javascript - 这样写为什么点击a标签，图片不会变化-PHP中文网问答

<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title>Busy - 电商</title>
<style>

body,ul{margin: 0; padding: 0;}
li{list-style: none;}

.banner{

width: 800px;
height: 450px;
margin: 100px auto;
position: relative;
overflow:hidden;

}
.banner_imgs{

width: 800px;
position: absolute;
left: 0; 
top:0px;

}
.banner_imgs li{

float: left;

}
.banner_imgs img{

width: 800px;
height: 100%;
display: block;

}
.number{

width: 100%;
text-align:center;
position: absolute;
left: 0;
bottom: 20px;

}
.number a{

display: inline-block;
width: 20px;
background-color: #fff;
height: 6px;
overflow: hidden;

}
</style>
<script>

window.onload =function(){
 var img=document.getElementById("img");
 var a=document.getElementById("number").getElementsByTagName("a");
 var b=[];
 for(var j=0;j<a.length;j++){
         b.push(j);
         b[j].onclick=function(){
             img.style.top=-450*this+'px';
         }
  }
}

</script>
</head>

<ul id="img" class="banner_imgs">
    <li><a href="javascript:;"><img src="http://img.mukewang.com/53ad2498000174af13660768.jpg" alt=""></a></li>
    <li><a href="javascript:;"><img src="http://img.mukewang.com/53ad25df0001300613660768.jpg" alt=""></a></li>
    <li><a href="javascript:;"><img src="http://img.mukewang.com/53ad26010001ac1113660768.jpg" alt=""></a></li>
    <li><a href="javascript:;"><img src="http://img.mukewang.com/53ad26170001a86013660768.jpg" alt=""></a></li>
    <li><a href="javascript:;"><img src="http://img.mukewang.com/53ad26320001289e13660768.jpg" alt=""></a></li>
</ul>
<p id="number" class="number">
    <a href="javascript:;"></a>
    <a href="javascript:;"></a>
    <a href="javascript:;"></a>
    <a href="javascript:;"></a>
    <a href="javascript:;"></a>
</p>

</p>
</body>
</html>

var img=document.getElementById("img"); var a=document.getElementById("number").getElementsByTagName("a"); var b=[]; for(var j=0;j<a.length;j++){ (function(j){ a[j].onclick= function(){ img.style.top=-450*j+'px'; } }(j)) }