I was using node.js for a while and having to deal with the same bugs over and over again. Usually the bug would be passing the wrong type or mistyping a name. I started using Typescript and things got a lot better. Scala.js is great and all, and probably a good choice if you're using Scala on the backend, but Typescript works a bit better because the Javascript it generates is pretty close to the original. If you search the HN archives, people seem pretty happy with it.
The difference is that Scala is a vastly better, designed language. It's not something like Typescript which tries to salvage JavaScript without being able to fix the worst parts.
Scala also has much better, more mature tooling and IDE support.
The build system is also amazing, not the mess cobbled together in JavaScript that is changing every second week.
In addition to that you have stable, existing and mature libraries which work on multiple platforms, achieving _real_ cross platform support. (Not this "if you make JavaScript run outside of the browser..." nonsense.)
Plus, the quality of libraries is so much higher because they aren't written by JS kids with a maximum attention span of 5 minutes.
