js放大镜效果

 <div class="wrap">
        <!-- 小图与遮罩 -->
        <div id="small">
            <img src="../styles/01.jpg" alt="">
            <div id="mark"></div>
        </div>
        <!-- 等比例放大的大图 -->
        <div id="big">
            <img src="../styles/01.jpg" alt="" id="bigimg">
        </div>
    </div>

 * {
        margin: 0;
        padding: 0;
    }
    .wrap {
        width: 1500px;
        margin: 10px auto;
    }
    #small {
        width: 500px;
        height: 768px;
        float: left;
        position: relative;
    }
    #small img {
        width: 100%;
        height: 100%;
    }
    #big {
        /* background-color: seagreen; */
        width: 768px;
        height: 90vh;
        float: left;
        /* 超出取景框的部分隐藏 */
        overflow: hidden;
        margin-left: 20px;
        position: relative;
        display: none;
    }
    #bigimg {
        /* width: 864px; */
        position: absolute;
        left: 0;
        top: 0;
    }
    #mark {
        width: 250px;
        height: 250px;
        background-color: #fff;
        opacity: .5;
        position: absolute;
        left: 0;
        top: 0;
        /* 鼠标箭头样式 */
        cursor: move;
        display: none;
    }

    // 获取小图和遮罩、大图、大盒子
    var small = document.getElementById("small")
    var mark = document.getElementById("mark")
    var big = document.getElementById("big")
    var bigimg = document.getElementById("bigimg")
    // 在小图区域内获取鼠标移动事件;遮罩跟随鼠标移动
    small.onmousemove = function (e) {
        // 得到遮罩相对于小图的偏移量(鼠标所在坐标-小图相对于body的偏移-遮罩本身宽度或高度的一半)
        var s_left = e.pageX - mark.offsetWidth / 2 - small.offsetLeft
        var s_top = e.pageY - mark.offsetHeight / 2 - small.offsetTop
        // 遮罩仅可以在小图内移动，所以需要计算遮罩偏移量的临界值（相对于小图的值）
        var max_left = small.offsetWidth - mark.offsetWidth;
        var max_top = small.offsetHeight - mark.offsetHeight;
        // 遮罩移动右侧大图也跟随移动（遮罩每移动1px，图片需要向相反对的方向移动n倍的距离）
        var n = big.offsetWidth / mark.offsetWidth
        // 遮罩跟随鼠标移动前判断：遮罩相对于小图的偏移量不能超出范围，超出范围要重新赋值（临界值在上边已经计算完成：max_left和max_top）
        // 判断水平边界
        if (s_left < 0) {
            s_left = 0
        } else if (s_left > max_left) {
            s_left = max_left
        }
        //判断垂直边界
        if (s_top < 0) {
            s_top = 0
        } else if (s_top > max_top) {
            s_top = max_top
        }
        // 给遮罩left和top赋值（动态的？因为e.pageX和e.pageY为变化的量），动起来！
        mark.style.left = s_left + "px";
        mark.style.top = s_top + "px";
        // 计算大图移动的距离
        var levelx = -n * s_left;
        var verticaly = -n * s_top;
        // 让图片动起来
        bigimg.style.left = levelx + "px";
        bigimg.style.top = verticaly + "px";
    }
    // 鼠标移入小图内才会显示遮罩和跟随移动样式，移出小图后消失
    small.onmouseenter = function () {
        mark.style.display = "block"
        big.style.display = "block"
    }
    small.onmouseleave = function () {
        mark.style.display = "none"
        big.style.display = "none"
    }