Check the field value Number or not in Javascript

<script type="text/javascript">
numbersonly(myfield, e, dec)
{
   var key;
   var keychar;

   if (window.event)
   { 
     key = window.event.keyCode;
   }
   else if (e)
   { 
     key = e.which;
   }
   else
   { 
    return true;
   }

   keychar = String.fromCharCode(key);
   // control keys
   if ((key==null) || (key==0) || (key==8) || (key==9) || (key==13) || (key==27) )
   {
      return true;
   }
   else if ((("0123456789").indexOf(keychar) > -1))
   { 
      return true; // decimal point jump
   }
   else if (dec && (keychar == "."))
   {
      myfield.form.elements[dec].focus();
      return false;
   }
   else
   {
      return false;
   }
}
</script>