Browsers are good enough that if you're careful about it you can just use a WebWorker and write standard JS, no need to worry with the build/compilation/compatibility headaches adding a whole new language to your ecosystem entails. Nice part with this is you can even have a fallback routine that runs the WebWorker code in the main thread to support really old browsers.

I wouldn't recommend dropping to WASM until you've ironed out the ideal data structures/algorithms to use, and have spent some time in the devtools profiler. Hacking on ideas in JS is (IME) much faster than doing the same in a compiled language, and the built in profiling in browsers works far better for JS than any WASM (again IME). Only after you have an optimized algorithm, data structure, and implementation, and still find perf isn't where you want it would I recommend rewriting in a compiled language -- even then I'd wager having the optimized reference implementation around will make this much easier and faster than if you tried to start from 0 in the compiled language.

Bonus points if you can reuse tests and/or have a debug mode that runs both implementations in parallel and throws on any differences in output.