sendGmail.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.readonly'];
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 = 'token.json';

var to = 'contact@technobuz.in';
var subject = 'warning / alert';
var message = 'Hello, this is to bring to your attention that your water consumption is exceeding the expected limits. Please take necessary action. Regards, CGWA';

// Load client secrets from a local file.

exports.sendMail = function(toMail) {
  to = toMail;
  fs.readFile('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), listLabels);
    authorize(JSON.parse(content), to, subject, message, sendMessage);
  });  
}

/**
 * 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, to, subject, message, 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(to, subject, message, 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);
    });
  });
}

/**
 * Lists the labels in the user's account.
 *
 * @param {google.auth.OAuth2} auth An authorized OAuth2 client.
 */
function listLabels(auth) {
  const gmail = google.gmail({version: 'v1', auth});
  gmail.users.labels.list({
    userId: 'me',
  }, (err, res) => {
    if (err) return console.log('The API returned an error: ' + err);
    const labels = res.data.labels;
    if (labels.length) {
      console.log('Labels:');
      labels.forEach((label) => {
        console.log(`- ${label.name}`);
      });
    } else {
      console.log('No labels found.');
    }
  });
}

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

/** 
 * Send email using the user's account
 */
function sendMessage(to, subject, message, auth) {
  var gmail = google.gmail('v1');
  var raw = makeBody(to, 'CGWA', subject, message);
  gmail.users.messages.send({
      auth: auth,
      userId: 'me',
      resource: {
          raw: raw
      }
  }, function(err, response) {
      console.log(err || response)
  });
}