nodemailer_gmail_oauth2.js

const nodemailer = require("nodemailer");
const { google } = require("googleapis");
const OAuth2 = google.auth.OAuth2;

const clientId = process.env.CLIENT_ID;
const clientSecret = process.env.CLIENT_SECRET;
const refreshToken = process.env.REFRESH_TOKEN;
const from = process.env.FROM;
const to = process.env.TO;

const createTransport = async () => {
	const oauth2Client = new OAuth2(
		clientId,
		clientSecret,
		"https://developers.google.com/oauthplayground"
	);

	oauth2Client.setCredentials({ refresh_token: refreshToken });

	const accessToken = await new Promise((resolve, reject) => {
		oauth2Client.getAccessToken((err, token) => {
			if (err) {
				reject();
			}
			resolve(token);
		});
	});

	const transport = nodemailer.createTransport({
		service: "gmail",
		auth: {
			type: "OAuth2",
			user,
			accessToken,
			clientId,
			clientSecret,
			refreshToken
		}
	});

	return transport;
};

/**
* Example:
* 
* const transport = await createTransport();
* await transport.sendMail({
*   subject: "This is the subject",
*   text: "Hello from nodemailer",
*   from,
*   to
* });
*
**/