It’s ubiquitous at the minute and can be used for development across the entire stack.
Functional programming is a relatively new concept to me and it’s not going to go away anytime soon. It’s bleeding into every major active OO language.
First up were the free browser based tutorials on CodeAcademy. These are good for anyone looking to have the basics of the language spelt out.
Third is reading Expert Javascipt – the opening paragraph sums it up perfectly.