为什么对话框会询问会出现两次？
<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>prompt</title>
<script type="text/javascript">
var myname=prompt("请输入你的姓名:");
if(myname!=null)
{ alert("你好"+myname); }
else
{ alert("你好 my friend."); }
</script>
</head>
<body>
<input name="button" type="button" onClick="prompt()" value="点击我，对成绩做评价!" />
</body>
</html>
