After some recent ambitious JS development, it's become clear to me that the language is just unsuited to large scale development and no library is going to change that.
As such, I am really keen on these compiler projects, but I think it's a mistake to avoid language features because they are difficult to implement in JS. Syntax is nice, but it's those language features that are really needed.
It depends on your goals. For me, it's not so much about difficult to implement as whether the generated JS is idiomatic enough for the various optimizing JS implementations (V8, SquirrelFish, TraceMonkey) to handle it. With Clamato, we've shied away from implementing language features until we could see how to do them in a way that wouldn't hurt optimization (the only exception I can think of is the Smalltalk cascade syntax, which is implemented naively and slowly for now, but it's obvious that with a bit more work it could be sped up).
After some recent ambitious JS development, it's become clear to me that the language is just unsuited to large scale development and no library is going to change that.
As such, I am really keen on these compiler projects, but I think it's a mistake to avoid language features because they are difficult to implement in JS. Syntax is nice, but it's those language features that are really needed.