I gave a try to this scala to javascript compiler last night, and was quite amazed by the level of maturity this has already reached !
While probably never a replacement for mainline javascript in web sites, I see this as a competitor to coffeescript for web applications.
Full demo of scala-js-fiddle so you can experiment with scala directly in your browser, with some IDE-like features.
This implementation of Langton’s ant is what I achieved from scratch in 20 min.