为什么使用小圆点切换几次图片后，图片出现的不对呢，比如点击第一个小圆点却出现了第三张图片，开始点击的几次是正常的
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>左右滑动轮播图</title>
<style type="text/css">
*{ margin: 0; padding: 0; text-decoration: none;}
body { padding: 20px;}
#container { width: 600px; height: 800px; border: 3px solid #333; overflow: hidden; position: relative;margin: 0 auto;}
#list { width: 4200px; height: 800px; position: absolute; z-index: 1;}
#list img { float: left;}
#buttons { position: absolute; height: 10px; width: 100px; z-index: 2; bottom: 20px; left: 250px;}
#buttons span { cursor: pointer; float: left; border: 1px solid #fff; width: 10px; height: 10px; border-radius: 50%; background: #333; margin-right: 5px;}
#buttons .on { background: 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: 380px; background-color: RGBA(0,0,0,.3); color: #fff;}
.arrow:hover { background-color: RGBA(0,0,0,.7);}
#container:hover .arrow { display: block;}
#prev { left: 20px;}
#next { right: 20px;}
</style>
</head>
<body>
<div id="container">
<div id="list" style="left: -600px;">
<img src="img/05.jpeg" alt="1"/>
<img src="img/01.jpeg" alt="1"/>
<img src="img/02.jpeg" alt="2"/>
<img src="img/03.jpeg" alt="3"/>
<img src="img/04.jpeg" alt="4"/>
<img src="img/05.jpeg" alt="5"/>
<img src="img/01.jpeg" 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" class="arrow"><</a>
<a href="javascript:;" id="next" class="arrow">></a>
</div>
<script type="text/javascript">
window.onload = function(){
var container = document.getElementById('container');
var list = document.getElementById('list');
var buttons = document.getElementById('buttons').getElementsByTagName('span');
var prev = document.getElementById('prev');
var next = document.getElementById('next');
var index = 1;
var animated = false;
var interval = 6000;
var timer;
function animate(offset){
var left = parseInt(list.style.left) + offset;
var animated = true;
var time = 300; //位移总时间
var interval = 10; //位移间隔时间
var speed = offset/(time/interval); //每次位移量
function go(){
if((speed < 0 && parseInt(list.style.left) > left) || (speed > 0 && parseInt(list.style.left) < left)){
list.style.left = parseInt(list.style.left) + speed + 'px';
setTimeout(go,interval);
}else{
list.style.left = left + 'px';
if(left > -600){
list.style.left = -3000 + 'px';
}
if(left < -3000){
list.style.left = -600 + 'px';
}
animated = false;
}
}
go();
}
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 play(){
timer=setTimeout(function(){
next.onclick();
play();
},interval)
}
function stop(){
clearTimeout(timer);
}
next.onclick = function(){
if (animated) {
return;
}
if (!animated) {
animate(-600);
}
if(index == 5){
index = 1;
}else{
index += 1;
}
showButton();
console.log(index);
}
prev.onclick = function(){
if (animated) {
return;
}
if (!animated) {
animate(600);
}
if(index == 1){
index = 5;
}else{
index -= 1;
}
showButton();
}
for (var i = 0; i < buttons.length ; i++) {
buttons[i].onclick = function(){
if (animated) {
return;
}
if(this.className == 'on') {
return;
}
var myIndex = parseInt(this.getAttribute('index'));
var offset = -600*(myIndex-index);
if (!animated) {
animate(offset);
}
index = myIndex;
showButton();
}
}
container.onmouseover = stop;
container.onmouseout = play;
play();
}
</script>
</body>
</html>
