[seanmcdirmid]

TypeScript is at the top of the list if you include unsound approaches.

[hellofunk]

Only problem there is that TS is still JS, with a lot of JS warts that bother many devs.

Other JS transpiling languages have entirely different semantics from JS altogether, which also make them attractive.

[seanmcdirmid]

That TS is basically JS with types is what makes it so popular.