Higher-Order Functions

Functions in JavaScript are first-class citizens. This means they can be passed as parameter values to other functions, or even assigned to a variable. This is one of the main characteristics that make JavaScript well-suited to the functional style of programming.

Higher-order functions are functions that operate on other functions. They can do this in one of three ways:

• If the function takes another function as an argument
• If the function returns another function as its result
• In both of these ways

In the previous chapters, we've already seen several higher-order functions, perhaps without you even realizing it. Remember the callback functions that get executed in response to DOM events, or the callbacks in Chapter 10, Accessing External Resources, which were called once the AJAX response was ready? These are all examples of higher-order functions since these functions are parameters that are passed into other functions...