前端JS电商放大镜效果

前端JS电商放大镜效果：
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>26-电商放大镜</title>
    <style type="text/css">
        
    *{
        padding: 0;
        margin: 0;
    }
    #left{
      padding: 0;
    margin: 0;
         400px;
        height: 400px;
        border: 2px solid blue;
        background: url(http://chuantu.biz/t6/17/1503469475x2063891122.jpg) no-repeat;
        float: left;
        cursor: crosshair;
        position: relative;
    box-sizing: border-box;
    }
    #box{
         200px;
        height: 200px;
        background: white;
        opacity: 0.6;
        position: absolute;
        top: 0;
        left: 0;
        display: none;
    box-sizing: border-box;
    }
    #cover{
         400px;
        height: 400px;
        background: red;
        position: absolute;
        left: 0;
        top: 0;
        opacity: 0;
    box-sizing: border-box;
    }
    #right{
         400px;
        height: 400px;
        border: 2px solid black;
        overflow: hidden;
        position: relative;
        display: none;
    box-sizing: border-box;
    }
    #rpic{
        position: absolute;
    }
    </style>

    <script type="text/javascript">
        
    window.onload = function(){
        var left = document.getElementById("left");
        var right = document.getElementById("right");
        var rpic = document.getElementById("rpic");
        var box = document.getElementById("box");
        var cover = document.getElementById("cover");

        // 给左侧加鼠标移动事件
        cover.onmousemove = function(){

            //获得事件对象
            var ev = window.event;
            var mouse_left = ev.offsetX || ev.layerX;
            var mouse_top = ev.offsetY || ev.layerY;
            // document.title = mouse_left + '|' +  mouse_top;

            //计算色块的位置
            var box_left = mouse_left - 100;
            var box_top = mouse_top - 100;

            // 判断是否超出
            if (box_left < 0) {
                box_left = 0;
            }
            if (box_left > 200) {
                box_left = 200;
            }
            if (box_top < 0) {
                box_top = 0;
            }
            if (box_top > 200) {
                box_top = 200;
            }

            // 让色块移动
            box.style.left = box_left + 'px';
            box.style.top = box_top + 'px';

            //计算右侧图片位置
            var rpic_left = box_left*-2;
            var rpic_top = box_top*-2;

            // 让右侧移动
            rpic.style.left = rpic_left + 'px';
            rpic.style.top = rpic_top + 'px';

        }

            //给左侧加鼠标移入事件
            cover.onmouseover = function(){
                // 让左侧色块和右侧隐藏
                box.style.display = 'block';
                right.style.display = 'block';
            }

            // 给左侧加鼠标移出事件
            cover.onmouseout = function(){
                // 让左侧色块和右侧隐藏
                box.style.display = 'none';
                right.style.display = 'none';
            }
    }

    </script>
</head>
<body>
    <div id="left">
        <div id="box"></div>
        <div id="cover"></div>
    </div>
    <div id="right">
        <img src="http://chuantu.biz/t6/17/1503469419x2063891122.jpg" id="rpic">
    </div>
</body>
</html>