main.js

const express = require('express');
const google = require('googleapis').google;
const jwt = require('jsonwebtoken');

// Google's OAuth2 client
const OAuth2 = google.auth.OAuth2;

// Including our config file
const CONFIG = require('./config');

// Creating our express application
const app = express();

// Allowing ourselves to use cookies
const cookieParser = require('cookie-parser');
app.use(cookieParser());

// Setting up Views
app.set('view engine', 'ejs');
app.set('views', __dirname);

app.get('/', function (req, res) {
  // Create an OAuth2 client object from the credentials in our config file
  const oauth2Client = new OAuth2(CONFIG.oauth2Credentials.client_id, CONFIG.oauth2Credentials.client_secret, CONFIG.oauth2Credentials.redirect_uris[0]);

  // Obtain the google login link to which we'll send our users to give us access
  const loginLink = oauth2Client.generateAuthUrl({
    access_type: 'offline', // Indicates that we need to be able to access data continously without the user constantly giving us consent
    scope: CONFIG.oauth2Credentials.scopes // Using the access scopes from our config file
  });
  return res.render("index", { loginLink: loginLink });
});

app.get('/auth_callback', function (req, res) {
  // Create an OAuth2 client object from the credentials in our config file
  const oauth2Client = new OAuth2(CONFIG.oauth2Credentials.client_id, CONFIG.oauth2Credentials.client_secret, CONFIG.oauth2Credentials.redirect_uris[0]);

  if (req.query.error) {
    // The user did not give us permission.
    return res.redirect('/');
  } else {
    oauth2Client.getToken(req.query.code, function(err, token) {
      if (err)
        return res.redirect('/');
      
      // Store the credentials given by google into a jsonwebtoken in a cookie called 'jwt'
      res.cookie('jwt', jwt.sign(token, CONFIG.JWTsecret));
      return res.redirect('/get_some_data');
    });
  }
});

app.get('/get_some_data', function (req, res) {
  if (!req.cookies.jwt) {
    // We haven't logged in
    return res.redirect('/');
  }

  // Create an OAuth2 client object from the credentials in our config file
  const oauth2Client = new OAuth2(CONFIG.oauth2Credentials.client_id, CONFIG.oauth2Credentials.client_secret, CONFIG.oauth2Credentials.redirect_uris[0]);

  // Add this specific user's credentials to our OAuth2 client
  oauth2Client.credentials = jwt.verify(req.cookies.jwt, CONFIG.JWTsecret);

  // Get the youtube service
  const service = google.youtube('v3');

  // Get five of the user's subscriptions (the channels they're subscribed to)
  service.subscriptions.list({
    auth: oauth2Client,
    mine: true,
    part: 'snippet,contentDetails',
    maxResults: 5
  }).then(response => {
    // Render the data view, passing the subscriptions to it
    return res.render('data', { subscriptions: response.data.items });
  });
});

// Listen on the port defined in the config file
app.listen(CONFIG.port, function () {
  console.log(`Listening on port ${CONFIG.port}`);
});