练习:鼠标感应伸缩隐藏层

　　鼠标感应伸缩隐藏层，实际上我没完全理解这个例子，但是相信以后会用的到，所以先把code贴上来，未来我要去理解的两个参数方法setInterval() ，clearInterval()，目前还不是很清楚这两个的用法 。如果有谁会欢迎留言解答，不胜感激。

!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"
html xmlns="http://www.w3.org/1999/xhtml"
head
meta http-equiv="Content-Type" content="text/html; charset=utf-8" /
title无标题文档/title
style type="text/css"
div{width:200px;word-break:break-all;border:1px solid red;overflow:hidden; cursor:pointer}
/style
script type="text/javascript"
var t = true;
var obj,a;
function openclose(openclose){
  var h = obj.offsetHeight;
  h += openclose*5;
  if(h=150){
   clearInterval(a);
   obj.style.height = "150px";
   t = true;
  }
  else if(h=50){
   clearInterval(a);
   obj.style.height = "50px";
   t = true;
  }
  else obj.style.height = h + "px";
 }
function getlink(){
 obj.onmouseout = function(){
   clearInterval(a);
   a = setInterval("openclose(-1)" , 5);
  }
 obj.onmouseover = function(){
   clearInterval(a);
   a = setInterval("openclose(1)" , 5);
  }
}
window.onload = function()
{
 obj = document.getElementById("box")
 obj.style.height = "50px";
 getlink();
}
/script
/head
body
 
 div id="box"
img src="http://www.webjx.com/images/Logo.gif"
/div
 
/body
/html