实现思路：
需要的事件：
鼠标移入 和 移出 移动 onmouseover onmouseout onmousemove
鼠标移入 ： 显示遮罩层mask和大图显示区
鼠标移出 ： 隐藏遮罩层mask和大图显示区
鼠标移动 ：
1、操作mask的left和top 让mask动起来
2、操作bigImg的left和top 让大图动起来
3、mask移动方向和bigImg
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title></title>
</head>
<style type="text/css">
*{
margin: 0;
padding: 0;
}
#box{
350px;
height: 350px;
border:1px solid #000;
margin: 200px;
position: relative;
}
#mask{
175px;
height: 175px;
background: pink;
opacity: .3;
position: absolute;
left: 0;
top: 0;
cursor: move;
display: none;
}
#big{
400px;
height: 400px;
border: 1px solid #000;
position: absolute;
top : 0;
left : 355px;
overflow: hidden;
display: none;
}
#bigImg{
position: absolute;
top: 0;
left: 0;
}
</style>
<body>
<div id="box">
<div id="small">
<img src="001.jpg" alt="" id="smallImg"/>
<div id="mask"></div>
</div>
<div id="big">
<img src="0001.jpg" alt="" id="bigImg"/>
</div>
</div>
</body>
</html>
<script>
function $id(id){
return document.getElementById(id)
}
//涉及的事件 ： onmouseover onmouseout onmousemove
var box = $id("box"), //整个放大镜容器
small = $id("small"),//小图区
mask = $id("mask"),//遮罩
smallImg = $id("smallImg"),//小图
bigImg = $id("bigImg"), //大图
big = $id("big"); //大图显示区
//第一步 ： 鼠标移入移出到small上 显示和隐藏mask和big
small.onmouseover = function(){
big.style.display = "block";
mask.style.display = "block";
}
small.onmouseout = function(){
big.style.display = "none";
mask.style.display = "none";
}
//第二步 ： 鼠标在small上移动 操作mask跟随鼠标移动
//改变mask的left和top
small.onmousemove = function(e){
var e = e || event;
var x = e.pageX - mask.offsetWidth/2-box.offsetLeft;
var y = e.pageY - mask.offsetHeight/2-box.offsetTop;
//得到mask移动时的最大的left和top
var maxL = small.offsetWidth-mask.offsetWidth;
var maxT = small.offsetHeight - mask.offsetHeight;
//边界处理
/*if( x<0 ){
x = 0;
}else if(x > maxL){
x = maxL;
}*/
x = x < 0 ? 0 : ( x > maxL ? maxL : x);
y = y < 0 ? 0 : ( y > maxT ? maxT : y);
//设置mask的移动
mask.style.left = x + "px";
mask.style.top = y + "px";
//分析大图向上或向左移动的距离
//大图的left/mask的left = 大图宽度/小图宽度 = 大图显示区宽度/mask宽度
/*var bigImgLeft = x*(bigImg.offsetWidth-big.offsetWidth)/(small.offsetWidth-mask.offsetWidth);
var bigImgTop = y*(bigImg.offsetHeight-big.offsetHeight)/(small.offsetHeight-mask.offsetHeight);*/
var bigImgLeft = x*(bigImg.offsetWidth)/(small.offsetWidth);
var bigImgTop = y*(bigImg.offsetHeight)/(small.offsetHeight);
//设置大图的移动
bigImg.style.left = -bigImgLeft + "px";
bigImg.style.top = -bigImgTop + "px";
}
</script>