The Book of JavaScript, 2nd Edition

isNaN() [FF, IE 4]

Returns true if the parameter is not a number, false otherwise. Example:

var zip_code = "none of your business";
if (isNaN(zip_code))
{
    alert("Please provide something that at least looks like a zip code!");
}

Since zip_code contains a string, isNaN() returns true, triggering the alert.