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>