The isNaN function in JavaScript determines whether a value is an illegal number (Not-a-Number).
This function returns true if the value is NaN, and false if not.
Examples:
<script> document.write(isNaN(123)+ "<br>"); // false document.write(isNaN(-1.23)+ "<br>"); // false document.write(isNaN(5-2)+ "<br>"); // false document.write(isNaN(0)+ "<br>"); // false document.write(isNaN("Hello")+ "<br>"); // true document.write(isNaN("2005/12/12")+ "<br>"); // true </script>