js simulation to achieve the effect of enlarging the picture on the Jingdong details page

<div class="preview_img">
 <img src="upload/s3.png" alt="">
 <div class="mask"></div>
 <div class="big">
  <img src="upload/big.jpg" alt="" class="bigImg">
 </div>
</div>

.preview_img {
 position: relative;
 height: 398px;
 border: 1px solid #ccc;
}
.mask {
 display: none;
 position: absolute;
 width: 300px;
 height: 300px;
 top: 0;
 left: 0;
 background: #FEFE4F;
 opacity: .5;
 border: 1px solid #ccc;
 cursor: move;
}
.big {
 display: none;
 position: absolute;
 width: 550px;
 height: 550px;
 top: 0;
 left: 410px;
 z-index: 999;
 border: 1px solid #ccc;
 overflow: hidden;
}
.bigimg {
 position: absolute;
 left: 0;
 top: 0;
}

window.addEventListener('load',function(){
    var preview_img = document.querySelector('.preview_img');
    var mask = this.document.querySelector('.mask');
    var big = this.document.querySelector('.big');
    var bigImg = this.document.querySelector('.bigImg');
    //Mouse over preview_img.addEventListener('mouseover',function(){
        mask.style.display = 'block';
        big.style.display = 'block';
    })
    //Mouse out preview_img.addEventListener('mouseout',function(){
        mask.style.display = 'none';
        big.style.display = 'none';
    })
    //When the mouse moves preview_img.addEventListener('mousemove',function(e){
        //The coordinates of the mouse in the box var x = e.pageX - this.offsetLeft;
        var y = e.pageY - this.offsetTop;
        //The distance the occlusion layer moves var maskX = x - mask.offsetWidth/2;
        var maskY = y -mask.offsetHeight/2;
        //Large image moving distance //Maximum moving distance of occlusion layer var maskMaxX = preview_img.offsetWidth - mask.offsetWidth;
        var maskMaxY = preview_img.offsetHeight - mask.offsetHeight;
        //If the X coordinate is less than 0, let it stay at 0 if (maskX <= 0) {
            maskX = 0;
        }else if (maskX >= maskMaxX) {
            maskX = maskMaxX;
        }
        //If the Y coordinate is less than 0, let it stay at 0 if (maskY <= 0) {
            maskY = 0;
        }else if (maskY >= maskMaxY) {
            maskY = maskMaxY;
        }
        //Occlusion layer movement mask.style.left = maskX + 'px';
        mask.style.top = maskY + 'px';
        
        //The maximum moving distance of the big picture var bigMaxX = bigImg.offsetWidth - big.offsetWidth;
        var bigMaxY = bigImg.offsetHeight - big.offsetHeight;
        //The moving distance of the large image XY
        // Moving distance of large image = Moving distance of occlusion layer * Maximum moving distance of large image / Maximum moving distance of occlusion layer var bigX = maskX * bigMaxX / maskMaxX;
        var bigY = maskY * bigMaxY / maskMaxY;
        //The big picture and the small picture (mouse movement) are in opposite directions bigImg.style.left = -bigX + 'px';
        bigImg.style.top = -bigY + 'px';
    })
})