用webAPI实现图片放大镜效果

<div class="w">
  <div class="fdj">
   <div class="leftBox" id="_leftBox">
    <!-- 小图 -->
    <img src="img/m.jpg" alt=""/>
    <!-- 小黄盒子 -->
    <div class="tool" id="_tool"></div>
   </div>
   <div class="rightBox" id="_rightBox">
    <img id="_bImg" src="img/b.jpg" alt=""/>
   </div>
  </div>
</div>

* {
 margin:0;
 padding:0;
}

.w {
 width: 1190px;
 margin: 0 auto;

}
.fdj {
 margin-top: 20px;
}

.fdj .leftBox {
 width: 400px;
 height: 400px;
 border: 1px solid #ccc;
 float: left;
 position: relative;
}

.fdj .tool {
 width: 250px;
 height: 250px;
 background:gold;
 opacity:.5;
 filter:alpha(opacity=50);
 position: absolute;
 top:0;
 left: 0;
 cursor: move;
 /* 默认隐藏 */
 display: none;

}
/* 给小黄加上active 就会显示 */
.fdj .tool.active {
 display: block;
}

.fdj .rightBox {
 width: 500px;
 height: 500px;
 border:1px solid #ccc;
 float: left;
 overflow: hidden;
 /* 隐藏 */
 display: none;
 position: relative;
}
/* 加上active表示显示 */
.fdj .rightBox.active {
 display: block;
}

.fdj .rightBox img {
 position: absolute;
}

// 获取元素
var leftBox = document.getElementById('_leftBox')
var tool = document.getElementById('_tool');
var rightBox = document.getElementById('_rightBox');
var bImg = document.getElementById('_bImg');
// 1、鼠标进入显示
  leftBox.onmouseenter = function () {
   tool.style.display = 'block';
   rightBox.style.display = 'block';
  }


// 2、鼠标离开隐藏
leftBox.onmouseleave = function () {
   tool.style.display = 'none';
   rightBox.style.display = 'none';
  }


// 3、鼠标在左盒子移动黄盒子和右图都要移动
leftBox.onmousemove = function (e) {
   // 修改黄盒子和右图left和top
   // 把鼠标的位置给黄盒子、获取鼠标位置
   var x = e.pageX - leftBox.offsetLeft - tool.offsetWidth / 2;
   var y = e.pageY - leftBox.offsetTop - tool.offsetHeight / 2;
   
   // 判断距离
   if (x < 0) {
    x = 0;
   } else if (x > leftBox.offsetWidth - tool.offsetWidth) {
    x = leftBox.offsetWidth - tool.offsetWidth;
   }

 

   if (y < 0) {
    y = 0;
   } else if (y > leftBox.offsetHeight - tool.offsetHeight) {
    y = leftBox.offsetHeight - tool.offsetHeight;
   }
   
   // 设置给黄盒子
   tool.style.left = x + 'px';
   tool.style.top = y + 'px';

   // console.log(x, y);
   // // left和top
   bImg.style.left = -2 * x + 'px';
   bImg.style.top = -2 * y + 'px';