JavaScript做了个弹出层的form ，请问如何关闭form

<script>
function signIn() {
addBg();
var alert_sign_in = document.getElementById("sign_in");
alert_sign_in.style.display = "block";
alert_sign_in.style.position = "absolute";
alert_sign_in.style.top = "50%";
alert_sign_in.style.left = "50%";
alert_sign_in.style.marginTop = "-75px";
alert_sign_in.style.marginLeft = "-150px";
alert_sign_in.style.width = "500px";
alert_sign_in.style.height = "700px";
alert_sign_in.style.zIndex = "501";

var mybg = document.createElement("div");
mybg.setAttribute("id","mybg");
mybg.style.background = "#000";
mybg.style.width = "100%";
mybg.style.height = "100%";
mybg.style.position = "absolute";
mybg.style.top = "0";
mybg.style.left = "0";
mybg.style.zIndex = "500";
mybg.style.opacity = "0.3";
mybg.style.filter = "alpha(opacity=30)";
document.body.appendChild(mybg);

document.body.style.overflow = "hidden";
}
</script>

这个是一个弹出式的登录功能，先有个遮罩层，然后弹出一个form供用户输入信息，请问这个这个form如何关闭？就是用户取消登录的时候，遮罩层自动取消，然后form自动关闭。

弹出式的登录功能是这样写的吗？有没有其他的写法，谢谢。

都是通过js操作class来实现的，可以在取消的时候，删除掉弹出层和登录form的class；

div.style.display = "none" 再看看别人怎么说的。