创建遮罩层

    //创建遮罩层
    function createMask(){
        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; 
            
            
            $("<div id=\"webMask\"></div>").appendTo("body");
            
            $("#webMask").css({posotion:"absolute",top:"0px",left:"0px",background:"#777",filter:"Alpha(opacity=60)",opacity:"0.6",width:sWidth+"px",height:sHeight+"px",zIndex:"100"});  
        }