Javascript as a functional language

JavaScript supports first class functions. See Use functional programming techniques to write elegant JavaScript.

Higher Order Javascript is a great way to get familiar with the functional aspects of javascript. It's also a relatively short read in case you want to get your feet wet without diving into a larger book.

I would say that although you can quickly grasp some functional programming concepts with JavaScript, using JavaScript consistently like a functional programming language is not a common practice. At least not obviously common. Most people don't post tutorials that pinpoint how to do functional programming with JavaScript -- the one marxidad pointed out is actually a pretty decent example, but you won't find a lot of that. The functional aspects are not often apparent, just like when people use closures in JavaScript, but are unaware that they are doing it.

The idea that you would pass two functions through as arguments to a third function, and then have the return value be some execution related to the first two functions is an advanced technique that almost always appears only in the core of full-blown libraries like jQuery. Self executing anonymous functions and the like have gained ground, but are still not used consistently. The majority of tutorials often focus instead on JavaScript's OO capabilities, like how to create properties and methods, scope, access control and also how to use the prototype property of constructors. Honestly, if functional programming is what you want, then I would choose a language known strictly for this capability.