[jerf]

"Well optimized Javascript", and more generally, "well-optimized code for a JIT/optimizer for language X", is a subset of language X, is an undefined subset of language X, is a moving subset of language X that is moving in ways unrelated to your project, is actually multiple such subsets at a minimum one per JIT and arguably one per version of JIT compilers, and is generally a subset of language X that is extremely complicated (e.g., you can lose optimization if your arrays grow in certain ways, or you can non-locally deoptimize vast swathes of your code because one function call in one location happened to do one thing the JIT can't handle and it had to despecialize everything touching it as a result) such that trying to keep a lot of developers in sync with the requirements on a large project is essentially infeasible.

None of these things say "this is a good way to build a large compiler suite that we're building for performance".