
structuredClone(new Set([1, 2, 3]));                      // => Set {1, 2, 3}
structuredClone(new Map([['a', 1]]));                      // => Map { 'a' => 1 }
structuredClone(new Int8Array([1, 2, 3]));                 // => Int8Array [1, 2, 3]
structuredClone(new TypeError('msg', { cause: 42 }));     // => TypeError

// Circular references
const circular = {};
circular.self = circular;
const c = structuredClone(circular);
c.self === c; // => true
The .transfer option for transferring ArrayBuffer is only polyfilled in environments with native structuredClone or MessageChannel support. Avoid relying on it in older environments.
Entry points: 
core-js(-pure)/stable|actual|full/structured-clone

​
queueMicrotask

Schedules a microtask — runs after the current task but before any macrotasks. 
import 'core-js/stable/queue-microtask';

queueMicrotask(() => console.log('microtask'));
console.log('sync');
// Output: 'sync', then 'microtask'
Entry point: 
core-js(-pure)/stable|actual|full/queue-microtask

​
setImmediate / clearImmediate

Schedules a callback to run after I/O events but before setTimeout. 
import 'core-js/stable/set-immediate';

const id = setImmediate((a, b) => {
  console.log(a, b); // => 'hello' 'world'
}, 'hello', 'world');

clearImmediate(id); // cancels it
Entry points: 
core-js(-pure)/stable|actual|full/set-immediate
core-js(-pure)/stable|actual|full/clear-immediate

​
setTimeout / setInterval

Fixes additional-arguments support in IE9 and below. 
import 'core-js/stable/set-timeout';

// Before (IE9 workaround):
setTimeout(log.bind(null, 42), 1000);

// After (cross-platform):
setTimeout(log, 1000, 42);
Entry points: 
core-js(-pure)/stable|actual|full/set-timeout
core-js(-pure)/stable|actual|full/set-interval

​
Base64 — atob / btoa

import 'core-js/stable/atob';
import 'core-js/stable/btoa';

btoa('hi, core-js');      // => 'aGksIGNvcmUtanM='
atob('aGksIGNvcmUtanM='); // => 'hi, core-js'
Entry points: 
core-js(-pure)/stable|actual|full/atob
core-js(-pure)/stable|actual|full/btoa

​
DOMException

Standard DOMException constructor for environments that don’t have it. 
import 'core-js/stable/dom-exception';

const err = new DOMException('message', 'DataCloneError');
err instanceof DOMException; // => true
err.name;    // => 'DataCloneError'
err.message; // => 'message'
Entry points: 
core-js(-pure)/stable|actual|full/dom-exception

​
self

Global self reference for environments that don’t expose it. 
import 'core-js/stable/self';

self.Array === Array; // => true
Entry point: 
core-js(-pure)/stable|actual|full/self

​
Iterable DOM collections

Makes NodeList, DOMTokenList, MediaList, StyleSheetList, and CSSRuleList iterable with for...of, spread, and destructuring. 
import 'core-js/stable/dom-collections';

for (const element of document.querySelectorAll('div')) {
  // works without Array.from()
}

const [first, ...rest] = document.querySelectorAll('a');
Entry points: 
core-js(-pure)/stable|actual|full/dom-collections
core-js(-pure)/stable|actual|full/dom-collections/iterator