元素跟随鼠标在指定区域运动通常是用在商城图片的放大镜中，下面是完整的Demo：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>鼠标区域</title>
<style type="text/css">
*{margin:0;padding:0;}
.area{
width:400px;
height:400px;
margin:30px auto;
background: #ccc;
overflow: hidden;
position: relative;
}
.hover{
border:1px dashed #000;
width:100px;
height:100px;
background: rgba(0,0,0,0.25);
position: absolute;
display: none;
cursor: move;
}
</style>
</head>
<body>
<div id="area" class="area">
<div class="hover" id="hover">
</div>
<script type="text/javascript">
var $box = document.getElementById("area");
var $hover = document.getElementById("hover");
var boxW=$box.offsetWidth,
boxH=$box.offsetHeight,
boxLeft=$box.offsetLeft,
boxTop=$box.offsetTop;
var startX,startY,hoverW,hoverH;
$box.onmouseover=function(event){
var event=event||window.event;
$hover.style.display="block";
hoverW=$hover.offsetWidth,
hoverH=$hover.offsetHeight;
var x = event.clientX;
var y = event.clientY;
$hover.style.left=(x-boxLeft-hoverW/2)+"px";
$hover.style.top=(y-boxTop-hoverH/2)+"px";
console.log(boxW+"<br/>"+hoverW)
}
$box.onmousemove=function(event){
var event=event||window.event;
var x = event.clientX;
var y = event.clientY;
var resultX,resultY;
if(x-boxLeft-50<0){
resultX=0;
}else if(x-boxLeft-50>=boxW-hoverW){
resultX=boxW-hoverW;
}else{
resultX=x-boxLeft-50;
}
if(y-boxTop-50<0){
resultY=0
}else if(y-boxTop-50>=boxH-hoverH){
resultY=boxH-hoverH;
}else{
resultY=y-boxTop-50;
}
$hover.style.left=resultX+"px";
$hover.style.top=resultY+"px";
}
$box.onmouseout = function(event) {
var event=event||window.event;
var x = event.clientX;
var y = event.clientY;
var divx1 = $box.offsetLeft;
var divy1 = $box.offsetTop;
var divx2 = $box.offsetLeft + $box.offsetWidth;
var divy2 = $box.offsetTop + $box.offsetHeight;
if (x < divx1 || x > divx2 || y < divy1 || y > divy2) {
$hover.style.display="none";
}
}
</script>
</body>
</html>
主要是两个关键点：
1、获取鼠标当前位置，然后设置运动元素的当前位置为鼠标位置；
2、判断元素是否靠近父级元素的边界，设置临界值；