app.js

var fs = require('fs');
var readline = require('readline');
var {google} = require('googleapis');
var OAuth2 = google.auth.OAuth2;

var SCOPES = [
      'https://www.googleapis.com/auth/yt-analytics.readonly',
      'https://www.googleapis.com/auth/youtube.readonly'
    ];

fs.readFile('client_secret.json', function processClientSecrets(err, content) {
  if (err) {
    console.log('Error loading client secret file: ' + err);
    return;
  }
  authorize(JSON.parse(content));
});

function authorize(credentials) {
  var clientSecret = credentials.installed.client_secret;
  var clientId = credentials.installed.client_id;
  var redirectUrl = credentials.installed.redirect_uris[0];
  var oauth2Client = new OAuth2(clientId, clientSecret, redirectUrl);

  getNewToken(oauth2Client);
}

function getNewToken(oauth2Client) {
  var authUrl = oauth2Client.generateAuthUrl({
    access_type: 'offline',
    scope: SCOPES
  });

  console.log('Authorize this app by visiting this url: ', authUrl);
  var rl = readline.createInterface({
    input: process.stdin,
    output: process.stdout
  });

  rl.question('Enter the code from that page here: ', function(code) {
    rl.close();
    oauth2Client.getToken(code, function(err, token) {
      if (err) {
        console.log('Error while trying to retrieve access token', err);
        return;
      }

      console.log('Copy and save entire JSON:')
      console.log(token);
    });
  });
}