JavaScript has a built-in function called isNAN that can be used to check if a value is numeric or not.

The isNan function returns true if the value is not numeric, and false if it is numeric:

function chkDigits(obj){   if (isNaN(document.getElementById(obj).value)==true)   {    alert("Please enter digits");    document.getElementById(obj).focus();    document.getElementById(obj).select();    return false;  }}

Here’s how to call it: