I use Haxe for a lot of side projects.
http://haxe.org/
It's a cross-compiling language, so it can output javascript, java, python, c#, c++, php, and lua. It covers every platform I'm currently interested in working with.
There's communities within Haxe that focus on things like game development for iOs/Android, or react-style web programming.
You can't expect Haxe to completely smooth out all of a given target's problems and quirks, so expect to sort out some target specific quirks if you're really trying to push performance (e.g. mobile gaming). However, having most of the core business logic in one language is incredibly gratifying. And, unless you're using Scala or Haskell, I bet Haxe ends up being the most expressive and powerful language that you use day to day.
There's communities within Haxe that focus on things like game development for iOs/Android, or react-style web programming.
You can't expect Haxe to completely smooth out all of a given target's problems and quirks, so expect to sort out some target specific quirks if you're really trying to push performance (e.g. mobile gaming). However, having most of the core business logic in one language is incredibly gratifying. And, unless you're using Scala or Haskell, I bet Haxe ends up being the most expressive and powerful language that you use day to day.