JS实现轮播图（一看就懂逻辑清晰）

轮播图有很多种实现方法，这是其中一种最清晰的方法。思路很清晰，代码很简单，欢迎大佬多指教。

先来看下效果图，嫌麻烦就不用具体图片来实现了，主要是理清思路。（自动轮播，左右按钮切换图片，小圆点切换图片，鼠标移入暂停轮播，鼠标移出继续轮播）

HTML

首先是html内容，布局很简单，一个图片列表，一个点列表，两个按钮。注意data-index使用HTML5中的data-xx属性来嵌入自定义数据（下面JS代码会提到）。记得给默认显示的图片和对应的小圆点加上active类哦。

 <div class="wrap">
        <ul class="list">
            <li class="item active">0</li>
            <li class="item">1</li>
            <li class="item">2</li>
            <li class="item">3</li>
            <li class="item">4</li>
        </ul>
        <ul class="pointList">
            <li class="point active" data-index = 0></li>
            <li class="point" data-index = 1></li>
            <li class="point" data-index = 2></li>
            <li class="point" data-index = 3></li>
            <li class="point" data-index = 4></li>
        </ul>
        <button class="btn" id="leftBtn"> < </button> 
        <button class="btn" id="rightBtn"> > </button>

    </div>

CSS

思路：父容器list相对定位，item绝对定位，实现让所有图片重叠在父容器内。利用z-index来设置图片高度，图片高度最高会显示在顶层上。那么整个容器中，左右切换按钮和小圆点始终要在最顶层，不能被图片覆盖，所以他们的z-index一定要比图片高。active是一个样式，给某张图片绑定这个样式就能在最上层显示。然后就是图片切换的渐变效果，opacity完全不透明到透明，再加个transition动画效果。最后就是cursor给小圆点添加“小手指”，其他就没什么好说的了。

<style>
        .wrap {
            width: 800px;
            height: 400px;
            position: relative;
        }

        .list {
            width: 800px;
            height: 400px;
            position: relative;
            padding-left: 0px;
        }

        .item {
            width: 100%;
            height: 100%;
            list-style: none;
            position: absolute;
            left: 0;
            opacity: 0;
            transition: all .8s;
        }

        .item:nth-child(1) {
            background-color: skyblue;
        }

        .item:nth-child(2) {
            background-color: yellowgreen;
        }

        .item:nth-child(3) {
            background-color: rebeccapurple;
        }

        .item:nth-child(4) {
            background-color: pink;
        }

        .item:nth-child(5) {
            background-color: orange;
        }

        .item.active {
            z-index: 10;
            opacity: 1;
        }

        .btn {
            width: 60px;
            height: 100px;
            z-index: 100;
            top: 150px;
            position: absolute;
        }

        #leftBtn {
            left: 0px;
        }

        #rightBtn {
            right: 0px;
        }

        .pointList {
            list-style: none;
            padding-left: 0px;
            position: absolute;
            right: 20px;
            bottom: 20px;
            z-index: 200;
        }

        .point {
            width: 10px;
            height: 10px;
            background-color: antiquewhite;
            border-radius: 100%;
            float: left;
            margin-right: 8px;
            border-style: solid;
            border-width: 2px;
            border-color: slategray;
            cursor: pointer;  
        }

        .point.active{
            background-color: cadetblue;
        }
    </style>

Javascript

Index可以说是整个代码里面的"核心"，先封装一个清除active的方法，这里面要清除图片的active（显示在最上层），比如切换到下一张图上张图的active就要清除。还有point的active（图片对应小圆点改变样式）。然后goIndex这个方法就是给图片和对应的小圆点同时加上active，左右按钮绑定的方法就不说了。

用getAttribute拿到刚才给li标签绑定的data-index属性，绑定图片index = pointindex，就能实现点击小圆点图片切换了。由于上面goIndex方法早已经绑定好了给图片添加active样式的时候也给小圆点添加的样式了，就可以实现图片切换小圆点跟随变化的效果。

<script>
        var items = document.querySelectorAll(".item");//图片节点
        var points = document.querySelectorAll(".point")//点
        var left = document.getElementById("leftBtn");
        var right = document.getElementById("rightBtn");
        var all = document.querySelector(".wrap")
        var index = 0;
        var time = 0;//定时器跳转参数初始化
        

        //封装一个清除active方法
        var clearActive = function () {
            for (i = 0; i < items.length; i++) {
                items[i].className = 'item';
            }
            for (j = 0; j < points.length; j++) {
                points[j].className = 'point';
            }
        }

        //改变active方法
        var goIndex = function () {
            clearActive();
            items[index].className = 'item active';
            points[index].className = 'point active'
        }
        //左按钮事件
        var goLeft = function () {
            if (index == 0) {
                index = 4;
            } else {
                index--;
            }
            goIndex();
        }

        //右按钮事件
        var goRight = function () {
            if (index < 4) {
                index++;
            } else {
                index = 0;
            }
            goIndex();
        }
        

