Prototype Object isUndefined() Method

Syntax:

Object.isUndefined(obj);

Return Value :

Returns true if obj is of type undefined, false otherwise.

Example:

<html>
<head>
<title>Prototype examples</title>
<script type="text/javascript" 
   src="/javascript/prototype.js">
</script>

<script>

function showResult(){

   alert( "test 1: " + Object.isUndefined() );
   alert( "test 2: " + Object.isUndefined(undefined) );
   alert( "test 3: " + Object.isUndefined(null) );
   alert( "test 4: " + Object.isUndefined(0) );
}

</script>
</head>

<body>

  <p>Click the button to see the result.</p>
  <br />
  <br />
  <input type="button" value="Result" onclick="showResult();"/>

</body>
</html>

To understand it in better way you can Try it yourself.