Changing global variable from Socket.io Callback

Client code:

var app = {
    data: {
        user: null
    }
};

var client = {
    connect: function () {
        client.data.socket.on('connectCallback', function (obj) {
            if (obj.userFound) {
                app.data.user = obj.user;
                return app.data.user;
            }
        });
        /** 
        Other callbacks 
        **/
    };
};

app.data.user is always null, I also tried a second function to change the variable. The obj returned by the server isn't null, I already checked that.