demo中不该先调用add()把，打开网页就直接弹出计算结果了。
<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312"
<title>function</title>
<script type="text/javascript">
function add2 (){
sum = 5 + 6;
alert(sum);
}
//add2();
</script>>
</head>
<body>
<form>
<input type="button" value="click" onclick="add2()">
</form>>
</body>
</html>>
