Functional JavaScript

Chapter 4. Higher-Order Functions

This chapter builds on Chapter 3 by extending the idea that functions are first-class elements. That is, this chapter will explain that functions can not only reside in data structures and pass as data; they can return from functions, too. Discussion of these first “higher-order” functions will comprise the bulk of this chapter.

A higher-order function adheres to a very specific definition:

• It’s first-class (refer back to Chapter 2 if you need a refresher on this topic)

• Takes a function as an argument

• Returns a function as a result

I’ve already shown many functions that take other functions as arguments, but it’s worth exploring this realm more deeply, especially since its dominance is palpable in functional programming style.

Functions That Take Other Functions

You’ve already seen a gaggle of functions that take other functions, the more prominent being _.map, _.reduce, and _.filter. All of these functions adhere to the definition of higher-order. However, simply showing a few uses of each is insufficient for getting a feel for the importance of function-taking functions in functional programming. Therefore, I’ll spend some time talking more about functions that take functions, and tie the practice together with a discussion of closures. Once again, whenever showing code utilizing a closure, I will capitalize the variable name of the captured value. It bears repeating that the capitalization of captured variables is not a recommended practice, but it ...