Javascript 只能輸入數值語法

<script>
function isNumberPressed(){
  var isValidKey = false;
  var intCode = window.event.keyCode;
  if (intCode >= 48 && intCode <= 57) isValidKey = true; //0-9
  if (intCode == 13) isValidKey = true; //13 enter
  if (intCode == 32) isValidKey = true; //32 spacebar
  window.event.returnValue = isValidKey;
}
</script>


<input type=text name=txt1 id=txt1 onkeypress="javascript:isNumberPressed();">