Rather than a Javascript 'Alternative', out of interest what is the best way of using Current / Future Javascript?

Looking at wikipedia (and I may be getting misinformed here) the most recent version of 'Javascript Extensions' is 1.8.1, and EMCA is on edition 5. However support for both of these seems patchy.

I often see mentions of all kinds of polyfills, but is there a nice library and/or compiler, which gives me access to the "Javascript of the future", on browsers which do not yet support it? Is this even a sensible thing to ask for?