I have read the question How to test and develop with asm.js?, and the accepted answer gives a link to http://kripken.github.com/mloc_emscripten_talk/#/.
The conclusion of that slide show is that Statically-typed languages and especially C/C++ can be compiled effectively to JavaScript, so we can expect the speed of compiled C/C++ to get to just 2X slower than native code, or better, later this year.
But what about non-statically-typed languages, such as regular JavaScript itself? Can it be compiled to asm.js?