JavaScript - Determines whether a value is an illegal number

The isNaN() function determines whether a value is an illegal number (Not-a-Number).

This function returns true if the value is NaN, and false if not.

Syntax

isNaN(value)

Example

Example

Check whether a number is an illegal number:

<script>

document.write(isNaN(123)+ "<br>");
document.write(isNaN(-1.23)+ "<br>");
document.write(isNaN(5-2)+ "<br>");
document.write(isNaN(0)+ "<br>");
document.write(isNaN("Hello")+ "<br>");
document.write(isNaN("2005/12/12")+ "<br>");

</script>

The output of the code above will be:

false
false
false
false
true
true

Reference:
http://www.w3schools.com/jsref/jsref_isnan.asp