js滑动验证

<style>
        * {
            margin: 0;
            padding: 0;
            list-style: none;
        }

        #box {
            width: 650px;
            height: 400px;
            margin: 50px auto;
            border: 1px solid red;
            position: relative;
        }

        #content {
            width: 650px;
            height: 350px;
            position: relative;
            background-image: url(1.jpeg);
            background-size: 650px 350px;
        }

        #shadow {
            height: 40px;
            width: 40px;
            position: absolute;
            background-color: aliceblue;
            left: 0;
            top: 0;
        }

        #block {
            height: 40px;
            width: 40px;
            position: absolute;
            left: 0;
            top: 355px;
            background-image: url(1.jpeg);
            background-size: 650px 350px;
        }

        #tip {
            height: 40px;
            width: 40px;
            position: absolute;
            left: 0;
            top: 0;
            background-image: url(1.jpeg);
            background-size: 650px 350px;
        }
    </style>

<div id="box">
        <div id="content">
            <div id="shadow"></div>
            <div id="tip"></div>
        </div>
        <div id="block"></div>
    </div>
    <script>
        var a = document.querySelector('#content');
        var b = document.querySelector('#shadow');
        var c = document.querySelector('#tip');
        var d = document.querySelector('#block');

        var maxTop = a.clientHeight - b.offsetHeight;
        var maxLeft = a.clientWidth - b.offsetWidth;
        setPosition();

        d.onmousedown = function (e) {
            var ev = event || e;
            var startX = ev.x;

            document.onmousemove = function (e) {
                var ev = event || e;
                var endX = ev.x;

                var left = endX - startX;

                if (left <= 0) {
                    left = 0;
                }
                if (left >= maxLeft) {
                    left = maxLeft;
                }

                d.style.left = left + 'px';
                c.style.left = left + 'px';
            }

            document.onmouseup = function () {
                document.onmousemove = null;
                if (c.offsetLeft === b.offsetLeft) {
                    alert('成功');
                    c.style.left = 0;
                    d.style.left = 0;
                    setPosition();
                } else {
                    c.style.left = 0;
                    d.style.left = 0;
                    setPosition();
                }
            }
        }

        function setPosition() {
            var y = Math.round(Math.random() * maxTop);
            var x = Math.round(Math.random() * maxLeft);

            b.style.top = y + 'px';
            b.style.left = x + 'px';

            c.style.top = y + 'px';

            c.style.backgroundPosition = `-${x}px -${y}px`;
            d.style.backgroundPosition = `-${x}px -${y}px`;
        }
        window.ondragstart = function (e) {
            return false;
        }
    </script>