Check number pada input form dengan javascript

Script Javascript :

<script type="text/javascript">
function checkNumber(val)
{
  var strPass = val.value;
  var strLength = strPass.length;
  var lchar = val.value.charAt((strLength) - 1);
  var cCode = CalcKeyCode(lchar);

  /* Check if the keyed in character is a number
      do you want alphabetic UPPERCASE only ?
      or lower case only just check their respective
      codes and replace the 48 and 57 */

  if (cCode < 48 || cCode > 57 )
{
    var myNumber = val.value.substring(0, (strLength) - 1);
    val.value = myNumber;
  }
  return false;
}
</script>

panggil function nya pada script HTML :

<input type="text" name="emplcode" size="10" id="emplcode" onKeyUp="checkNumber(form_name.emplcode);">