So is scala, and scala already interoperates with java code. But it's still nice to be able to turn your legacy java code into scala and have all your code in a single language.
I'd say it's an somewhat important side point... But Clojure already cross-compiles to Javascript.
There have been a number of not-quite-successful attempts in the past to compile Scala to Javascript. The latest and most promising seems to be https://github.com/lampepfl/scala-js