Send email via Google, Nodemailer

email.js

require('dotenv').config();
const fs = require('fs'),
	path = require('path'),
	Handlebars = require('handlebars'),
	nodemailer = require('nodemailer'),
	{ google } = require('googleapis'),
	OAuth2 = google.auth.OAuth2,
	{
		GMAIL_REFRESH_TOKEN,
		GMAIL_CLIENT_SECRET,
		GMAIL_CLIENT_ID,
		GMAIL_CLIENT_EMAIL,
		SIGNED_UP_EMAILS,
	} = process.env,
	EMAIL_FOLDER = '../static/'; // This is the path to your email templates (Handle bars)

const oauth2Client = new OAuth2(
	GMAIL_CLIENT_ID, // ClientID
	GMAIL_CLIENT_SECRET, // Client Secret
	'https://developers.google.com/oauthplayground' // Redirect URL
);

oauth2Client.setCredentials({
	refresh_token: GMAIL_REFRESH_TOKEN,
});

const send = (options) => {
	const accessToken = oauth2Client.getAccessToken();

	const smtpTransport = nodemailer.createTransport({
		service: 'gmail',
		auth: {
			type: 'OAuth2',
			user: GMAIL_CLIENT_EMAIL,
			clientId: GMAIL_CLIENT_ID,
			clientSecret: GMAIL_CLIENT_SECRET,
			refreshToken: GMAIL_REFRESH_TOKEN,
			accessToken: accessToken,
		},
		tls: {
			rejectUnauthorized: false,
		},
	});

	smtpTransport.sendMail(options, (error, response) => {
		error ? console.log(error) : console.log(response);
		smtpTransport.close();
	});
};

const email = (email, locals) => {
	return {
		from: `Support <support@example.com>`,
		to: email,
		subject: 'Subject',
		html: Handlebars.compile(
			fs.readFileSync(
				path.join(__dirname,  , 'template.hbs'), // name of the email template
				'utf8'
			)
		)(locals),
		text: '',
	};
};

exports.sendEmail = (email, locals) => send(resetEmailOptions(email, locals));

See https://gist.github.com/joshatxantie/8a5e3a600392b8c3a9a1a4e5dc488d6e for email template