<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html><head> <meta http-equiv="Content-Type" content="text/html; charset=gb2312" /> <title>无标题</title><link href="style/l.css" style="text/css" rel="stylesheet"/><script type="text/javascript" src="js/l.js"></script><style type="text/css">*{ margin:0px; padding:0px;}body{ font-family:Verdana, Geneva, sans-serif; font-size:14px;}#nav{ width:600px; height:40px; background-color:#eee; margin:0 auto;}ul{ list-style:none;}ul li{ float:left; line-height:40px; text-align:center; width:100px;}a{ text-decoration:none; color:#000; display:block;}a:hover{ color:#F00; background-color:#666;}ul li ul li{ float:none;background-color:#eee; margin:2px 0px;}ul li ul{ display:none; }</style> <script type="text/javascript"> window.onload = function(){ var nav_item = document.getElementsByClassName("nav_item"); for(var i=0;i<nav_item.length;i++){ nav_item[i].onmouseover = function(){ this.getElementsByTagName('ul')[0].style.display = "block"; } nav_item[i].onmouseout = function(){ this.getElementsByTagName('ul')[0].style.display = "none"; }; } } </script></head><body> <div id="nav"> <ul> <li><a href="#">首页</a></li> <li ><a href="#">符合规范化</a> <ul> <li><a href="#">首页</a></li> <li><a href="#">首页</a></li> <li><a href="#">首页首页首页</a></li> </ul> </li> <li><a href="#">古语我们</a> <ul> <li><a href="#">首页</a></li> <li><a href="#">首页</a></li> <li><a href="#">首页</a></li> <li><a href="#">首页</a></li> </ul> </li> <li><a href="#">首页</a></li> <li><a href="#">古语我们</a></li> </ul> </div></body></html>菜单能正常显示，但是在循环的时候，比如i=0的时候，鼠标放在第一个li标签上时会报错：this.getElementsByTagName('ul')[0] is not defined.怎么解决
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