Functional programming is a style or pattern. If a language gives you basic stuff, you can do that pattern.

I learned a bit of haskell a long time ago - mind blown kind of time - then found javascript had closures (erm, kind of a way of turning functions into objects-with-state-that-still-look-like-functions) and functions as first class objects (meaning a function can be passed around exactly like and integer or a string) and so I used it in javascript. Google up "javascript functional" and see what happens. Do some exercises.

Beware there are libraries out there that make simple functional Jscript stuff look entirely unnecessarily complicated.

And when you're ready, pick up this guy's book!

Edit: that may have come across a bit insulting towards the author. All I'm saying is you can learn the functional stuff while you wait for the book to come out. And when you have the functional stuff under your belt, get the book and you'll be one large step better prepared to dive into it - scala has a lot more to offer than just functional programming, and the book will surely cover a lot of ground.