xref-js2 adds an xref backend for JavaScript files. Instead of using a tag system, it relies on `ag' to query the codebase of a project. This might sound crazy at first, but it turns out that `ag' is so fast that jumping using xref-js2 is most of the time instantaneous, even on fairly large JavaScript codebase (it successfully works with 50k lines of JS code). Because line by line regexp search has its downside, xref-js2 does a second pass on result candidates and eliminates possible false positives using `js2-mode''s AST, thus giving very accurate results.