For type systems you have Flow and TypeScript, coming with their IDEs. The Chrome devtools and various plugins helps having a good IDE too. Multi threading is possible but rarely used because the concurrency model of JavaScript is event based.

Overall, I think JavaScript has a very rich ecosystem because, if you do Web programming, you do not really have alternatives to JavaScript. So the community made the language support various programming styles, tools and libraries.

I do not believe WASM will help bringing more language to the Web than today because it is already possible to compile to JavaScript with good enough performances for most use cases.