function $Id(obj){return document.getElementById(obj);}
function $Tag(obj,Tag){return obj.getElementsByTagName(Tag);}
/**
 * Js.extend()
 * @target
 * @options
 */
var Js = {
	extend : function(target, options) {
		for (name in options) target[name] = options[name];
		return target;
	} 
}
/**
 * but_backTop()
 * @ addTag 加入区域
 * @ t 指定Top距离 隐藏
 * @ s 样式
 */
function but_backTop(addTag,t,s){
	var cr = null;
	var bt = (document.body.scrollTop || document.documentElement.scrollTop);
	var back = document.createElement('div');
	var e = {
		display : "none"
		,color : "#000"
		,position : "absolute"
		,top : 600 +"px"
		,right : 10 +"px"
		,width : 16 +"px"
		,paddingTop : 5 +"px"
		,paddingRight : 0 +"px"
		,paddingBottom : 5 +"px"
		,paddingLeft : 0 +"px"
		,borderWidth : 1 +"px"
		,borderColor : "#DDD"
		,borderStyle : "solid"
		,textAlign : "center"
		,lineHeight : 15 +"px"
		,backgroundColor : "#EEE"
	};	
	Js.extend(e,s)
	Js.extend(back.style,e); 
	back.innerHTML = "<a style='color:#000;' href='#'>返回顶部</a>";
	addTag.appendChild(back);
	window.onscroll = function(){
		bt = document.body.scrollTop || document.documentElement.scrollTop;
		back.style.display = (bt > t) ? "" : "none";
		clearInterval(cr);
		cr = setInterval(function(){
			var cy = parseInt(back.style.top);
			back.style.top = (cy + (bt -(cy-parseInt(e.top)))*0.1) +"px";
		},10);
	}	
}