Inserir popup dinamicamente com javascript

popup.js

/**
 *  Popup javascript sem depender do jQuery
 *  
 */

var popup = {
	img:'', // new Array()
	img_close:'popup/close2.png',
	style:'', //new Array(),
	config:function(){
		var div = document.createElement("div");
		div.setAttribute("class", "popup_container");
		div.setAttribute("id", "popup");
		div.innerHTML = 
				'<div class="popup" align="center">'+
					'<img class="close" src="'+ this.img_close +'" alt="close" onclick="popup.fadeOut(\'popup\',0.1);" />'+
					'<img src="'+ this.img +'" alt="popup"/>'+
				'</div>';
		document.body.insertBefore(div,document.body.firstChild);
	},
	fadeIn:function(id, time) {
		target = document.getElementById(id);
		alpha = 0;
		timer = (time*1000)/50;
		var i = setInterval(
				function() {
					if (alpha >= 100)
						clearInterval(i);
					popup.setAlpha(target, alpha);
					alpha += 2;
				}, timer);
	},
	fadeOut:function(id, time) {
		target = document.getElementById(id);
		alpha = 100;
		timer = (time*1000)/50;
		var i = setInterval(
				function() {
					if (alpha <= 0)
						clearInterval(i);
					popup.setAlpha(target, alpha);
					alpha -= 2;
					if(alpha==0){
						target.parentNode.removeChild(target);
					}
				}, timer);
	},
	setAlpha:function(target, alpha) { 
    	target.style.filter = "alpha(opacity="+ alpha +")"; 
    	target.style.opacity = alpha/100;
	},
	setup:function(img) {
		popup.img = img;
		popup.config();
		popup.fadeIn('popup',0.1);
	}	
};