Out of the Alt-JS languages I've played with, #Scala.js seems to be the best.

Given it's probably not widely adopted, its maturity is surprising. E.g., ScalablyTyped can convert TypeScript definitions, and actually works. Interop is good. And the compiler is, dare I say it, pretty fast compared to 2–3 years ago. Scala 3 also helps. E.g., it has untagged unions, just like TypeScript.

I'm surprised that it's in better shape than #KotlinJS, for all its multi-platform marketing.