如何判断窗口已打开或已关闭？

用ASP编写一短消息功能，对有新消息时会自动弹出窗口，如果已有该窗口则不弹出！
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 1<script>   
 2var newwin   
 3function makenewwin() {   
 4newwin=window.open("","vv","height=300,width=200")   
 5newwin.document.writeln("ok")   
 6newwin.document.close()   
 7}   
 8function check() {   
 9if (newwin&&newwin.open&&!newwin.closed){   
10alert("开的")   
11}   
12else{   
13alert("关的")   
14}   
15}   
16</script>

1<input onclick="makenewwin()" type="button" value="open"/>

1<input onclick="check()" type="button" value="检查"/>

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