js模拟实现京东详情页图片放大效果

<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;
}
.big img {
 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');
    //鼠标经过
    preview_img.addeventlistener('mouseover',function(){
        mask.style.display = 'block';
        big.style.display = 'block';
    })
    //鼠标移出去
    preview_img.addeventlistener('mouseout',function(){
        mask.style.display = 'none';
        big.style.display = 'none';
    })
    //鼠标移动的时候
    preview_img.addeventlistener('mousemove',function(e){
        //鼠标在盒子内的坐标
        var x = e.pagex - this.offsetleft;
        var y = e.pagey - this.offsettop;
        //遮挡层移动的距离
        var maskx = x - mask.offsetwidth/2;
        var masky = y -mask.offsetheight/2;
        //大图移动距离
        //遮挡层的最大移动距离
        var maskmaxx = preview_img.offsetwidth - mask.offsetwidth;
        var maskmaxy = preview_img.offsetheight - mask.offsetheight;
        //如果x坐标小于0  就让它停留在0 的位置
        if (maskx <= 0) {
            maskx = 0;
        }else if (maskx >= maskmaxx) {
            maskx = maskmaxx;
        }
        //如果y坐标小于0  就让它停留在0 的位置
        if (masky <= 0) {
            masky = 0;
        }else if (masky >= maskmaxy) {
            masky = maskmaxy;
        }
        //遮挡层移动
        mask.style.left = maskx + 'px';
        mask.style.top = masky + 'px';
        
        //大图片最大移动距离
        var bigmaxx = bigimg.offsetwidth - big.offsetwidth;
        var bigmaxy = bigimg.offsetheight - big.offsetheight;
        //大图片的移动距离 x y
        // 大图片的移动距离 = 遮挡层移动距离 * 大图片最大移动距离 / 遮挡层的最大移动距离
        var bigx = maskx * bigmaxx / maskmaxx;
        var bigy = masky * bigmaxy / maskmaxy;
        //大图和小图（鼠标移动）方向相反
        bigimg.style.left = -bigx + 'px';
        bigimg.style.top = -bigy + 'px';
    })
})