边栏高度：offsetHeight的高度为空，取不到值是为什么？
var $ = function(id){
return document.getElementById(id);
}
var addEvent = function(obj,event,fn){
if(obj.addEventListener){
obj.addEventListener(event,fn,false);
}else if(obj.attachEvent){
obj.attachEvent('on'+event,fn);
}
}
var domSider = $('J_BdSide');
var scrollEvent = function(){
var sideHeight = domSider.offsetHeight;
var screenHeight = document.documentElement.clientHeight||document.body.clientHeight;
var scrollHeight = document.documentElement.scrollTop||document.body.scrollTop;
if(scrollHeight+screenHeight>sideHeight){
domSider.style.cssText = 'position:fixed;right:0px;top:'+(-(sideHeight-screenHeight))+'px';
}else{
domSider.style.position='static';
}
}
addEvent(window,'scroll',function(){
scrollEvent();
});
addEvent(window,'resize',function(){
scrollEvent();
});
