It's written in JS, not compiled to JS. WASM isn't a good target to compile JS to, and there's little point in doing so.

So what you're saying is we need to rewrite it in Rust, then compile it to WASM, then compile an ASM.js backup as well? /s

Now all the browsers support wasm so you don't need to compile it to asm.js. You may use any language if you don't like Rust.

I think the reason they did not do that is because it's a JavaScript project and they didn't want to rewrite it in C or another language since there are already lots of emulators in C. But it could be faster if it was compiled to wasm.