<script type="text/javascript">
        window.onload = function () {
            var oBtn = document.getElementById("btnLogin");
            oBtn.onclick = function () {
                //获取页面的宽度和高度
                var sHeight = document.documentElement.scrollHeight;
                var sWidth = document.documentElement.scrollWidth;
                //可视区域的高度和宽度
                //如果说页面是一个竖向的页面，那么可视区域的宽度和页面的宽度是一样的
                var wHeight = document.documentElement.clientHeight;

var oMask = document.createElement("div");
                oMask.id = "mask";
                oMask.style.height = sHeight + "px";
                oMask.style.width = sWidth + "px";
                document.body.appendChild(oMask);

                var oLogin = document.createElement("div");
                oLogin.id = "login";
                oLogin.innerHTML = "<div class='loginCon'><div id='div1'></div><div id='div2'></div><div id='close'></div></div>"
                document.body.appendChild(oLogin);
                //获取login的宽度和高度
                var dHeight = oLogin.offsetHeight;
                var dWidth = oLogin.offsetWidth;
                oLogin.style.left(sWidth-dWidth) / 2 + "px";
                oLogin.style.top(wHeight-dHeight) / 2 + "px";
                //获取id为close的关闭元素
                var oClose = document.getElementById("close");
                oClose.onclick = function () {
                    document.body.removeChild(oMask);
                    document.body.removeChild(oLogin);
                }
            }
        }
    </script>