index.js

const fs = require('fs');
const readline = require('readline');
const {google} = require('googleapis');
const Mail = require('./mail.js');
const Contacts = require('./contacts.js');

// If modifying these scopes, delete token.json. The code will automatically
// generate a new token when re-run.
const SCOPES = [
  'https://www.googleapis.com/auth/gmail.readonly',
  'https://www.googleapis.com/auth/gmail.modify',
  'https://www.googleapis.com/auth/gmail.compose',
  '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 = 'token.json';

// Load client secrets from a local file.
fs.readFile('credentials.json', (err, content) => {
  if(err){
    return console.log('Error loading client secret file:', err);
  }

  // Authorize the client with credentials, then trigger mailer using the Gmail API.
  authorize(JSON.parse(content), sendMailers);
});

/**
 * 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);
    });
  });
}

// Iterate through contacts, create attachments for each, and send
// or create draft of generated email.
function sendMailers(auth){
  let contacts = new Contacts();
  contacts.getContacts(function(formattedContacts){
    for(let i = 0; i < formattedContacts.length; i++){
      setTimeout(function() {
        new Mail(auth, formattedContacts[i]).createMail();
      }, (i * 250));
    }
  });
}