javascript - 关于JS图片轮播的问题
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
<style>
*{margin:0;padding:0;border:0;}
p{
width:1200px;
height:510px;
overflow: hidden;
position: relative;
}
ul{
width:3600px;
list-style: none;
}
li{
float:left;
}
a{
display:block;width:30px;height:50px;background:#FCEBB6;color:#5E412F;font-size:30px;font-weight:bold;line-height:50px;text-decoration: none;
}
#prev{
position: absolute;
left:0;
top:50%;
}
#next{
position: absolute;
right:0;
top:50%;
text-align: center;
}
</style>
<script>
window.onload = function(){
var prev = document.getElementById('prev');
var next = document.getElementById('next');
var aImg = document.getElementsByTagName('img');
var oUl = document.getElementsByTagName('ul')[0];
var aLi = oUl.getElementsByTagName('li');
var num = 0;
var index =0;
next.onclick = function(){
num = num - 1200; //一张图片1200宽
index++;
if(index >= aImg.length){
index = 0;
num = 0;
oUl.style.marginLeft = num;
}else{
oUl.style.marginLeft = num +'px';
}
}
prev.onclick = function(){
if(index == 0){
//在这里应该怎么写
}
}
</script>
</head>
<body>
<p>
<ul>
<li><img src="1.png" alt=""></li>
<li><img src="2.png" alt=""></li>
<li><img src="3.png" alt=""></li>
</ul>
<a id="prev" href="javascript:;"><</a>
<a id="next" href="javascript:;">></a>
</p>
</body>
</html>把next的功能实现了，但是prev的功能没想出来，怎么当第一张图片的时候点击prev返回图片最后一张？应该怎么判断？
