The general consensus is that you won't (meaningfully) be able to "reuse the JS that a user might already have" anyway. The idea of well-know-domain.com/well-known-framework-well-known-version-0-1.js being cashed is interesting -- but a) it doesn't appear to work to well in practice (I seem to recall 30% hit rate for the "best case" of jquery-latest on some big CDN) b) It's not that much data anyway.

Lets say you have 500kb of compressed js. If you have 2 million unique visitors, that'll just eat 1TB of bandwidth -- and if you don't screw up your own cache headers -- that'll be that -- it'll be cached.