请问下面的代码为什么无法实现自动播放？
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>焦点图轮播特效</title>
<style type="text/css">
#container{
width: 680px;
height: 680px;
overflow: hidden;
position: relative;
}
#list{
width: 4760px;
height: 680px;
position: absolute;
z-index: 1;
}
#list img { float: left;}
#buttons{
position: absolute;
left: 300px;
bottom: 20px;
width: 100px;
height: 10px;
z-index: 2;
}
#buttons span{
cursor: pointer;
width: 10px;
height: 10px;
background-color: #333;
border: solid #fff 1px;
border-radius: 50%;
margin-right: 5px;
float: left;
}
#buttons .on{
background-color: orangered;
}
.arrow{
cursor: pointer;
display: none;
line-height: 39px;
text-align: center;
font-size: 36px;
font-weight: bold;
width: 40px;
height: 40px;
position: absolute;
z-index: 2;
top: 320px;
background: rgba(0,0,0,0.3);
color: #fff;
text-decoration: none;
}
.arrow:hover{
background-color: rgba(0,0,0,0.7);
}
#container:hover .arrow{
display: block;
}
#prev{
left: 20px;
}
#next{
right: 20px;
}
</style>
<script type="text/javascript">
window.onload=function(){
var container = document.getElementById('container');
var list=document.getElementById('list');
var prev=document.getElementById('prev');
var next=document.getElementById('next');
var buttons=document.getElementById('buttons').getElementsByTagName('span');
var index=1;
var animated=false;
var timer;
function showButton(){
for (var i = 0; i < buttons.length; i++) {
if(buttons[i].className=='on'){
buttons[i].className="";
break;
}
}
buttons[index-1].className="on"
}
function animate(offset){
animated=true;
var newLeft=parseInt(list.style.left) + offset;
var time=340;
var interval=10;//位移间隔时间
var speed=offset/(time/interval);
var go = function (){
if ( (speed > 0 && parseInt(list.style.left) < newLeft) || (speed < 0 && parseInt(list.style.left) > newLeft)) {
list.style.left = parseInt(list.style.left) + speed + 'px';
setTimeout(go, interval);
}
else{
animated = false;
list.style.left=newLeft+'px';
if (newLeft>-680) {
list.style.left =-3400 + 'px';
}
if (newLeft<-3400) {
list.style.left =-680 + 'px';
}
}
}
go();
}
function play(){
timer = setInterval(function(){
next.onclick();
},2000);
}
function stop(){
clearInterval(timer);
}
prev.onclick=function(){
if (index==1) {
index=5;
}
else{
index-=1;
}
showButton();
if (!animated) {
animate(680);
}
}
next.onclick=function(){
if (index==5) {
index=1;
}
else{
index+=1;
}
showButton();
if (!animated) {
animate(-680);
}
}
for(var i =0 ;i<=buttons.length;i++){
if (this.className=="on") {
return;
}
buttons[i].onclick=function(){
var myindex=parseInt(this.getAttribute('index'));
var offset=-680*(myindex-index);
if (!animated) {
animate(offset);
}
index=myindex;
showButton();
}
}
container.onmouseover = stop;
container.onmouseout = play;
play();
}
</script>
</head>
<body>
<div id="container">
<div id="list" style="left: -680px";
>
<img src="img/235158io8t6qpuv8ev7tto.jpg" alt="1">
<img src="img/235158io8t6qpuv8ev7tto.jpg" alt="1">
<img src="img/235331hadx3yniynfxyl1d.jpg" alt="2">
<img src="img/235423iaeyknz18tdj66zc.jpg" alt="3">
<img src="img/235518ittnnkr0xuruttnx.jpg" alt="4">
<img src="img/235543ewinowim8nqtln7o.jpg" alt="5">
<img src="img/235543ewinowim8nqtln7o.jpg" alt="5">
</div>
<div id="buttons">
<span index="1" class="
on"></span>
<span index="2"></span>
<span index="3"></span>
<span index="4"></span>
<span index="5"></span>
</div>
<a href="javascript:;" id="prev"><</a>
<a href="javascript:;" id="next">></a>
</div>
</body>
</html>
