By using a coffeescript approach of ES.next to ES.prev compilation. There are several such compilers already, including one from Google called Traceur, and the

By using a coffeescript approach of ES.next to ES.prev compilation. There are several such compilers already, including one from Google called Traceur, and the one as old as Javascript itself, Narcissus. They have been using Traceur and Narcissus for something like "reference" implementations of ES.next, to try out how features work before adding them permanently to the spec.