core-js is the more common of the two in our index: it runs on 18,642 websites against 17 for Locomotive Scroll, and 9 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.
Locomotive Scroll is an opinionated JavaScript library that provides smooth scrolling animations and advanced scroll interactions for web applications.
9 sites run both core-js and Locomotive Scroll. That is 53% of the sites using Locomotive Scroll, 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.