Chapter 3: Data Types and Variables

Key Concepts

A variable can be thought of as a container that holds data. It’s called a “variable” because the data it contains—its value—varies depending on your script. For example, you might place the total price of items a customer is buying in a variable, and then add tax to this amount, storing the result back in the variable. The type of a variable describes the nature of the data stored. For example, the type of a variable holding the value 3.14 would be number while the type of a variable holding a sentence would be string. Note that “string” is programming language lingo for a sequence of characters—in other words, some text.

Since you need to have some way to refer to variables, each variable is given an identifier, a name that refers to the container and allows the script to access and manipulate the data it contains. Not surprisingly, a variable’s identifier is often referred to as its name. When scripts are run, the JavaScript interpreter (the facility within the browser that executes JavaScript) needs to allocate space in memory to store a variable’s value. Declaring a variable is the process of telling the interpreter to get ready to store data in a new variable. In JavaScript, variables are declared using the var keyword with the name of the variable you wish to declare. For example, you might write

var firstName;

You can now store data in the variable known by the identifier firstName. Presumably, you’d be storing a string here. We could then assign a value like "Thomas" to the variable. We call the string "Thomas" a literal, which describes any data appearing directly in the source code. The complete example is now

var firstName;
firstName = "Thomas";

The illustration here demonstrates all the terms used so far together.

Although it is good programming practice to declare variables before use, JavaScript allows the implicit declaration of variables by using them on the left-hand side of an assignment. That is, when the interpreter sees that a script would likely stick data into a variable that hasn’t been declared, it automatically allocates space for the variable without the programmer having to use the var keyword. For example, you might just assign a variable, like so:

lastName = "Schneider";

Many programmers use this type of implicit declaration to save time when coding. It’s faster and easier to not bother declaring variables before using them. Unfortunately, it’s also not a good idea. Scripts written without variable declarations are significantly harder to read than those that use explicit declarations. Implicit declaration can also lead to subtle, hard-to-find errors involving variable scope, a topic we’ll discuss later in the chapter. Unless you’re writing a very simple script (less than a dozen lines), always explicitly declare your variables.

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var favNumber;
favNumber = 3;
favNumber = "San Diego";

var favNumber : number;

favNumber = 3;

favNumber = "San Diego";