Speaking JavaScript

Chapter 15. Functions

Functions are values that can be called. One way of defining a function is called a function declaration. For example, the following code defines the function id that has a single parameter, x:

function id(x) {
    return x;
}

The return statement returns a value from id. You can call a function by mentioning its name, followed by arguments in parentheses:

> id('hello')
'hello'

If you don’t return anything from a function, undefined is returned (implicitly):

> function f() { }
> f()
undefined

This section showed just one way of defining and one way of calling a function. Others are described later.

The Three Roles of Functions in JavaScript

Once you have defined a function as just shown, it can play several roles:

Nonmethod function (“normal function”)

You can call a function directly. Then it works as a normal function. Here’s an example invocation:

id('hello')

By convention, the names of normal functions start with lowercase letters.

Constructor

You can invoke a function via the new operator. Then it becomes a constructor, a factory for objects. Here’s an example invocation:

new Date()

By convention, the names of constructors start with uppercase letters.

Method

You can store a function in a property of an object, which turns it into a method that you can invoke via that object. Here’s an example invocation:

obj.method()

By convention, the names of methods start with lowercase letters.

Nonmethod functions are explained in this chapter; constructors and methods are explained ...