CoffeeScript is a subset of JavaScript with bindings to an alternate syntax (CoffeeScript). John's suggestion is an enforced subset of JavaScript, with the adva

CoffeeScript is a subset of JavaScript with bindings to an alternate syntax (CoffeeScript). John's suggestion is an enforced subset of JavaScript, with the advantage that (some) JavaScript in the wild is understandable; error messages are in terms of the source language (very helpful, more so for beginners); and yields a skill with greater marketability (so far). Why not use the same subset as CoffeeScript? In some cases, this simply forbids some constructs; in other cases it allows a construct, but only in certain contexts. A mathematically precise but implementation-unfriendly definition is: only allow the subset of JavaScript that is the compilation of some CoffeeScript program.