Express server that looks for files in /common, then figures out the useragent and goes somewhere else.

server.js

// Load dependencies
var express = require('express');
var is = require('browseris');
var fs = require('fs');

// Build me an app!
var app = express();
app.set('port', process.env.PORT || 3000);
app.use(express.compress());

// What are some URLs we should use?
var url = {
	common: 'common',
	mobile: 'mobile',
	desktop: 'desktop'
};

// Show me an app!
app.get('/', function(request, response) {

	// Which file to render?
	var location;
	if (is(request).mobile) {
		location = url.mobile + '/app.html';
	} else {
		location = url.desktop + '/app.html';
	}

	// Send me a file
	response.sendfile(location);

});

// What about everything else?
app.get('*', function(request, response) {

	// Look for it in /common
	var commonPath = url.common + request.path;
	fs.exists(commonPath, function(exists) {

		// Which file to render?
		var location;
		if (exists) {
			location = commonPath;
		} else {
			if (is(request).mobile) {
				location = url.mobile + request.path;
			} else {
				location = url.desktop + request.path;
			}
		}

		// Render that puppy
		response.sendfile(location);

	});

});

// Start that app
app.listen(app.get('port'));
console.log('Server listening on port ' + app.get('port'));