I'm a big fan of Elm but it's still experimental. Major non-backward-compatible changes to the languages still happen sometimes in new releases. Not the same thing, really.

(Don't know that much about the others.)

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.

Aren't those, with the exception of Elm, much younger and less mature than Dart? I think a language has to reach a certain level before the majority of people will use it in production. And the tooling should be stable also. I think PureScript looks great but it has a different syntax (Haskell like) which may make it a hard sell and I'm not sure they have any debugger at all. When you use a compile-to-js language I think the debugger and editors are important, at least if your language is not very close to js (which Dart is).

Whoo, seems I am getting old. Thats the first time I hear anything of those languages.