求教，点击按钮后为什么没有弹出对话框？有什么问题？
<!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>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312" />
<title>无标题文档</title>
<script>
function message(){
alert("hello!");
}
var btn=document.getElementById("btn");
var eventUtil={
add:function(element,type,handler){
if(element.addEventListener){
element.addEventListener(type,handler,false);
}
else if(element.attachEvent){
element.attachEvent("on"+type,handler);
}
else{
element["on"+type]=handler;
}
}
}
eventUtil.add(btn,"click",message);
</script>
</head>
<body>
<input type="button" value="button" id="btn" />
</body>
</html>
