Core JavaScript

Variables

A variable holds an item of data that can change or vary. In many programming languages, a variable can change its value but not its type. In JavaScript, the value and even the type of data can change.

The program below begins with a declared variable named "X" that is undefined. This variable is then assigned the values of an integer, a string, and a boolean. Incidentally, the value "undefined" is a legitimate JavaScript value.

Variables.html

<!DOCTYPE html>
<html>
<head>
    <title>XoaX.net's Javascript</title>
</head>
<body>
    <script type="text/javascript" src="Variables.js"></script>
</body>
</html>

Variables.js

// Declare the variable with no value.
var X;
document.writeln("X = "+ X + "<br />");

// Assign it an integer value.
X = 4;
document.writeln("X = "+ X + "<br />");

// Assign it a string value.
X = "XoaX.net";
document.writeln("X = "+ X + "<br />");

// Assign it a boolean value.
X = true;
document.writeln("X = "+ X + "<br />");
 

Output

 
 

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