JavaScript: The Definitive Guide, Fourth Edition

Chapter 4. Variables

A variable is a name associated with a value; we say that the variable stores or contains the value. Variables allow you to store and manipulate data in your programs. For example, the following line of JavaScript assigns the value 2 to a variable named i:

i = 2;

And the following line adds 3 to i and assigns the result to a new variable, sum:

var sum = i + 3;

These two lines of code demonstrate just about everything you need to know about variables. However, to fully understand how variables work in JavaScript, you need to master a few more concepts. Unfortunately, these concepts require more than a couple of lines of code to explain! The rest of this chapter explains the typing, declaration, scope, contents, and resolution of variables. It also explores garbage collection and the variable/property duality.^[11]

Variable Typing

An important difference between JavaScript and languages such as Java and C is that JavaScript is untyped. This means, in part, that a JavaScript variable can hold a value of any data type, unlike a Java or C variable, which can hold only the one particular type of data for which it is declared. For example, it is perfectly legal in JavaScript to assign a number to a variable and then later assign a string to that variable:

i = 10;
i = "ten";

In C, C++, Java, or any other strongly typed language, code like this is illegal.

A feature related to JavaScript’s lack of typing is that the language conveniently and automatically converts values ...