Number validation in javascript

javascript code:
<script type="text/javascript">
function number_validation(s)
{
  var i;
  s = s.toString();
  for (i = 0; i < s.length; i++)
  {
     var c = s.charAt(i);
     if (isNaN(c))
       {
        alert("Given value is not a number");
        return false;
       }
  }
  return true;
}
</script>

html code:
<input type="text" name="number" id="number" onkeyup="number_validation(this.value);" />