I noted at a Twinkle issue that there seems to be a complete absence of documentation on MediaWiki's JavaScript hook functionality, other than the generated documentation on doc.wikimedia.org (which has almost zero discoverability).

The documentation of MediaWiki's JavaScript libraries is pretty patchy; the material on doc.wikimedia.org is generally good, but like I said, that's only useful if you already know what you're looking for.

If someone is looking for something to do on MW Documentation Day, it'd make me really happy if the JavaScript documentation on MW.org (most of which currently lives in subpages of https://www.mediawiki.org/wiki/ResourceLoader) could be thoroughly updated and expanded, and possibly ordered into a topical index alongside the strictly alphabetic ordering that is currently available.