关于this的使用疑问
<!doctype html>
<html>
<head>
<meta charset="UTF-8">
<title>Untitled Document</title>
<style type = "text/css">
*{margin:0; padding:0; font:normal 12px "微软雅黑"；}
ul{list-style-type: none;}
#tablelist{width:275px;
height:155px;
margin:20px auto;
}
#tablelist #ul1{border-bottom: 2px solid #8B4517;
height:32px;}
#tablelist #ul1 li{display:inline-block;
width:60px;
height:30px;
line-height:30px;
margin-left:3px;
border:1px solid #999;
border-bottom:none;
text-align:center;
}
#tablelist #ul1 li:hover{cursor:pointer;}
//#ul1 li:hover{cursor:pointer;}
#tablelist #ul1 li.active{border-top:2px solid #8B4517;
border-bottom:2px solid #FFF;
}
#tablelist div{border:1px solid #999;
border-top:none;
}
#tablelist div ul li{height:30px;
line-height:30px;
text-indent:8px;
}
.hide{display:none;}
.show{display:block;}
</style>
<script type = "text/javascript">
window.onload = function(){
var oul = document.getElementById("ul1");
var aul = oul.getElementsByTagName("li");
var oDiv = document.getElementById("tablelist");
var aDiv = oDiv.getElementsByTagName("div");
for (var i = 0;i < aul.length;i++){
aul[i].index = i;
aul[i].onmouseover = function(){
for(var n = 0;n < aul.length;n++){
aul[n].className = "";
}
this.className = "active";
for(var j = 0;j < aDiv.length;j++){
aDiv[j].className = "hide";
}
aDiv[this.index].className = "show";
}
}
}
</script>
</head>
<body>
<div id="tablelist">
<ul id="ul1">
<li class="active">房产</li>
<li>家居</li>
<li>二手房</li>
</ul>
<div class= "">
<ul>
<li>275万购昌平邻铁三居 总价20万买一居</li>
<li>200万内购五环三居 140万安家东三环</li>
<li>北京首现零首付楼盘 53万购东5环50平</li>
</ul>
</div>
<div class= "hide">
<ul>
<li>40平出租屋大改造 美少女的混搭小窝</li>
<li>经典清新简欧爱家 90平老房焕发新</li>
<li>新中式的酷色温情 66平撞色活泼家居</li>
</ul>
</div>
<div class= "hide">
<ul>
<li>通州豪华3居260万 二环稀缺2居250w甩</li>
<li>西3环通透2居290万 130万2居限量抢购</li>
<li>黄城根小学学区仅260万 121平70万抛!</li>
</ul>
</div>
</div>
</body>
</html>这段代码中，javascript代码中的一句aul[i].index = i;为什么有存在的必要，如果删掉只有下面的大块div就不显示了，但是我不理解这是为什么啊,求解
