JavaScript: how to know if a variable is defined ?

Here’s a sample that shows how to use the typeof operator to determine a variable’s type.

<html>
<head>

<script language="javascript">
function test()
{

var a;
alert("The next pop-up displays the sample code,\n the ones following that display the result");
alert(test);
a="hello";
alert("typeof(a) = " + typeof(a));
alert("typeof(b) = " + typeof(b));

}
</script>

</head>

<body background="images/bg_light_gc.gif">

<input type=button onclick='test()' value="Test">
</body>
</html>

Paste the code into an HTML file, open the file using a browser, and click the button to see the result. The first thing the code does is show you the code for the test function, it then shows the types of variables a and b - only variable a is declared and defined.

Unfortunately you cannot determine if a variable has simply been declared using the same approach because all JavaScript variables start life as undefined objects, even if they have been declared. JavaScript variables work like this because declaring variables is completely optional in JavaScript, meaning that you can introduce a new variable at any time and have JavaScript use it right away.

Essam Ahmed
___________________________________________________
Author of JScript .NET Programming - Now Avaialble!
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