        //绑定点击事件监听
        left.addEventListener('click', function () {
            goLeft();
            time = 0;//计时器跳转清零
        })

        right.addEventListener('click', function () {
            goRight();
            time = 0;//计时器跳转清零
        })

        for(i = 0;i < points.length;i++){
            points[i].addEventListener('click',function(){
                var pointIndex = this.getAttribute('data-index')
                index = pointIndex;
                goIndex();
                time = 0;//计时器跳转清零
            })
        }
        //计时器轮播效果
        var timer;
        function play(){
         timer = setInterval(() => {
            time ++;
            if(time == 20 ){
                goRight();
                time = 0;
            }    
        },100)
    }
        play();
        //移入清除计时器
        all.onmousemove = function(){
            clearInterval(timer)
        }
       //移出启动计时器
        all.onmouseleave = function(){
            play();
        }
    </script>

总结：这个简单的轮播图实现例子是第一次写最容易理解，逻辑最清晰的一种。下面我把完整的代码块放出来，直接复制粘贴就可以运行。

<!DOCTYPE html>
<html>

<head>
    <style>
        .wrap {
            width: 800px;
            height: 400px;
            position: relative;
        }

        .list {
            width: 800px;
            height: 400px;
            position: relative;
            padding-left: 0px;
        }

        .item {
            width: 100%;
            height: 100%;
            list-style: none;
            position: absolute;
            left: 0;
            opacity: 0;
            transition: all .8s;
        }

        .item:nth-child(1) {
            background-color: skyblue;
        }

        .item:nth-child(2) {
            background-color: yellowgreen;
        }

        .item:nth-child(3) {
            background-color: rebeccapurple;
        }

        .item:nth-child(4) {
            background-color: pink;
        }

        .item:nth-child(5) {
            background-color: orange;
        }

        .item.active {
            z-index: 10;
            opacity: 1;
        }

        .btn {
            width: 60px;
            height: 100px;
            z-index: 100;
            top: 150px;
            position: absolute;
        }

        #leftBtn {
            left: 0px;
        }

        #rightBtn {
            right: 0px;
        }

        .pointList {
            list-style: none;
            padding-left: 0px;
            position: absolute;
            right: 20px;
            bottom: 20px;
            z-index: 200;
        }

        .point {
            width: 10px;
            height: 10px;
            background-color: antiquewhite;
            border-radius: 100%;
            float: left;
            margin-right: 8px;
            border-style: solid;
            border-width: 2px;
            border-color: slategray;
            cursor: pointer;  
        }

        .point.active{
            background-color: cadetblue;
        }
    </style>
</head>

<body>
    <div class="wrap">
        <ul class="list">
            <li class="item active">0</li>
            <li class="item">1</li>
            <li class="item">2</li>
            <li class="item">3</li>
            <li class="item">4</li>
        </ul>
        <ul class="pointList">
            <li class="point active" data-index = 0></li>
            <li class="point" data-index = 1></li>
            <li class="point" data-index = 2></li>
            <li class="point" data-index = 3></li>
            <li class="point" data-index = 4></li>
        </ul>
        <button class="btn" id="leftBtn"> < </button> 
        <button class="btn" id="rightBtn"> > </button>

    </div>
    <script>
        var items = document.querySelectorAll(".item");//图片
        var points = document.querySelectorAll(".point")//点
        var left = document.getElementById("leftBtn");
        var right = document.getElementById("rightBtn");
        var all = document.querySelector(".wrap")
        var index = 0;
        var time = 0;//定时器跳转参数初始化
        

        //清除active方法
        var clearActive = function () {
            for (i = 0; i < items.length; i++) {
                items[i].className = 'item';
            }
            for (j = 0; j < points.length; j++) {
                points[j].className = 'point';
            }
        }

        //改变active方法
        var goIndex = function () {
            clearActive();
            items[index].className = 'item active';
            points[index].className = 'point active'
        }
        //左按钮事件
        var goLeft = function () {
            if (index == 0) {
                index = 4;
            } else {
                index--;
            }
            goIndex();
        }

        //右按钮事件
        var goRight = function () {
            if (index < 4) {
                index++;
            } else {
                index = 0;
            }
            goIndex();
        }
        

        //绑定点击事件监听
        left.addEventListener('click', function () {
            goLeft();
            time = 0;//计时器跳转清零
        })

        right.addEventListener('click', function () {
            goRight();
            time = 0;//计时器跳转清零
        })

        for(i = 0;i < points.length;i++){
            points[i].addEventListener('click',function(){
                var pointIndex = this.getAttribute('data-index')
                index = pointIndex;
                goIndex();
                time = 0;//计时器跳转清零
            })
        }
        //计时器
        var timer;
        function play(){
         timer = setInterval(() => {
            time ++;
            if(time == 20 ){
                goRight();
                time = 0;
            }    
        },100)
    }
        play();
        //移入清除计时器
        all.onmousemove = function(){
            clearInterval(timer)
        }
       //移出启动计时器
        all.onmouseleave = function(){
            play();
        }
    </script>
</body>

</html>

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