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.^[*]

^[*] These are tricky concepts, and a complete understanding of this chapter requires an understanding of concepts introduced in later chapters of the book. If you are relatively new to programming, you may want to read only the first two sections of this chapter and then move on to Chapters 5, 6, and 7 before returning to finish up the remainder of this chapter.