遮罩层问题

我知道能实现这样效果的是AjaxControlToolkit里面有个控件 可以实现，可以去查查

<script type="text/javascript" language="javascript">
        function ShowNo()                        //隐藏两个层
        {
            document.getElementById("doing").style.display="none";
            document.getElementById("divLogin").style.display="none";
        }
        function $(id)        
        {
            return (document.getElementById) ? document.getElementById(id) : document.all[id] ;
        }
        function showFloat()                    //根据屏幕的大小显示两个层
        {
            var range = getRange();
            $('doing').style.width = range.width + "px";
            $('doing').style.height = range.height + "px";
            $('doing').style.display = "block";
            document.getElementById("divLogin").style.display="";
        }
        function getRange()                      //得到屏幕的大小
        {
              var top     = document.body.scrollTop;
              var left    = document.body.scrollLeft;
              var height = document.body.clientHeight;
              var width   = document.body.clientWidth;

              if (top==0 && left==0 && height==0 && width==0)
              {
                top     = document.documentElement.scrollTop;
                left    = document.documentElement.scrollLeft;
                height = document.documentElement.clientHeight;
                width   = document.documentElement.clientWidth;
              }
              return {top:top ,left:left ,height:height ,width:width } ;
        }
    </script>
    <style type="text/css">
          body{margin:0px;}
    </style>

<form id="form1" runat="server">
    <a href="javascript:void(0)" onclick="showFloat()">登陆</a>     //登陆链接
    <!--加一个半透明层-->
    <div id="doing" style="filter:alpha(opacity=30);-moz-opacity:0.3;opacity:0.3;background-

color:#000;width:100%;height:100%;z-index:1000;position: absolute;left:0;top:0;display:none;overflow: hidden;">
    </div>   
    <!--加一个登录层-->
    <div id="divLogin" style="border:solid 10px #898989;background:#fff;padding:10px;width:780px;z-index:1001; position:

absolute; display:none;top:50%; left:50%;margin:-200px 0 0 -400px;">
            <div style="padding:3px 15px 3px 15px;text-align:left;vertical-align:middle;" >
                <div>
                    用户：
                  <asp:TextBox ID="TxtUserName" runat="server" > </asp:TextBox>
                </div>
                <div>
                    密码：
                  <asp:TextBox ID="TxtUserPwd" runat="server" TextMode="Password" > </asp:TextBox>
                </div>
                <br/>
                <div> &nbsp; &nbsp;
                    <asp:Button ID="BttLogin" runat="server" Text=" 登 陆 "/>&nbsp;
                    <input id="BttCancel" type="button" value=" 取 消 " onclick="ShowNo()" />
                </div>
            </div>
      </div>
    </form>

老大，这个函数中 return (document.getElementById) ?  是什么意见啊？不是应该写成 document.getElementById(id) 吗？