offsetParent算法分析
<!DOCTYPE HTML>
<html id="html">
<head>
<title>offsetParent by 司徒正美</title>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<script src="mass.js">
</script>
<style type="text/css">
body,html{
height:100%;
background:orange;
}
#relative {
position: relative;
}
div{
width:100px;
height:100px;
background: red;
margin:10px;
}
#aaa,#eee,#iii,#mmm,#qqq{
position: relative;
}
#bbb,#fff,#jjj,#nnn,#rrr{
position: absolute;
}
#ccc,#ggg,#kkk,#ooo,#sss{
position: fixed;
background: blue;
}
#td2 {
position: relative;
}
td{
background: green;
}
form{
margin-left:160px;
}
</style>
<script>
window.onload = function(){
var add = "<ul>"
var i = 0
var hash = {
1: "自身position: relative的DIV元素",
2: "自身position: absolute的DIV元素",
3: "自身position: fixed的DIV元素",
0: "自身position: static的DIV元素"
}
"abcdefghijklmnopqrst".replace(/\w/g, function(id){
i++;
add += "<li>"
if( i < 5){
add += "没有已定位的父节点，且"
} if(i >= 5 && i < 9){
add += "拥有一个已定位的父节点，且"
}else if( i >= 9 && i < 13){
add += "在table之内，td与table都没有定位，且"
}else if( i >= 13 && i < 17){
add += "在table之内，td相对定位，且"
}else if( i >= 17){
add += "在table之内，table相对定位，且"
}
add += hash[ i % 4];
id = id + id + id;
var el = document.getElementById(id).offsetParent
var ret = (el && el.id)+"";
switch(ret){
case "html":
add += "的offsetParent为HTML"
break
case "body":
add += "的offsetParent为BODY"
break
case "relative":
add += "的offsetParent为其最近被定位的祖先"
break
case "td":
case "td2":
case "td3":
add += "的offsetParent为其最近的<strong>TD、TH</strong>元素"
break
case "table":
add += "的offsetParent为其最近的<strong>TABLE</strong>元素"
break
default:
add += "的offsetParent为null"
break
}
add += "</li>"
})
add += "</ul>"
console.log(add)
var el = document.createElement("form");
document.body.appendChild( el );
el.innerHTML = add;
}
</script>
</head>
<body id="body">
<blockquote>
<div id="aaa">
</div>
<div id="bbb">
</div>
<div id="ccc">
</div>
<div id="ddd">
</div>
</blockquote>
<blockquote id="relative">
<div id="eee">
</div>
<div id="fff">
</div>
<div id="ggg">
</div>
<div id="hhh">
</div>
</blockquote>
<table border="1" id="table">
<tr>
<td id="td"><div id="iii"></div><div id="jjj"></div><div id="kkk"></div><div id="lll"></div></td>
</tr>
<tr>
<td id="td2" ><div id="mmm"></div><div id="nnn"></div><div id="ooo"></div><div id="ppp"></div></td>
</tr>
</table>
<table style="float:left;border:1px solid black; position: relative;" id="table">
<tr>
<td id="td3"><div id="qqq"></div><div id="rrr"></div><div id="sss"></div><div id="ttt"></div></td>
</tr>
</table>
</body>
</html>
根据其自身定位情况与父节点的标签类型与定位情况，分为以下二十种情况（IE8， chrome23, opera12的结果）：
我们可以总结以下几条规律：
a) position为fixed元素是没有offsetParent，但firefox统一返回body。 b) position为absolute, relative的元素的offsetParent总是为其最近的已定位的元素,没有找最近的td,th元素，再没有找body。 c) position为static的元素的offsetParent则是先找最近的td,th元素，再没有找body。 d) body为最顶层的offsetParent。
