Then pick Scala.js.

Solid, mature, great libraries and one of the best languages out there, even when competing against real (in the sense of not compile-to-JavaScript-only) languages.

People are different and not all thinks Scala is great language...

I'm unlikely to give it a serious try because I find the complexity of the Scala language to be pretty scary. Still, I'm curious: how fast is the compiler? Does it generate compact code? Clear error messages?

I started using Scala a few weeks ago. I'm finding it's pretty easy to ease yourself into it. So far I'm not doing much that I couldn't do in C#, but it's more concise in Scala. I was productive with it the first day.

I haven't tried scala.js yet, but I've gone through some presentation slides...they're claiming compact code that runs as fast as javascript, clear error messages that point to the line of scala sourcecode, and even some advantages over jvm scala (though I forget the details). Don't know about compile speed.

Compilation to JS is a few additional seconds during development, and the rest of your comment is right to the point.