Javascript comes with mandatory garbage collection. I suppose you could compile any language to an allocation-free semantic subset of Javascript, but it's probably going to be even less pretty than transpiling to Javascript already is.

WASM, and asm.js before it, roughly exist because Javascript is such a bad compile target.

WASM allows you to run some parts of the application a bit faster. ;)

WASM works with any language and can be much faster than javascript