core-js is the more common of the two in our index: it runs on 19,033 websites against 6 for TanStack, and 3 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.
TanStack is a set of headless JavaScript libraries for building web application features such as data fetching, tables, routing, virtualization, and state management.
3 sites run both core-js and TanStack. That is 50% of the sites using TanStack, 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.