It looks like web workers is the way for JavaScript to do multi-threading.
Async has always been enough for what I need to do in the front end, as most of my long running processes are just calling a back end.
Edit to add: for context, I am a full stack developer and know what threads are... I just never have needed them in the browser.
On the other hand, the isolation guarantees are strong. There aren't really any footguns. Messaging is straightforward, works with a lot of data types and supports channels for inter-worker communication.