Open Popup window javascript

function fn_openwindow() {      

            var url= 'popupwindow.aspx';           

            var x = screen.width / 2 - 800 / 2;

            var y = screen.height / 2 - 500 / 2;

            newwindow = window.open(url, 'Report', 'height=500,width=800,resizable=1,left='+x+',top='+y);

            newwindow.focus();

            return false;

        }