js 放大镜效果

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Document</title>
    <link rel="stylesheet" type="text/css" href="放大镜.css">
</head>
<body>
    <div id="box">
        <div id="small_box">
            <img src="../img/small.png">
            <span id="mask"></span>
        </div>
        <div id="big_box">
            <img src="../img/big.png">
        </div>
    </div>
    <script type="text/javascript">
        var box = document.getElementById('box')
        var small_box = box.children[0]
        var mask = small_box.children[1]
        var small_img = small_box.children[0]
        var big_box = box.children[1]
        var big_img = big_box.children[0]

        small_box.onmouseover = function(){
            mask.style.display = 'block'
            big_box.style.display = 'block'
            small_box.onmousemove = function(e){
                e = e || window.event

                // 遮罩层移动的水平垂直距离
                var moveX = e.clientX - small_box.offsetLeft - box.offsetLeft - mask.offsetWidth * 0.5
                var moveY = e.clientY - small_box.offsetTop - box.offsetTop - mask.offsetHeight * 0.5

                // 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
                }

                // 让遮罩层移动起来
                mask.style.left = moveX + 'px'
                mask.style.top = moveY + 'px'

                // 让大图移动起来
                // 公式：moveX / 大图移动的距离 = （small_img宽度 - mask宽度）/ (big_img宽度 - big_box宽度)
                var x = (big_img.offsetWidth - big_box.offsetWidth) / (small_box.offsetWidth - mask.offsetWidth)
                var y = (big_img.offsetHeight - big_box.offsetHeight) / (small_box.offsetHeight - mask.offsetHeight)
                big_img.style.left = -moveX * x + 'px'
                big_img.style.top = - moveY * y + 'px'

            }
            small_box.onmouseout = function(){
                mask.style.display = 'none'
                big_box.style.display = 'none'
            }

        }

    </script>
    
</body>
</html>
效果演示
没有图片，所以随便找了两张张，比例不对，所以效果不怎么好
图片和样式文件在如下这里