来写一个轮播图

轮播图一直都是JS的经典实现:
有以下要素:
1.html和css布局的考察浮动, 定位等
2.Dom操作
3.定时器使用与清除
4.递归函数的使用
下面是源代码
<!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>
    <style>
        * {
            margin: 0;
            padding: 0;
        }

        .container {
            height: 300px;
            width: 600px;
            background-color: #ccc;
            margin: 100px auto;
            position: relative;
            overflow: hidden;
        }

        .picture-list {
            height: 300px;
            width: 3000px;
            list-style: none;
            position: absolute
        }

        .picture-list li {
            height: 300px;
            width: 600px;
            float: left;
        }

        img {
            height: 300px;
            width: 600px;
        }

        .left,
        .right {
            display: inline-block;
            height: 30px;
            width: 20px;
            background-color: #666;
            text-align: center;
            opacity: 0.5;
            line-height: 30px;
            position: absolute;
        }

        .left {
            top: 135px;
        }

        .right {
            top: 135px;
            right: 0px;
        }

        .point {
            height: 15px;
            width: 100px;
            position: absolute;
            left: 50%;
            margin-left: -50px;
            bottom: 10px;
        }

        .point span {
            display: inline-block;
            height: 15px;
            width: 15px;
            border: 3px solid #000;
            box-sizing: border-box;
            border-radius: 50%;
        }

        .selected {
            background-color: #f40;
        }
    </style>
</head>

<body>
    <div class="container" id="container">
        <ul class="picture-list" id="list" style="left: -600px">
            <li>
                <a href="">
                    <img src="./images/3rd.jpg" alt="">
                </a>
            </li>
            <li>
                <a href="">
                    <img src="./images/1st.jpg" alt="">
                </a>
            </li>
            <li>
                <a href="">
                    <img src="./images/2nd.jpg" alt="">
                </a>
            </li>
            <li>
                <a href="">
                    <img src="./images/3rd.jpg" alt="">
                </a>
            </li>
            <li>
                <a href="">
                    <img src="./images/1st.jpg" alt="">
                </a>
            </li>
        </ul>

        <span class="left" id="prev">
            <</span>
                <span class="right" id="next">></span>

                <div class="point" id="buttons">
                    <span class="selected" index="1"></span>
                    <span class="" index="2"></span>
                    <span class="" index="3"></span>
                </div>
    </div>
    <script>
        function $(ele) {
            return document.getElementById(ele);
        }
        var container = $('container'),
            list = $('list'),
            buttons = $('buttons').getElementsByTagName('span'),
            prev = $('prev'),
            next = $('next'),
            index = 1;
        function showButton() {
            for (let i = 0; i <= buttons.length; i++) {
                if (buttons[i].className == 'selected') {
                    buttons[i].className = '';
                    break;
                }
            }

            buttons[index - 1].className = 'selected';
        }

        function prevNext(offset) {
            var newLeft = parseInt(list.style.left) + offset;
                time = 300,
                interval = 10,
                speed = offset / (time / interval);

            function go() {
                if (speed < 0 && parseInt(list.style.left) > newLeft || speed > 0 && parseInt(list.style.left) < newLeft) {
                    list.style.left = parseInt(list.style.left) + speed + 'px';
                    setTimeout(go, interval);
                } else {
                    list.style.left = newLeft + 'px';
                    console.log(1)
                    if (newLeft > -600) {
                        list.style.left = -1800 + 'px';
                    } else if (newLeft < -1800) {
                        list.style.left = -600 + 'px';
                    }
                }
            }
            go();
        }
        function autoPlay() {
            timer = setInterval(function () {
                next.onclick();
            }, 3000);
        }
        function stop() {
            clearInterval(timer);
        }
        container.onmouseover = stop;
        container.onmouseout = autoPlay;
        autoPlay();
        prev.onclick = function () {
            index -= 1;
            if (index < 1) {
                index = 3;
            }
            showButton(index);
            prevNext(600);
        }
        next.onclick = function () {
            index += 1;
            if (index > 3) {
                index = 1;
            }
            showButton(index);
            prevNext(-600);
        }
        for (let i = 0; i <= buttons.length - 1; i++) {

            buttons[i].onclick = function () {
                if (this.className == 'selected') {
                    return;
                }
                var myIndex = parseInt(buttons[i].getAttribute('index'));
                var offSet = -600 * (myIndex - index);
                index = myIndex;
                prevNext(offSet);
                showButton();
            }
        }
    </script>
</body>
</html>
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