Spotify Authorization server

spotify-server.js

var express = require('express'); // Express web server framework
var request = require('request'); // "Request" library
var cors = require('cors');
var querystring = require('querystring');
var cookieParser = require('cookie-parser');

var client_id = '*****'; // Your client id
var client_secret = '****'; // Your secret
var redirect_uri = 'http://localhost:8888/callback'; // Your redirect uri

var frontend_redirect_uri = 'http://localhost:4200'; // Your redirect uri

var port = 8888;

var generateRandomString = function (length) {
    var text = '';
    var possible = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';

    for (var i = 0; i < length; i++) {
        text += possible.charAt(Math.floor(Math.random() * possible.length));
    }
    return text;
};

var stateKey = 'spotify_auth_state';

var app = express();

app.use(express.static(__dirname + '/public'))
    .use(cors())
    .use(cookieParser());

app.get('/login', function (req, res) {

    var state = generateRandomString(16);
    res.cookie(stateKey, state);

    // your application requests authorization
    var scope = 'user-read-private user-read-email user-library-read playlist-read-private user-top-read user-read-recently-played user-modify-playback-state streaming user-read-playback-state user-follow-read';
    res.redirect('https://accounts.spotify.com/authorize?' +
        querystring.stringify({
            response_type: 'code',
            client_id: client_id,
            scope: scope,
            redirect_uri: redirect_uri,
            state: state
        }));
});

app.get('/ping', function (req, res) {

    res.writeHead(200, {'Content-Type': 'text/plain'});
    res.end('Hello World\n');
});

app.get('/callback', function (req, res) {
    var code = req.query.code || null;
    var state = req.query.state || null;
    var storedState = req.cookies ? req.cookies[stateKey] : null;

    if (state === null || state !== storedState) {
        res.redirect('/#' +
            querystring.stringify({
                error: 'state_mismatch'
            }));
    } else {
        res.clearCookie(stateKey);
        var authOptions = {
            url: 'https://accounts.spotify.com/api/token',
            form: {
                code: code,
                redirect_uri: redirect_uri,
                grant_type: 'authorization_code'
            },
            headers: {
                'Authorization': 'Basic ' + (new Buffer(client_id + ':' + client_secret).toString('base64'))
            },
            json: true
        };

        request.post(authOptions, function (error, response, body) {
            if (!error && response.statusCode === 200) {

                var access_token = body.access_token,
                    refresh_token = body.refresh_token;

                var options = {
                    url: 'https://api.spotify.com/v1/me',
                    headers: {'Authorization': 'Bearer ' + access_token},
                    json: true
                };
                // use the access token to access the Spotify Web API
                request.get(options, function (error, response, body) {
                    // we can also pass the token to the browser to make requests from there
                    res.redirect(frontend_redirect_uri + '#' +
                        querystring.stringify({
                            spotify_access_token: access_token,
                            spotify_refresh_token: refresh_token,
                            region: body.country
                        }));
                });

            } else {
                res.redirect(frontend_redirect_uri + '/#' +
                    querystring.stringify({
                        error: 'invalid_token'
                    }));
            }
        });
    }
});

app.get('/refresh_token', function (req, res) {

    // requesting access token from refresh token
    var refresh_token = req.query.refresh_token;
    var authOptions = {
        url: 'https://accounts.spotify.com/api/token',
        headers: {'Authorization': 'Basic ' + (new Buffer(client_id + ':' + client_secret).toString('base64'))},
        form: {
            grant_type: 'refresh_token',
            refresh_token: refresh_token
        },
        json: true
    };

    request.post(authOptions, function (error, response, body) {
        if (!error && response.statusCode === 200) {
            var access_token = body.access_token;
            res.json({
                'spotify_access_token': access_token
            });
        }
    });
});

app.listen(port);