代码中为何总显示"return numKeyDown"缺少对象

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" >
<head>
    <title>Untitled Page</title>
    <script type="text/javascript">
        function numKeyDown() {
            var k = window.event.keyCode;
            return isValidNum(k);
            alert;
        }
        function isValidNum(k) {
            return ((k == 9) || (k == 13) || (k == 46) || (k == 8) || (k == 189) || (k == 109) ||
        (k == 190) || (k == 110) || (k >= 48 && k <= 57) || (k >= 96 && k <= 105) || (k >= 37 && k <= 40));
        }
        function commafy() {
        var re=/\d(1,3)(?=(\d+)((\.\d+)?)$/g;
        var n1=n.replace(/"(\d+)?)$/,function(s,s1,s2){return s1.replace(re,"$&,")+s2;});
        return n1;
        }
        function numPaste(){
        var text=clipboardData.getData("Text");
        for(var i=0;i<text.length;i++){
          var asc=text.charCodeAt(i);
          if(!isValidNum(asr){
          return false;
          }
        }
       }
    </script>
</head>
<body>
<input id="t" type="text" onpaste="return numPaste()" style="ime-mode:disabled" onkeydown="return numKeyDown() "
onfocus="this.style.textAlign='left':this.value=this.value.replace(/,/g,'')";
onblur="this.style.textAlign='right':this.value=commafy(this.value)" />
</body>
</html>
代码中为何总显示"return numKeyDown"缺少对象 --------------------编程问答-------------------- numKeyDown 是什么  在页面能找到吗？
--------------------编程问答-------------------- 我自己定义的一个函数 --------------------编程问答--------------------   function numKeyDown() {
  var k = window.event.keyCode;
  return isValidNum(k);
  alert;//去掉
  }

补充：.NET技术 ,  ASP.NET