js之间变量传递问题

给做了个示例:
主页面Index.htm<!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>Index.htm</title>
</head>
<script>
function num(n)
{
alert("parent alert!" + n);
}
</script>
<body>
<iframe src="frame.htm"></iframe>
</body>
</html>
Frame页面 frame.htm<!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>frame.htm</title>
</head>
<script>
window.onload=function begin(){var num = 100;alert("frame alert:" + num);parent.num(num);}
</script>
<body>
</body>
</html>