Js.图片切换

<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>无标题文档</title>
<style>
p { margin:0; }
body { text-align:center; }
#box { 400px; height:400px; border:10px solid #ccc; margin:50px auto 0; position:relative; }
a { 40px; height:40px; background:#fff; filter:alpha(opacity:80); opacity:0.8; position:absolute; top:160px; font-size:18px; color:#000; text-align:center; line-height:40px; text-decoration:none; }
a:hover { filter:alpha(opacity:30); opacity:0.3; }
#prev { left:10px; }
#next { right:10px; }
#p1 { 400px; height:30px; line-height:30px; text-align:center; background:#000; color:#fff; font-size:14px; filter:alpha(opacity:80); opacity:0.8; position:absolute; bottom:0; left:0; }
strong { 400px; height:30px; line-height:30px; text-align:center; background:#000; color:#fff; font-size:14px; filter:alpha(opacity:80); opacity:0.8; position:absolute; top:0; left:0; }
#img1 { 400px; height:400px; }
span { position:absolute; 400px; height:30px; line-height:30px; text-align:center; top:-50px; left:0; font-family:'微软雅黑'; }
</style>
<script>
window.onload = function (){
    var oPrev = document.getElementById('prev');
    var oNext = document.getElementById('next');
    var oP = document.getElementById('p1');
    var oStrong = document.getElementById('strong1');
    var oImg = document.getElementById('img1');
    var aBtn = document.getElementsByTagName('input');
    
    var arrUrl = ['img/1.jpg','img/2.jpg','img/3.jpg','img/4.jpg'];
    var arrText = [ '文字一', '文字二', '文字三', '文字四' ];
    var num = 0;
    var onOff = true;
    
    aBtn[0].onclick = function (){ onOff = true; document.getElementsByTagName('span')[0].innerHTML = '图片可从最后一张跳转到第一张循环切换'; };
    aBtn[1].onclick = function (){ onOff = false; document.getElementsByTagName('span')[0].innerHTML = '图片只能到最后一张或只能到第一张切换'; };
    
    // 初始化
    function fnTab(){
        oP.innerHTML = arrText[num];
        oStrong.innerHTML = num+1 + ' / ' + arrText.length;
        oImg.src = arrUrl[num];
    }
    fnTab();
    
    oPrev.onclick = function (){
        num --;
        if( num == -1 ){
            if(onOff){num = arrText.length-1;}else{
                alert('这已经是第一张了，不能再往前了~~');
                num = 0;
            }
            //num = arrText.length-1;
        }
        fnTab();
    };
    oNext.onclick = function (){
        num ++;
        if( num == arrText.length ){
            if(onOff){num = 0;}else{
                alert('已经到最后一张啦~');
                num = arrText.length-1;
            }
        }
        fnTab();
    };
    
};
</script>
</head>

<body>

<input type="button" value="循环切换" />
<input type="button" value="顺序切换" />


<div id="box"><span>图片可从最后一张跳转到第一张循环切换</span>
    <a id="prev" href="javascript:;"><</a>
    <a id="next" href="javascript:;">></a>
  <p id="p1">图片文字加载中……</p>
  <strong id="strong1">图片数量计算中……</strong>
  <img id ="img1" src="img/1.jpg" />
</div>

</body>
</html>
js.切换图片代码