core-js
core-js is a modular standard library for JavaScript, with polyfills for cutting-edge ECMAScript features.
What detecting core-js tells you about a site
Detecting core-js on a site tells you it ships a javascript libraries — a concrete, named part of the stack rather than a guess. Its fingerprints live in the client — runtime globals, DOM markers and asset URLs — so detection happens straight from the rendered page. core-js is open source, so the detection signals are stable and well-documented across versions.
About
core-js is a modular standard library for JavaScript, with polyfills for cutting-edge ECMAScript features.
Categories: JavaScript libraries
Quick facts
How we detect core-js
Sourcemap Explorer carries 5 fingerprint signals for core-js, spread across 1 channel — javascript global. The exact patterns are listed below, and you can replay each one in Chrome DevTools to confirm a match by hand.
Each signal alone is rarely conclusive — Sourcemap Explorer cross-references all of them and weights by confidence. You can reproduce any of these checks yourself in Chrome DevTools.
Window-level global the technology installs on page. Reproducible by typing the path into the DevTools console.
window.__core-js_shared__
window.__core-js_shared__.versions.0.version = ^(.+)$
window._babelPolyfill
window.core
window.core.version = ^(.+)$
FAQ
How do I check if a website is using core-js?
Open the page in Chrome, click the Sourcemap Explorer toolbar icon, and read the Stack tab. core-js's specific fingerprints here are javascript global, and the popup flags core-js whenever any combination of them is found. The same checks can be reproduced manually in DevTools — see the "How we detect" section above.
What core-js version can Sourcemap Explorer detect?
core-js ships as a hosted javascript libraries rather than a bundled npm package, so version-specific detection isn't always possible. Where the platform leaks a version in response headers (`X-Powered-By`, `Server`, generator meta tags) we surface it; otherwise we report presence only.
Is core-js open source?
Yes, core-js is open source. The license details are listed on the official site (https://github.com/zloirock/core-js), and the codebase typically lives on a public source-control host.
Where can I read more about core-js?
Official site: https://github.com/zloirock/core-js. For Sourcemap Explorer's detection guide, see the deep-dive link below or the related guides in the cross-link section.
Detected by Sourcemap Explorer
Open the popup on any page running core-jsand you'll see the exact version pulled from the bundled package.json when sourcemaps are exposed.