core-js is the more common of the two in our index: it runs on 15,036 websites against 28 for HeadJS, and 10 sites use both. Counts come from real scans, not vendor claims.
core-js is a modular standard library for JavaScript, with polyfills for cutting-edge ECMAScript features.
HeadJS is a JavaScript library that optimizes web performance by managing the loading and execution of scripts and stylesheets to ensure faster page loads and efficient resource handling.
10 sites run both core-js and HeadJS. That is 36% of the sites using HeadJS, the smaller of the two.
Want to know which one a particular site uses? Look it up and see its whole stack, or compare two sites side by side.