Speaking JavaScript

Chapter 16. Variables: Scopes, Environments, and Closures

This chapter first explains how to use variables and then goes into detail on how they work (environments, closures, etc.).

Declaring a Variable

In JavaScript, you declare a variable via a var statement before you use it:

var foo;
foo = 3; // OK, has been declared
bar = 5; // not OK, an undeclared variable

You can also combine a declaration with an assignment, to immediately initialize a variable:

var foo = 3;

The value of an uninitialized variable is undefined:

> var x;
> x
undefined

Background: Static Versus Dynamic

There are two angles from which you can examine the workings of a program:

Statically (or lexically)

You examine the program as it exists in source code, without running it. Given the following code, we can make the static assertion that function g is nested inside function f:

function f() {
    function g() {
    }
}

The adjective lexical is used synonymously with static, because both pertain to the lexicon (the words, the source) of the program.

Dynamically

You examine what happens while executing the program (“at runtime”). Given the following code:

function g() {
}
function f() {
    g();
}

when we call f(), it calls g(). During runtime, g being called by f represents a dynamic relationship.

Background: The Scope of a Variable

For the rest of this chapter, you should understand the following concepts:

The scope of a variable

The scope of a variable are the locations where it is accessible. For example:

function foo() {
    var