I've used javascript for many many years and, as of 6mo ago, started using Scala (I do a lot of distributed programming on hadoop, storm, and recently spark). Scala is now one of my favorite languages because of Spark (http://spark-project.org/). Scala very nicely blends the functional and imperative styles and it always favors programming without side-effects. Anything you can do in java you can do in fewer lines and you will see fewer bugs.

That said- it's not a superior language, just a different paradigm. With Scala you get very strong typing, functional constructs (so incredibly powerful), matching, options, very clean closure syntax, etc.. You'll eventually fall in-love with _, too.

I haven't used scala.js yet, but here are some downsides to Scala proper: * slow the compile * tooling isn't great (this true of anything jvm to some degree though)