It compares pretty well. It is possible to define Scala bindings to JS libraries, so you can use them more or less seamlessly. Bindings to the most popular ones (jQuery, React, etc.) are already defined, and actively maintained. It is relatively simple to add your own. A nice thing is that, since Scala is a strongly-typed language, you get code-completion and refactoring "for free". However, if you prefer to use dynamic typing, ScalaJS lets you do that easily too.

Also, like the grandparent comment said, ClojureScript is a very nice language too, with powerful tooling.

The nice thing about ScalaJS and ClojureScript is that they both can run on the browser, and on a JVM backend. Scala also has an LLVM backend that is being actively developed, but on its early stages. (ScalaJS is very mature and performant)