html代码：

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <script type="text/javascript" src="测试js.js"></script>
</head>
<body>
    <div id="showpic">
        <img src="./img/p-detail-1.jpg" alt="" id="img" style="width: 100px;height: 100px">
    </div>
    <div class="conpic">
        <button id="previousBut">上一页</button>
        <button id="nextBut">下一页</button>
    </div>
</body>
</html>

js代码：

var cur = 1;
function changPic(){
    var img = document.getElementById('img');
    img.src = "./img/p-detail-" + cur + ".jpg";
}
window.onload = function(){
    document.getElementById('previousBut').addEventListener('click',function(){
        if(cur > 1){
            cur--;
        }
        changPic();
    },false);
    document.getElementById('nextBut').addEventListener('click',function(){
        if(cur < 4){
            cur++;
        }
        changPic();
    },false);
}

 效果：

 

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js上一页和下一页的绑定事件