JavaScript—分步学做遮罩层弹出框

//创建遮罩层
    function AlertMsg(){
        var sWidth,sHeight;
        sWidth = window.screen.availWidth;
        //屏幕可用工作区高度： window.screen.availHeight;
        //屏幕可用工作区宽度： window.screen.availWidth;
        //网页正文全文宽：     document.body.scrollWidth;
        //网页正文全文高：     document.body.scrollHeight;
        if(window.screen.availHeight > document.body.scrollHeight){  //当高度少于一屏
            sHeight = window.screen.availHeight;  
        }else{//当高度大于一屏
            sHeight = document.body.scrollHeight;  
        }
        //创建遮罩背景
        var maskObj = document.createElement("div");
        maskObj.setAttribute('id','BigDiv');
        maskObj.style.position = "absolute";
        maskObj.style.top = "0";
        maskObj.style.left = "0";
        maskObj.style.background = "#777";
        maskObj.style.filter = "Alpha(opacity=30);";
        maskObj.style.opacity = "0.3";
        maskObj.style.width = sWidth + "px";
        maskObj.style.height = sHeight + "px";
        maskObj.style.zIndex = "10000";
        document.body.appendChild(maskObj);
    }

var MyDiv =document.getElementById("MyDiv");
        var MyDiv_w = getStyle(MyDiv,"width");
        var MyDiv_h = getStyle(MyDiv,"height");
        
        MyDiv_w = parseInt(MyDiv_w); //去掉 单位 "px"
        MyDiv_h = parseInt(MyDiv_h);

        var width = pageWidth();
        var height = pageHeight();
        var left = leftPosition();
        var top = topPosition();

        var Div_topposition = top + (height / 2) - (MyDiv_h / 2); //计算上边距
        var Div_leftposition = left + (width / 2) - (MyDiv_w / 2); //计算左边距

        MyDiv.style.left = Div_leftposition + "px";  //拼接上 单位"px"
        MyDiv.style.top =   Div_topposition + "px";
        MyDiv.style.display = "block";  //设置弹出div显示
    }

   function CloseDiv(){
        var Bigdiv = document.getElementById("BigDiv");
        var Mydiv = document.getElementById("MyDiv");
        document.body.removeChild(Bigdiv);
        Mydiv.style.display="none";
    }

  /** 第四步：弹出层拖动。*/
     window.onload = function(){
       if(document.getElementById("MyDiv")){
            var MyDiv = document.getElementById("MyDiv");
            var dragMe   = document.getElementById("dragMe");
            Drag.init(dragMe, MyDiv);
        }
    }

      /**第五步：弹出层跟随滚动条滚动。*/
    window.onscroll = window_onscroll;
    function window_onscroll(){
        var MyDiv =document.getElementById("MyDiv");
        var MyDiv_h = getStyle(MyDiv,"height");
        MyDiv_h = parseInt(MyDiv_h);
        var height = pageHeight();
        var top = topPosition();
        var Div_topposition = top + (height / 2) - (MyDiv_h / 2); //计算上边距
        MyDiv.style.top =  Div_topposition + "px";
    }