JavaScript实现放大镜详细

 <divclass="wrap">
    <!-- 小图与遮罩 -->
    <div id="small">
      <img src="img/1.jpg" alt="" >
      <div id="mark"></div>
    </div>
    <!-- 等比例放大的大图 -->
    <div id="big">
      <img src="img/2.jpg" alt="" id="bigimg">
    </div>
  </div>

 * {
      margin: 0;
      padding: 0;
    }
    .wrap {
      width: 1500px;
      margin: 100px auto;
    }

    #small {
      width: 432px;
      height: 768px;
      float: left;
      position: relative;
    }

    #big {
      
      width: 768px;
      height: 768px;
      float: left;
      
      overflow: hidden;
      margin-left: 20px;
      position: relative;
      display: none;
    }

    #bigimg {
      
      position: absolute;
      left: 0;
      top: 0;
    }

    #mark {
      width: 220px;
      height: 220px;
      background-color: #fff;
      opacity: .5;
      position: absolute;
      left: 0;
      top: 0;
      
      cursor: move;
      display: none;
    }

// 获取小图和遮罩、大图、大盒子
    var small = document.getElementById("small")
    var mark = document.getElementById("mark")
    var big = document.getElementById("big")
    var bigimg = document.getElementById("bigimg")
    // 在小图区域内获取鼠标移动事件;遮罩跟随鼠标移动
    small.onmousemove = function (e) {
      // 得到遮罩相对于小图的偏移量(鼠标所在坐标-小图相对于body的偏移-遮罩本身宽度或高度的一半)
      var s_left = e.pageX - mark.offsetWidth / 2 - small.offsetLeft
      var s_top = e.pageY - mark.offsetHeight / 2 - small.offsetTop
      // 遮罩仅可以在小图内移动，所以需要计算遮罩偏移量的临界值（相对于小图的值）
      var max_left = small.offsetWidth - mark.offsetWidth;
      var max_top = small.offsetHeight - mark.offsetHeight;
      // 遮罩移动右侧大图也跟随移动（遮罩每移动1px，图片需要向相反对的方向移动n倍的距离）
      var n = big.offsetWidth / mark.offsetWidth
      // 遮罩跟随鼠标移动前判断：遮罩相对于小图的偏移量不能超出范围，超出范围要重新赋值（临界值在上边已经计算完成：max_left和max_top）
      // 判断水平边界
      if (s_left < 0) {
        s_left = 0
      } else if (s_left > max_left) {
        s_left = max_left
      }
      //判断垂直边界
      if (s_top < 0) {
        s_top = 0
      } else if (s_top > max_top) {
        s_top = max_top
      }
      // 给遮罩left和top赋值（动态的？因为e.pageX和e.pageY为变化的量），动起来！
      mark.style.left = s_left + "px";
      mark.style.top = s_top + "px";
      // 计算大图移动的距离
      var levelx = -n * s_left;
      var verticaly = -n * s_top;
      // 让图片动起来
      bigimg.style.left = levelx + "px";
      bigimg.style.top = verticaly + "px";
    }
    // 鼠标移入小图内才会显示遮罩和跟随移动样式，移出小图后消失
    small.onmouseenter = function () {
      mark.style.display = "block"
      big.style.display= "block"
    }
    small.onmouseleave = function () {
      mark.style.display = "none"
      big.style.display= "none"
    }