js 轮播图效果

<div class="pics-ul">
                       <div class="pics-ulleft">
                           <ul id="allImg">
                               <li><img src="img/img01.png"/></li>
                               <li><img src="img/img2.jpg"/></li>
                               <li><img src="img/img1.jpg"/></li>
                               <li><img src="img/img3.jpg"/></li>
                               <li><img src="img/img4.jpg"/></li>
                           </ul>
                           <a name="btn" id="prev" class="prev" style="display: block;"></a>
                           <a name="btn" id="next" class="next" style="display: block;"></a>
                           <ul id="btn" class="pagination">
                               <li class="hover"><a href="#0">1</a></li>
                               <li><a href="#1">2</a></li>
                               <li><a href="#2">3</a></li>
                               <li><a href="#3">4</a></li>
                               <li><a href="#4">5</a></li>
                           </ul>
                       </div>
                       <div class="pics-ulright"><img src="img/imgright.png"/></div>
                   </div>

/*图片轮播*/
var but1 = document.getElementById("prev");
var abtn = document.getElementById("btn").getElementsByTagName("li"); //获取所有的按钮

var lis = document.getElementById('allImg').getElementsByTagName('li');
var but2 = document.getElementById("next");
var index = 0;
var timer = null;
but2.onclick = function() {

    index++;
    if (index > lis.length - 1) {
        index = 0;
    }
    setImg(index);

}
but1.onclick = function() {
    index--;
    if (index < 0) {
        index = lis.length - 1;
    }
    setImg(index);

}

function changeImg() {
    if (index == lis.length - 1) { //当到最后一张图片时
        index = 0
    } else {
        index++; //图片索引发生改变
    }
    setImg(index);
}

function setImg(index) {
    for (j = 0; j < lis.length; j++) {
        lis[j].style.display = "none";
    }

    lis[index].style.display = "block";
    //按钮的样式要与图片对应
    for (var i = 0; i < abtn.length; i++) {
        abtn[i].className = ""
    }
    abtn[index].className = "hover";
}

//自动切换
timer = setInterval(changeImg, 3000);

//按钮
for (var i = 0; i < abtn.length; i++) {
    (function() {
        var p = i
        abtn[p].onclick = function() {
            index = p;
            setImg(index);
        }

    })();

}