JavaScript实现可控制方向的无缝滚动广告图

<!DOCTYPE html>
<html lang="en">
 <head>
 <meta charset="utf-8">
 <style type = "text/css">
 *{margin: 0; padding: 0;}
  li { list-style: none; }
  .box {
  width: 800px;
  height: 450px;
  margin: 50px auto;
  overflow: hidden; 
  position: relative;
  }
  .box span {
  width: 40px;
  height: 60px;  
  display: block;
  position: absolute;
  top: 225px;
  margin-top: -20px;
  cursor: pointer;
  z-index: 1;
  }
  .box #left {
 background: url('http://cdn.attach.qdfuns.com/notes/pics/201701/14/042311cbd7gbjd7sggkd2b.png') no-repeat;
 left: 0;
    display: none;
  }
  .box #right {
  background: url('http://cdn.attach.qdfuns.com/notes/pics/201701/14/042241w8z4hx4m4pjhyjzs.png') no-repeat;
  right: 0;
   display: none;
  }
  #ad {
  width: 4000px;
  height: 450px;
  position: absolute;
  }
  #ad li {
 float: left;
  }
 </style>
 <script type = "text/javascript">
  window.onload = function(){
  var ad = document.getElementById("ad");
  var lef = document.getElementById("left");
  var rig = document.getElementById("right");
  var timer = null; //管理定时器
 var aspect = true;
  function animate(obj,speed){
   //关闭上一个定时器
   clearInterval(obj.timer);
   //管理定时器
 obj.timer = setInterval(autoAd,30);
   function autoAd(){
   //判断左走或者右走
   if(speed > 0){
    aspect = true;
    if(obj.offsetLeft >= 0){
    obj.style.left = -3200 + 'px';
    }
 }else {
    aspect = false;
    if(obj.offsetLeft <= -3200){
    obj.style.left = 0;
    }
 }
   //匀速动画： 盒子当前的位置 + 步长
   obj.style.left = obj.offsetLeft + speed +"px";
   }
 }
 animate(ad,-5);
 //鼠标划入显示控制按钮并关闭
  ad.parentNode.onmouseover = function(){
   clearInterval(ad.timer);
 lef.style.display = "block";
   rig.style.display = "block";
 };
  //鼠标离开隐藏控制按钮并启动定时器
  ad.parentNode.onmouseout = function(){
   clearInterval(ad.timer);
   clearInterval(timer);
   lef.style.display = "none";
   rig.style.display = "none";
 if(aspect){
 animate(ad,5);
 }else{
 animate(ad,-5);
 }
  };
  ad.onclick = function(event){
   //关闭自动轮播定时器
   clearInterval(ad.timer);
   clearInterval(timer);
   var event = event || window.event;
 if(event.target){
   var target = - parseInt(event.target.alt) * 800;
 }else{
   var target = - parseInt(event.srcElement.alt) * 800;
 }
   timer = setInterval(function(){
   var step = (target - ad.offsetLeft) / 10;
   step = step > 0 ? Math.ceil(step):Math.floor(step);
   ad.style.left = ad.offsetLeft + step + "px";
   if(ad.offsetLeft%800 == 0){
    clearInterval(timer);
 }
 },20)
 }
  //右移动
  lef.onclick = function(){
   clearInterval(timer);
   animate(ad,5);
  };
  //左移动
  rig.onclick = function(){
   clearInterval(timer);
 animate(ad,-5);
 };
  }
 </script>
 </head>
 <body>
 <div class="box"> 
 <ul id="ad">
 <li><img src="http://cdn.attach.qdfuns.com/notes/pics/201701/14/042153lscsitwp7sszb6zs.jpg" alt="0"></li>
 <li><img src="http://cdn.attach.qdfuns.com/notes/pics/201701/14/042152pfbkbfe8vbtvulfu.jpg" alt="1"></li>
 <li><img src="http://cdn.attach.qdfuns.com/notes/pics/201701/14/042153u3jgn0ds43ndd3dz.jpg" alt="2"></li>
 <li><img src="http://cdn.attach.qdfuns.com/notes/pics/201701/14/042152juuohne22z60hbsb.jpg" alt="3"></li>
 <li><img src="http://cdn.attach.qdfuns.com/notes/pics/201701/14/042153lscsitwp7sszb6zs.jpg" alt="4"></li>
 </ul>
 <span id="left"></span>
 <span id="right"></span>
 </div>
 </body>
</html>