JavaScript - What is the opposite of isNaN?

Question

I am very new to JavaScript, and trying to run a simple script which gives my field a red border if it is incorrectly completed (i.e. if the entry is not a number), but then turns the border colour green if the field is then correctly completed, but the form cannot be submitted because a different field has been filled out incorrectly.

So far i have managed to do this for a text field by using this:

function validateForm()
{
var x=document.forms["myForm"]["FN"].value;
if (x==null || x=="")
  {
  alert("First name must be completed");
  document.forms["myForm"]["FN"].style.border="1px solid red";
  return false;
  }
  else (x!==null || x!=="")
  {
  document.forms["myForm"]["FN"].style.border="1px solid green";
  }

but I am now trying to do the same thing but limit the field to only accept numbers. What I have so far is this:

var x=document.forms["myForm"]["mobile"].value;
if (isNaN(x) || x==null || x=="")
  {
  alert("Contact number must be completed with numbers only");
  document.forms["myForm"]["mobile"].style.border="1px solid red";
  return false;
  }
  else (x!==null || x!=="" || // need to write here is not isNaN !isNaN//)
  {
  document.forms["myForm"]["SN"].style.border="1px solid green";
  }

This restricts the fie;ld to only allowing numbers, but once the form box border turns red, it will not turn to green once the validation has been met.

Any help/tips on how to write is not isNaN, or any work arounds would be appreciated.

Please try to keep as simple as poss, as am a complete beginner with web design & JavaScript.

Thanks!

Answer 1

The literal opposite of isNaN(num) is !isNaN(num). However, this won't always verify that something is numeric. To check if something is numeric, use jQuery's $.isNumeric(num), or this

function isNumeric(num) {
  return !isNaN(parseFloat(num)) && isFinite(num);
}