get_google_oauth.js

const fs = require('fs');
const readline = require('readline');
const {google} = require('googleapis');

const SCOPES = [
    'https://www.googleapis.com/auth/admin.directory.group.member'
    ,'https://www.googleapis.com/auth/admin.directory.group'
    ,'https://mail.google.com/'
    ,'https://www.googleapis.com/auth/gmail.modify'
    ,'https://www.googleapis.com/auth/gmail.compose'
    ,'https://www.googleapis.com/auth/gmail.send'
];


// Load client secrets from a local file.
fs.readFile('google-credentials.json', (err, content) => {
  if (err) return console.log('Error loading client secret file:', err);
  // Authorize a client with credentials, then call the Gmail API.
  authorize(JSON.parse(content));
});

/**
 * Create an OAuth2 client with the given credentials, and then execute the
 * given callback function.
 * @param {Object} credentials The authorization client credentials.
 * @param {function} callback The callback to call with the authorized client.
 */
function authorize(credentials) {
  const {client_secret, client_id, redirect_uris} = credentials.installed;
  const oAuth2Client = new google.auth.OAuth2(
      client_id, client_secret, redirect_uris[0] );

  getNewToken(oAuth2Client);
}

/**
 * Get and store new token after prompting for user authorization, and then
 * execute the given callback with the authorized OAuth2 client.
 * @param {google.auth.OAuth2} oAuth2Client The OAuth2 client to get token for.
 * @param {getEventsCallback} callback The callback for the authorized client.
 */
function getNewToken(oAuth2Client) {
  const authUrl = oAuth2Client.generateAuthUrl({
    access_type: 'offline',
    scope: SCOPES,
  });
  console.log('Get client key at:', authUrl);
}