The undefined data type

The undefined type is generally assigned to those variables that are declared but not yet initialized (i.e. var text;).

Keep in mind that if a variable is not declared, the script will stop causing an error.

Undeclared identifiers are not the same as undefined ones.

Following examples shows the difference between them:

var text; // undefined

// var zipcode; //left out due comments

alert (test); //undefined

alert (zipcode); // cause an error!

However the operator typeof may be applied to undeclared identifiers and it will, confusingly, return undefined as the type.

Example

The undefined data type example:

<!DOCTYPE html>
<html>
<head>
<title>JS tutorial</title>
<style type="text/css">
body {
	margin-top:20px;
	background-color:#ccccec;
}
</style>
<script type="text/javascript">
	var text = "String";
	// var numb = 20;
	document.write("1. The variable <i>text</i> is a " + typeof(text) + " (var text = 'String')<br />");
	document.write("2. The variable <i>numb</i> is a " + typeof(numb) + " (//var numb = 20)<br /><br />");
	document.write("In example above, the variable <i>numb</i> is actually not declared but the <i>typeof()</i> operator returned <i>undefined</i> instead.");
</script>	

</head>
<body>

</body>
</html>

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The undefined data type

Example

The undefined data type example:

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