Of course Rails took YEARS getting to the point of scalability. People are acting like it was scalable on the very first alpha release.

But will JavaScript be able to scale? It has come a long way from the days of onMouseOver but it seems like it's going to be hard unless it gets turned into a different language (i.e. Dart).

What do you even mean by scale in this context?

https://en.wikipedia.org/wiki/V8_%28JavaScript_engine%29

V8 increases performance by compiling JavaScript to native machine code (x86,[2] ARM,[2] or MIPS[3] CPUs), before executing it, versus executing bytecode or interpreting it. Further performance increases are achieved by employing optimization methods such as inline caching.

I mean the language itself, not the performance of the individual implementations. For example, I can't imagine prototype based OO being used for large scale projects.