An excerpt from http://www.sitepoint.com/higher-order-functions-javascript/, by @mdavidgreen
One of the characteristics of JavaScript that makes it well-suited for functional programming is the fact that it can accept higher-order functions. A higher-order function is a function that can take another function as an argument, or that returns a function as a result.
First Class Functions
You may have heard it said that JavaScript treats functions as first-class citizens. What this means is that functions in JavaScript are treated as objects. They have the type Object, they can be assigned as the value of a variable, and they can be passed and returned just like any other reference variable.
This native ability gives JavaScript special powers when it comes to functional programming. Because functions are objects, the language supports a very natural approach to functional programming. In fact, it’s so natural, that I’ll bet you’ve been using it without even thinking about it.
Taking Functions as Arguments
If you’ve done much web-based JavaScript programming or front-end development, you’ve probably come across functions that use a callback. A callback is a function that gets executed at the end of an operation, once all of the other operations of been completed. Usually this callback function is passed in as the last argument in the function. Frequently, it’s defined inline as an anonymous function.
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