Js 实现图片放大镜``````

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>放大镜</title>
    <style>
        * {
            padding: 0;
            margin: 0;
        }
        .box {
            width: 430px;
            height: 430px;
            border: 1px solid #dddddd;
            position: relative;
            margin: 50px;
        }
        .small_box {
            width: 430px;
            height: 430px;
            position: relative;
            
        }
        .small_box img {
           width: 430px;
           height: 430px;
        }
        .small_box #mask {
            position: absolute;
            left: 0;
            top: 0;
            width: 210px;
            height: 210px;
            background: rgba(255, 0, 0, 0.3);
            display: none;
        }
        .big_box {
            position: absolute;
            left: 440px;
            top: 0;
            width: 430px;
            height: 430px;
            border: 1px solid #dddddd;
            overflow: hidden;
            display: none;
        }
        .big_box img {
            position: absolute;
            z-index: 5;
        }
    </style>
</head>
<body>
    <div class="box" id="box">
        <div class="small_box" id="small_box">
            <img src="https://scpic.chinaz.net/files/pic/pic9/202011/apic29270.jpg" alt="">
            <span id="mask"></span>
        </div>
        <div class="big_box" id="big_box">
            <img src="https://scpic.chinaz.net/files/pic/pic9/202011/apic29270.jpg" alt="">
        </div>
    </div>
    <script>
        window.onload = function() {
            // 1.获取需要的标签
            var box = document.getElementById('box');
            var small_box = box.children[0]; //小盒子
            var big_box = box.children[1];
            var small_img = small_box.children[0];
            var mask = document.getElementById('mask') //遮罩层
            var big_img = big_box.children[0];

            // 2.监听鼠标在小盒子上的移动
            small_box.onmouseover = function() { //鼠标进入
                // 2.1 鼠标移入时让盒子显示出来
                mask.style.display = 'block';
                big_box.style.display = 'block';
                // 2.2 鼠标移动时
                small_box.onmousemove = function(e) {
                    e = e || window.event;
                    // console.log(e.clientX);
                    // 求出当前鼠标坐标,然后求出遮罩层移动的距离
                    // console.log(small_img.offsetLeft);
                    // console.log(mask.offsetWidth);
                    console.log(big_img.offsetWidth);
                    var moveX = e.clientX - small_box.offsetLeft - mask.offsetWidth*0.5;
                    var moveY = e.clientY - small_box.offsetTop - mask.offsetHeight*0.5;

                    // 2.3边界处理
                    if (moveX < 0) {
                        moveX = 0;
                    } else if(moveX >= small_box.offsetWidth - mask.offsetWidth) {
                        moveX = small_box.offsetWidth - mask.offsetWidth;
                    }
                    
                    if (moveY < 0) {
                        moveY = 0;
                    } else if (moveY >= small_box.offsetHeight - mask.offsetHeight) {
                        moveY = small_box.offsetHeight - mask.offsetHeight;
                    }
                    // 遮罩层移动
                    mask.style.left = moveX + 'px';
                    mask.style.top = moveY + 'px';

                    // 让大图移动起来
                    // 公式： moveX / 大图的移动距离 === (small.box宽度 - mask宽度) / (big_img图片的宽度 - big_box宽度)
                    var x = moveX / (small_box.offsetWidth - mask.offsetWidth);
                    var y = moveY / (big_img.offsetWidth - big_box.offsetWidth);
                    console.log("x:"+ x + "y:" + y);

                    big_img.style.left = -x * (big_img.offsetWidth - big_box.offsetWidth) + 'px';
                    big_img.style.top = -y * (big_img.offsetHeight - big_box.offsetHeight) + 'px';

                }
            }
            small_box.onmouseout = function () {
                mask.style.display = 'none';
                big_box.style.display = 'none';
            } 
        }
        
    </script>
</body>
</html>