<!DOCTYPE html><html> <head> <meta charset="UTF-8"> <title></title> <style> ul{ position: relative; list-style: none; } ul li{ width: 100px; height: 50px; background: #CCCCCC; margin: 2px; } div{ width: 200px; height: 200px; background: greenyellow; position: absolute; top: 0px; left: 150px; display: block; } </style> <script> window.onload=function(){ var oUl=document.getElementsByTagName("ul")[0]; var oLi=document.getElementsByTagName("li"); var oDiv=document.getElementsByTagName("div")[0]; for (var i=0;i<oLi.length;i++) { oLi[i].onmousemove=function(){ if (oUl.offsetHeight-oLi[i].offsetTop>200) { oDiv.style.top=oLi[i].offsetTop+"px"; oDiv.style.display="block"; } else{ oDiv.style.top=this.offsetTop+oLi[i].offsetHeight-200+"px"; } } } } </script> </head> <body> <ul> <li>1111</li> <li>2222</li> <li>3333</li> <li>4444</li> <li>5555</li> <li>6666</li> <li>7777</li> <li>8888</li> <li>9999</li> <li>1010</li> <div></div> </ul> </body></html>这个程序运行为什么会报错---Cannot read property 'offsetTop' of undefined"？但是把oLi【i】改成this反而可以运行为什么会这样
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