当页面离开时，判断页面的内容是否修改
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当页面离开时，判断页面的内容是否修改，如果修改，则提示是否保存，否则直接离开。
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<!DOCTYPE html public "-//w3c//dtd html 4.0 transitional//EN"><html> <head> <title> new document </title> <meta http-equiv="Content-Type" content="text/html;charset=GBK"> <meta http-equiv="pragma" content="no-cache"> <meta http-equiv="cache-control" content="no-store"> <meta http-equiv="expires" content="0"> <script language="javascript"src="jquery.js"></script><script language="javascript" src="jquery_validate/jquery.metadata.js"></script><script language="javascript" src="jquery_validate/jquery.validate.js"></script> </head> <script language="JavaScript">//绑定beforeunload事件window.onbeforeunload=function(){ var b=$("#aaaa").val(); var f=""; if(b!=""){ f="数据没有保存"; return f; }else{ }} </script> <body > <h1>bbbbbbbbbbbbbbbbbbb </h1><form method=post action=""> <input type="text" name="" id="aaaa" ></form><a href="Noname1.html">Noname1.html</a> </body></html>?