放大镜效果

用原生js，实现放大镜效果

css

	<style>
		*{
			margin:0;
			padding:0;
		}
		#box{
			350px;
			height:350px;
			border:1px solid #000;
			margin:100px;
			position:relative;
		}
		#big{
			400px;
			height:400px;
			border:1px solid #000;
			position:absolute;
			top : 0;
			left : 360px;
			overflow:hidden;
			display:none;
		}
		#mask{
			175px;
			height:175px;
			background:pink;
			opacity:0.3;
			position:absolute;
			top:0;
			left:0;
			cursor:move;
			display:none
		}
		#small{
			position:relative;
		}
		#bigImg{
			position:absolute;
			left:0;
			top:0;
		}
	</style>

　　html

<div id="box">
	<div id="small">
		<img src="images/iphone.jpg" alt="" id="smallImg"/>
		<div id="mask"></div>
	</div>
	<div id="big">
		<img src="images/iphone_big.jpg" alt="" id="bigImg"/>
	</div>
</div>

　　js

<script>
	var small = document.getElementById("small");
	var smallImg = document.getElementById("smallImg");
	var mask = document.getElementById("mask");
	var big = document.getElementById("big");
	var bigImg = document.getElementById("bigImg");
	var box = document.getElementById("box");
	small.onmouseover = function(){
		mask.style.display = "block";
		big.style.display = "block";
	}
	small.onmouseout = function(){
		mask.style.display = "none";
		big.style.display = "none";
	}
	small.onmousemove = function(e){
		var e = e || event;
		var x = e.pageX - box.offsetLeft - mask.offsetWidth / 2 ;
		var y = e.pageY - box.offsetTop - mask.offsetHeight / 2 ;	
		var maxL = box.offsetWidth - mask.offsetWidth;
		var maxT = box.offsetHeight - mask.offsetHeight;
		x = x < 0 ? 0 : x > maxL ? maxL : x;
		y = y < 0 ? 0 : y > maxT ? maxT : y;
		mask.style.left = x + "px";
		mask.style.top = y + "px";
		
		var bigL = x * (bigImg.offsetWidth - big.offsetWidth) / (box.offsetWidth - mask.offsetWidth);
		var bigT = y * (bigImg.offsetHeight - big.offsetHeight) / (box.offsetHeight - mask.offsetHeight);
		bigImg.style.left = - bigL + "px";
		bigImg.style.top = - bigT + "px";
	}
</script>