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                        /JavaScript進(jìn)階篇

關(guān)于this的使用疑問

<!doctype?html>
<html>
<head>
<meta?charset="UTF-8">
<title>Untitled?Document</title>
<style?type?=?"text/css">
????*{margin:0;?padding:0;?font:normal?12px?"微軟雅黑"；}
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">房產(chǎn)</li>
????????<li>家居</li>
????????<li>二手房</li>
????</ul>
????
????<div?class=?"">
????<ul>
????????????<li>275萬購昌平鄰鐵三居?總價20萬買一居</li>
????????????<li>200萬內(nèi)購五環(huán)三居?140萬安家東三環(huán)</li>
????????????<li>北京首現(xiàn)零首付樓盤?53萬購東5環(huán)50平</li>
????????</ul>
????</div>
????
????<div?class=?"hide">
????<ul>
????????????<li>40平出租屋大改造?美少女的混搭小窩</li>
????????????<li>經(jīng)典清新簡歐愛家?90平老房煥發(fā)新</li>
????????????<li>新中式的酷色溫情?66平撞色活潑家居</li>
????????</ul>
????</div>
????<div?class=?"hide">
????<ul>
????????????<li>通州豪華3居260萬?二環(huán)稀缺2居250w甩</li>
????????????<li>西3環(huán)通透2居290萬?130萬2居限量搶購</li>
????????????<li>黃城根小學(xué)學(xué)區(qū)僅260萬?121平70萬拋!</li>
????????</ul>
????</div>
</div>
</body>
</html>

這段代碼中，javascript代碼中的一句aul[i].index = i;為什么有存在的必要，如果刪掉只有下面的大塊div就不顯示了，但是我不理解這是為什么啊,求解