renew.js

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

// If modifying these scopes, delete token.json.
const SCOPES = [
  'https://www.googleapis.com/auth/gmail.send'
];
// The file token.json stores the user's access and refresh tokens, and is
// created automatically when the authorization flow completes for the first
// time.
const TOKEN_PATH = process.env.TOKEN_PATH;

fs.readFile('./_secret.json', function processClientSecrets(err, content) {
  if (err) {
    console.log('Error loading client secret file: ' + err);
    return;
  }
  // Authorize a client with the loaded credentials, then call the
  // Gmail API.
  authorize(JSON.parse(content), sendMessage);
});

function makeBody(to, from, subject, message) {
  var str = ["Content-Type: text/plain; charset=\"UTF-8\"\n",
    "MIME-Version: 1.0\n",
    "Content-Transfer-Encoding: 7bit\n",
    "to: ", to, "\n",
    "from: ", from, "\n",
    "subject: ", subject, "\n\n",
    message
  ].join('');

  var encodedMail = Buffer.from(str).toString("base64").replace(/\+/g, '-').replace(/\//g, '_');
  return encodedMail;
}

function sendMessage(auth) {
  var raw = makeBody(process.env.TO, process.env.TO, 'Token renewed', `Token ${process.env.TOKEN_PATH} had been saved.`);
  const gmail = google.gmail({ version: 'v1', auth });
  gmail.users.messages.send({
    auth: auth,
    userId: 'me',
    resource: {
      raw: raw
    }

  }, function (err, response) {
    return (err || response)
  });
}

/**
 * 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, callback) {
  const { client_secret, client_id, redirect_uris } = credentials.installed;
  const oAuth2Client = new google.auth.OAuth2(
    client_id, client_secret, redirect_uris[0]);

  // Check if we have previously stored a token.
  fs.readFile(TOKEN_PATH, (err, token) => {
    if (err) return getNewToken(oAuth2Client, callback);
    oAuth2Client.setCredentials(JSON.parse(token));
    callback(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, callback) {
  const authUrl = oAuth2Client.generateAuthUrl({
    access_type: 'offline',
    scope: SCOPES,
  });
  console.log('Authorize this app by visiting this url:', authUrl);
  const rl = readline.createInterface({
    input: process.stdin,
    output: process.stdout,
  });
  rl.question('Enter the code from that page here: ', (code) => {
    rl.close();
    oAuth2Client.getToken(code, (err, token) => {
      if (err) return console.error('Error retrieving access token', err);
      oAuth2Client.setCredentials(token);
      // Store the token to disk for later program executions
      fs.writeFile(TOKEN_PATH, JSON.stringify(token), (err) => {
        if (err) return console.error(err);
        console.log('Token stored to', TOKEN_PATH);
      });
      callback(oAuth2Client);
    });
  });
}