+1 on ScalaJS. It's pretty amazing piece of technology (Thank's team!) using it in combination with David Barri's bindings for ReactJS[1] and Li Haoyi's uPickle[2]. It makes for elegant solutions with a shared codebase.

[1] https://github.com/japgolly/scalajs-react

[2] https://github.com/lihaoyi/upickle