Take advantage of user-defined variables in JavaScript

Learn how to master variables, the special information-holding areas that are crucial to a programming language

By Gordon Mccomb, JavaWorld.com, 01/01/97

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"I am a string"

or

'I am a string'

Note that double or single quotes are acceptable; unlike some languages, such as Perl, JavaScript makes no distinction between the two forms of quote marks. This working example shows how to place a string into a variable:

MyVar = "This is JavaScript";

Boolean values in variables
There are two Boolean values: true and false. Some programming languages don't have a separate set of Boolean values, and instead use 0 for false, and 1 or -1 (or any other non-zero value) for true. JavaScript can use these numbers to represent true and false but, in addition, reserves the words "true" and "false" to mean Boolean true and false. You can think of the Boolean true and false values as being equivalent to on/off or yes/no. To assign a Boolean value to a variable, provide just the word true or false, without quotes. Here's an example:

MyVar = true;

Objects in variables
Variables can contain objects, including JavaScript objects. There are basically two kinds of object variables:

• Variables that contain built-in browser-related objects -- window, document, navigator, and so on -- actually are references to the original object. They are like copies, but the copies change if the original changes. In some cases, making a change to the object in the variable affects the original JavaScript object.
  
  
• Variables that contain user-defined objects represent the actual object. Make a change to the object in the variable, and you change only that object.

To assign a JavaScript object to a variable, provide the name of the object, as in:

MyVar = navigator;

To assign a new copy of a user-defined object to a variable, use the new statement, and provide the name of the object function:

MyVar = new myObject();

SUBHEAD Variable name limits

When it comes to the names you can give variables, JavaScript offers a great deal of latitude. JavaScript variables can be almost unlimited in length, although for practical reasons you'll probably keep your variable names under 10 or 15 characters. Shorter variable names help JavaScript execute the program faster. Keep the following in mind when naming your variables:

• Variable names should consist of letters only -- without spaces. You can use numbers as long as the name doesn't start with a digit. For example, MyVar1 is acceptable, but 1MyVar is not.
  
  
• Don't use punctuation characters in variable names. Exception: the underscore character ( _ ). That is, the variable My_Var is acceptable, but My*Var is not. Variables can begin with the underscore character.
  
  
• Variable names are case-sensitive. The variable MyVar is a distinctly different variable from myVar, myvar, and other variations.

Understanding JavaScript's "loose" variable data types

Unlike some other programming languages, in JavaScript there is no need to explicitly define the type of variable you want to create. This JavaScript behavior is called "loose data typing," and it differs from C and Java, both of which use strict data typing.

In JavaScript there is no need to differentiate variable types by appending special characters to the end of the variable name, such as MyVar$ for a string variable (or, for that matter, $MyVar for a scalar variable, a la Perl). JavaScript internally decodes the variable type based on its content.