title: JavaScript - What is the opposite of isNaN?
body: <p>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.</p>
  
  <p>So far i have managed to do this for a text field by using this:</p>
  
  <pre><code>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";
    }
  </code></pre>
  
  <p>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:</p>
  
  <pre><code>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";
    }
  </code></pre>
  
  <p>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.</p>
  
  <p>Any help/tips on how to write is not isNaN, or any work arounds would be appreciated.</p>
  
  <p>Please try to keep as simple as poss, as am a complete beginner with web design &amp; JavaScript.</p>
  
  <p>Thanks!</p>