for里面可以不添加 break跳出循环，buttons也可以不加index自定义属性
<div id="buttons">
<span class="on"></span>
<span ></span>
<span ></span>
<span ></span>
<span ></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=0;
function showButton(){
for(i=0;i<buttons.length;i++){
if(buttons[i].className=="on"){
buttons[i].className="";
}
}
buttons[index].className="on"
}
