See the Elm to JS compiler, it does deep unused code elimination at the function level so you only ship the code you actually use (or the functions u actually use, even inside libraries).

The closure js compiler is also pretty good if you prefer writing js.

If we only start using better languages in our applications we could improve performance quite a bit.

Dead code elimination has been available in C compilers since... so long ago I can't remember. It's not that compilers can't do it, but much like the halting problem, they really can't catch it all.