知识点：
1.window.onload网页全部加载完后再执行
2.获取元素 设置属性
3.临界情况判断
运行效果：
点击上一张下一章切换图片
代码：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Title</title>
</head>
<style>
#box{
width: 1200px;
margin: 0 auto;
}
</style>
<body>
<div id="box">
<img id="icon" src="images/阿鲁1.gif" alt="">
<p></p>
<button id="prep">上一张</button>
<button id="next">下一张</button>
</div>
<script>
window.onload = function (ev) {
// 1. 获取标签
var prep = document.getElementById('prep');
var next = document.getElementById('next');
var icon = document.getElementById('icon');
// 2. 点击
var currentIndex = 1, minIndex=1, maxIndex=10;
prep.onclick = function (ev1) {
if (currentIndex === minIndex){
currentIndex = maxIndex;
}else{
currentIndex--;
}
icon.setAttribute('src','images/阿鲁'+ currentIndex +'.gif');
};
next.onclick = function (ev1) {
if (currentIndex === maxIndex){
currentIndex = minIndex;
}else {
currentIndex++;
}
icon.setAttribute('src','images/阿鲁'+ currentIndex +'.gif');
}
}
</script>
</body>
</html>
