index.js

// routing

'use strict';

const express = require('express');
const http = require('http');
const app = express();
const server = http.createServer(app);
const cors = require('cors');
const bodyParser = require('body-parser');
const cookieParser = require('cookie-parser');

const config = require('./config');
const sessions = require('./sessions');

app.use(cors({
	credentials: true,
	origin: true
}));

app.use(bodyParser.json());
app.use(cookieParser());

// Connect to database
require('./models');

// Set up router endpoints
const userRouter = require('./routes/user');
app.use('/user', userRouter);


server.listen(config.server.port, () => {
	console.log('Listening on port ' + config.server.port);
});

sessions.js

const sessions = {};

/** 
 * returns a random string ID 
 * */
function generateID() {
    return '' + Math.random();
}

/** 
 * return the session state for the ID 
 * */
function lookupSession(sessionId) {
    return sessions[sessionId];
}

function setSessionStateById(id, state) {
    sessions[id] = state;
}

module.exports = {
    generateID,
    lookupSession,
    setSessionStateById
}

SpotifyAdapter.js

function getAccessToken(code) {
    var authOptions = {/**/};
    
    return new Promise(function(resolve, reject) {
        request.post(authOptions, function(error, response, body) {
            if (!error && response.statusCode === 200) {
                var access_token = body.access_token,
                    refresh_token = body.refresh_token;

                resolve({
                    access_token: access_token,
                    refresh_token: refresh_token
                });
            }
            else {
                reject('invalid_token');
            }
        });
    });
}

function getUserInfo(access_token) {
    var options = {/**/};

    // use the access token to access the Spotify Web API
    return new Promise(function(resolve, reject) {
        request.get(options, function(error, response, body) {
            resolve(body);
        });
    });
}

module.exports = {
    getAccessToken,
    getUserInfo
};

user.js

'use strict';

const express = require('express');
let router = express.Router();

const userController = require('../controllers/UserController');

router.get('/', userController.index);
router.get('/name', userController.getName);
router.post('/token', userController.getSpotifyUserData);

module.exports = router;

UserController.js

const SpotifyAdapter = require('../adapters/SpotifyAdapter');
const sessions = require('../sessions');

function getSpotifyUserData(req, res) {
	var code = req.body.code;
	var accessTokenPromise;
	const sessionId = req.cookies.session;
	
  // get accessToken from session
	if (sessionId) {
		accessTokenPromise = new Promise(function(resolve, reject) {
			var lookup = lookupSession(sessionId);
			return lookup.access_token;
		});
	}
  // get access token from spotify
	else {
		accessTokenPromise = SpotifyAdapter
			.getAccessToken(code)
			.then(function(tokens) {
				const id = sessions.generateID();
				sessions.setSessionStateById(id, tokens);
				res.cookie('session', id);
				return tokens.access_token;
			});
	}

	accessTokenPromise
		.then(SpotifyAdapter.getUserInfo)
		.then(function(spotifyData) {
			res.json(spotifyData);
		})
		.catch(function(error) {
			res.json({'error': error})
		});
}