JavaScript Cookbook, 3rd Edition

Chapter 6. Functions

Functions are the building blocks that you use to assemble a program out of discrete, reusable code routines. But in JavaScript, that’s only part of the story.

JavaScript functions are also genuine objects—instances of the Function type. They can be assigned to variables and passed around your code. They can be declared in an expression, without a function name, and optionally using a streamlined arrow syntax. You can even wrap one function in another to create a private package that includes the function’s state (called a closure).

Functions are also at the core of JavaScript’s object-oriented support. That’s because custom classes are really just a special type of constructor function (as you’ll see in Chapter 8). Sooner or later, everything in JavaScript comes back to functions.

Passing a Function as an Argument to Another Function

Problem

You’re calling a function that expects you to provide your own function. What’s the best way to pass it?

Solution

Many functions in JavaScript accept, or even require, a function that’s passed as an argument. Some operations ask for a callback function that will be triggered when a task is complete. Others need to use your function to complete a broader task. For example, many methods of the Array object ask you to provide a function for sorting, converting, combining, or selecting data. The array then uses your function multiple times, until it has processed every element.

There are several different ...