I guess it really depends on our definition of fundamental. Certainly, it doesn't turn JS into Lisp. But it does introduce concepts that while possible in JS, are not nearly as easy and transparent to use. E.g. the 'do' keyword. When I write CS, I think using 'do' and not "let me insert a closure here to ensure variable values aren't overwritten in the loop." Same with comprehension, function binding (=>), and classes in CS.

Syntactically, I can't live without Heredocs because my project involves a lot of HTML generated dynamically through JS/CS.