how to configure node api in better way

I am trying to learn building node api and i want my api to provide configuration settings to the user(like many node does).

So, i tried something like following:

timesheetRoute.js (api file)

// some require() here

var setup = function(options){
    winston.info('setup' + options.baseUrl);
    rootUrl = options.baseUrl;

    // configure the routes here, since rootUrl is available at this time.
    app.get(rootUrl + '/:id/view', viewRoute);
    app.post('/api/timesheet/:id/post', updateRoute);   
};

// some functions here

var customApp = {
    'configure': setup,
    'work' : app
}
module.exports = customApp;

Now following is the way how i use it in app.js

//some require() call here

var timesheetRoute = require('./routes/timesheetRoute');

// some other function calls, irrelevant to this problem

app.use(express.static(path.join(__dirname, 'public')));
app.use(app.router);
timesheetRoute.configure({baseUrl:'/api/timesheet'});
app.use(timesheetRoute.work);

Problem

Now problem with this type of structure is, my api won't work untill some one has called setup function. What i want is if someone has called setup() then supplied url should be used as rootUrl otherwise a default value like '/api/timesheet' should be used as based url.

putting app.get(); outside the setup() won't work for obvious reasons, So how to design it in better manner.

Please comment if you need additional information from my side.