isNaN() This method is used to assertain whether or not a number is actually "Not A Number". That is, the value of 'true' will be returned if the expression passed as the parameter doesn't evaluate to a numeric value. syntax: isNaN(expression) EXAMPLE function isNum() { var n = documnet.form1.text1.value; if(isNaN(n) == true) { alert("Your Entry is Not a Number"); } } The example basically shows an alert box if the number evaluates to true, which indicates that the number inputted to the form control isn't a number.