If you ask me, this is all about (b) if you consider only JavaScript vs Scala.js.

If you compare Scala.js to similar languages (CoffeeScript, TypeScript, ClojureScript to name a few), it definitely helps to have the same language both on server and client, especially if logic must be replicated on both sides. So in that sense, there is some (a) too.

But that is only my opinion. Any developer or team can make up her own mind on the matter.