function logout() {
        var oXMLHttpRequest = new XMLHttpRequest; 
        oXMLHttpRequest.open("GET", "webservice/main.php?" + "format=json&cmd=UserLogoutService", true); 
        oXMLHttpRequest.onreadystatechange = function() { 
            if (this.readyState == XMLHttpRequest.DONE) { 
                var obj = eval(oXMLHttpRequest.responseText);
                eraseCookie('userId');
                window.location = "index.php";
            } 
        } 
        oXMLHttpRequest.send(null); 
    }