// 加入透明背景 
var body = document.body;
var backgroundDiv = document.createElement("div"); 
backgroundDiv.style.cssText = "left:0;top:0;
position:fixed;
background:#ddd;opacity:0.8;width:100%;height:100%"; 
body.appendChild(backgroundDiv); 
// 加入居中弹出框 
var newDiv = document.createElement("div"); 
newDiv.style.cssText = "position:absolute;margin:auto;
left:0;right:0;top:0;
bottom:0;width:400px;
height:200px;background:white;";
backgroundDiv.appendChild(newDiv); // 丰富弹出框内容 ......