例如：

let imgs = {
   small: ["imgA_1.jpg", "imgB_1.jpg", "imgC_1.jpg"],
   middle: ["imgA_2.jpg", "imgB_2.jpg", "imgC_2.jpg"],
   large: ["imgA_3.jpg", "imgB_3.jpg", "imgC_3.jpg"]
};

handleSmall();
handleMiddle();
handleMove();



function handleSmall(){
   // 渲染小圖
   smallImg.innerHTML = imgs.small.map((item, index) => {
      return `<li><a href="#"><img src="./images/${item}" _id="${index}" class="imgLi" /></a></li>`
   }).join("");

   smallImg.addEventListener("mouseover", (e) => {
      if(e.target.nodeName === "IMG"){

         let imgArr = document.getElementsByClassName("imgLi");
         for(let i = 0;i < imgArr.length;i++){
            imgArr[i].style.borderColor = "transparent";
         }
         e.target.style.borderColor = "black";

         let i = e.target.getAttribute("_id");
         middleImg.style.background = `url("./images/${imgs.middle[i]}") no-repeat`;
         largeImg.style.background = `url("./images/${imgs.large[i]}") no-repeat`;
      }
   }, false);
}

function handleMiddle(){
   middleImg.addEventListener("mouseover", () => {
      largeImg.style.display = "block";
      enlarge.style.display = "block";
   }, false)
   middleImg.addEventListener("mouseout", () => {
      largeImg.style.display = "none";
      enlarge.style.display = "none";
   }, false)
}

function handleMove(){
   middleImg.addEventListener("mousemove", (e) => {
      // 鼠標相對於文檔顯示區的坐標
      let mouseX = e.clientX;
      let mouseY = e.clientY;
      // middleImg 相對於文檔顯示區的坐標
      let middleX = middleImg.offsetLeft;
      let middleY = middleImg.offsetTop;

      let moveX = mouseX - middleX - enlarge.offsetWidth / 2;
      let moveY =mouseY -middleY - enlarge.offsetHeight / 2;

      if(moveX <= 0){
         moveX = 0;
      }else if(moveX >= middleImg.clientWidth - enlarge.offsetWidth){
         moveX = middleImg.clientWidth - enlarge.offsetWidth
      }
      if(moveY <= 0){
         moveY = 0;
      }else if(moveY >= middleImg.clientHeight - enlarge.offsetHeight){
         moveY = middleImg.clientHeight - enlarge.offsetHeight
      }

      enlarge.style.left = moveX + "px";
      enlarge.style.top = moveY + "px";

      largeImg.style.backgroundPositionX = -moveX * (800 / 430) + "px";
      largeImg.style.backgroundPositionY = -moveY * (800 / 430) + "px";

   }, false)
}

放大鏡如何用js