js广告图片轮播

<div class="box">
<div class="box1"></div>
<div class="box2" id="box2">
<ul class="list">
<li>1</li>
<li>2</li>
<li>3</li>
<li>4</li>
<li>5</li>
<li>6</li>
<li>7</li>
<li>8</li>
<li>9</li>
<li>10</li>
<li>11</li>
<li>12</li>
</ul>
</div>
</div>
<script type="text/javascript">
window.onload=function(){
var box2=document.getElementById('box2');
var oUl=box2.getElementsByTagName('ul')[0];
var aLi=oUl.getElementsByTagName('li');
var len=aLi.length;
var w=aLi[0].offsetWidth;
var index=0;
var b=true;
var timer=null;
function next(){
b?index++:index--;
(index==0 || index==len-1) && (b=!b);
startMove(-index*w);
}

setInterval(next,2000);
function doMove(iTarget){
var iSpeed=(iTarget-oUl.offsetLeft)/14;
iSpeed = iSpeed > 0 ? Math.ceil(iSpeed) : Math.floor(iSpeed);
oUl.offsetLeft == iTarget ? clearInterval(timer) : oUl.style.left = oUl.offsetLeft + iSpeed + "px";
};
function startMove(iTarget){//alert(1);
clearInterval(timer);
timer=setInterval(function(){
doMove(iTarget);
},30)
};
};
</script>