jQuery

jQuery is stable, actively maintained, and still receives updates. It solved cross-browser DOM manipulation when browsers were inconsistent, and it continues to work reliably in the codebases that depend on it.

Don't start new projects with it. React handles DOM updates declaratively, and vanilla JavaScript covers everything jQuery does via document.querySelector, fetch, and CSS transitions. But ripping jQuery out of a working legacy application rarely justifies the effort. Let it be until a larger rewrite makes sense.