let dataArr = [{
    small: "./images/imgA_1.jpg",
    middle: "./images/imgA_2.jpg",
    big: "./images/imgA_3.jpg"
}, {
    small: "./images/imgB_1.jpg",
    middle: "./images/imgB_2.jpg",
    big: "./images/imgB_3.jpg"
}, {
    small: "./images/imgC_1.jpg",
    middle: "./images/imgC_2.jpg",
    big: "./images/imgC_3.jpg"
}];

let artEle = document.querySelector("article");
//拼接article的内容
let str = "";

//初始化页面
function renderImg() {
    str += `<section>
                <div class="box">
                    <img class="middleimg" src="${dataArr[0].middle}" alt="${0}">
                </div>
                <div class="bigimg"></div>
            </section>`;
            
    str += `<section>
    <input type="button" value="-">`;

    //添加小图片（并添加下标）
    dataArr.map((item, index) => {
        str += `<img src="${item.small}" alt="${index}">`
    })
    
    str += `<input type="button" value="+">
           </section>`;
    artEle.innerHTML = str;
    //先生成在获取节点
    let bigEle = document.getElementsByClassName("bigimg")[0];
    bigEle.style.backgroundImage = `url('${dataArr[0].big}')`;
}

renderImg();

//获取小图父节点、中图、大图的节点
let bigEle = document.getElementsByClassName("bigimg")[0];
bigEle.style.display = "none";
let middleimg = document.getElementsByClassName("middleimg")[0];
let secEle = document.getElementsByTagName("section");
let boxEle = document.getElementsByClassName("box")[0];
//生成蒙板
function meng() {
    //生成蒙板节点
    let asiEle = document.createElement("aside");
    asiEle.className = "meng";
    asiEle.style.width = (430 / 800) * 430 + "px";
    asiEle.style.height = (430 / 800) * 430 + "px";
    asiEle.style.backgroundImage = `url("./images/bg.png")`;
    asiEle.style.position = "absolute";
    asiEle.style.left = 0;
    asiEle.style.top = 0;
    asiEle.style.display = "none";
    //添加蒙板到节点
    boxEle.appendChild(asiEle);
}
meng();
//点击小图片，中图和大图进行更换
secEle[1].addEventListener("click", e => {
    let event = e || window.event;
    //获取被点击下图的下标
    if (event.target.localName == "img") {
        bigEle.style.backgroundImage = `url(${dataArr[event.target.alt].big})`;
        middleimg.src = dataArr[event.target.alt].middle;
        //点击加号或减号时判断中途的下标
        middleimg.alt = event.target.alt;
    }
    if (event.target.value == "+") {
        //获取当前图片下标
        let index = ++middleimg.alt;
        //判断是不是最后一张，如果是则停止
        index = index >= dataArr.length - 1 ? dataArr.length - 1 : index;
        bigEle.style.backgroundImage = `url(${dataArr[index].big})`;
        middleimg.src = dataArr[index].middle;
    }
    if (event.target.value == "-") {
        //获取当前图片下标
        let index = --middleimg.alt;
        //判断是不是最后一张，如果是则停止
        index = index <= 0 ? 0 : index;
        bigEle.style.backgroundImage = `url(${dataArr[index].big})`;
        middleimg.src = dataArr[index].middle;
    }
});

//蒙板的显示、隐藏、移动、大图的移动
boxEle.addEventListener("mouseover", e => {
    let mengEle = document.getElementsByClassName("meng")[0];
    mengEle.style.display = "block";
    bigEle.style.display = "block";
    boxEle.addEventListener("mousemove", e => {
        let event = e || window.event;
        mengEle.style.left = event.clientX - boxEle.offsetLeft - mengEle.clientWidth / 2 + "px";
        mengEle.style.top = event.clientY - boxEle.offsetTop - mengEle.clientHeight / 2 + "px";
        if (parseInt(mengEle.style.left) <= 0) {
            mengEle.style.left = 0;
        }
        if (parseInt(mengEle.style.top) <= 0) {
            mengEle.style.top = 0;
        }

        if (parseInt(mengEle.style.left) >= boxEle.clientWidth - mengEle.clientWidth) {
            mengEle.style.left = boxEle.clientWidth - mengEle.clientWidth + "px";
        }
        if (parseInt(mengEle.style.top) >= boxEle.clientHeight - mengEle.clientHeight) {
            mengEle.style.top = boxEle.clientHeight - mengEle.clientHeight + "px";
        }

        bigEle.style.backgroundPositionX = -((800 / 430) * parseInt(mengEle.style.left)) + "px";
        bigEle.style.backgroundPositionY = -((800 / 430) * parseInt(mengEle.style.top)) + "px";
    })

    boxEle.addEventListener("mouseout", e => {
        mengEle.style.display = "none";
        bigEle.style.display = "none";
    })

})

<!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>
    <link rel="stylesheet" href="./index.css">
</head>

<body>
    <article>

    </article>
    <input type="button" value="加入购物车">
    <script src="./index.js"></script>
</body>

</html>