core-js is the more common of the two in our index: it runs on 15,932 websites against 55 for NProgress, and 23 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.
NProgress is a JavaScript library that displays a nanoscopic progress bar at the top of websites or web applications. It includes trickle animations to indicate loading activity, enhancing user experience by showing that content is being loaded.
23 sites run both core-js and NProgress. That is 42% of the sites using NProgress, 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.