                        课程
                    
                        /前端开发
                        
                            /JavaScript
                        
                        /焦点图轮播特效

老师！我怎么没有切换的效果。。

<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>query焦点轮播图</title>
<style type="text/css">
*{ margin: 0; padding: 0; text-decoration: none;}
body { padding: 20px;}
#container { width: 600px; height: 400px; border: 3px solid #333; overflow: hidden; position: relative;}
#list { width: 4200px; height: 400px; 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: 180px; 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>
<script type="text/javascript">
window.onload = function(){
var container = document.getElementById('container');
var list = document.getElementById('list');//获取列表
var buttons = document.getElementById('buttons').getElementByTagment('span');//获取按钮
var prev = document.getElementById('prev');//获取箭头
var next = document.getElementById('next');//绑定事件
function animate(offset){
list.style.left = parseInt(list.style.left) + offset + 'px';
}
next.onclick = function(){
animate(-600);//点击时向左移动600像素
}
prev.onclick = function(){
animate(600);//点击时向右移动600像素
}
}
</script>
</head>
<body>

<div id="container">
<div id="list" style="left: -600px;">
<img src="img/5.jpg" alt="1"/>
<img src="img/1.jpg" alt="1"/>
<img src="img/2.jpg" alt="2"/>
<img src="img/3.jpg" alt="3"/>
<img src="img/4.jpg" alt="4"/>
<img src="img/5.jpg" alt="5"/>
<img src="img/1.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" class="arrow"><</a>
<a href="javascript:;" id="next" class="arrow">></a>
</div>

</body>
</html>