Unquoted literal constant NaN is a special value representing Not-a-Number. Since NaN always compares unequal to any number, including NaN, it is usually used to indicate an error condition for a function that should return a valid number.
Example
You can try to run the following example to learn how to use NaN −
Live Demo
<html>
<head>
<script>
<!--
function showValue() {
var dayOfMonth = 50;
if (dayOfMonth < 1 || dayOfMonth > 31) {
dayOfMonth = Number.NaN
alert("Day of Month must be between 1 and 31.")
}
Document.write("Value of dayOfMonth : " + dayOfMonth );
}
//-->
</script>
</head>
<body>
<p>Click the following to see the result:</p>
<form>
<input type="button" value="Click Me" onclick="showValue();" />
</form>
</body>
</html>