Cloud function to send custom emails using nodemailer and the Firebase adminSDK when a user is created

index.js

const functions = require('firebase-functions');
const admin = require('firebase-admin');
admin.initializeApp();
const nodemailer = require('nodemailer');

// Configure the email transport using the default SMTP transport and a GMail account.
// For other types of transports such as Sendgrid see https://nodemailer.com/transports/
// TODO: Configure the `gmail.email` and `gmail.password` Google Cloud environment variables.
const gmailEmail = functions.config().google.email;
const gmailPassword = functions.config().google.password;
const mailTransport = nodemailer.createTransport({
  service: 'gmail',
  auth: {
    user: gmailEmail,
    pass: gmailPassword,
  },
});

// Sends an email confirmation when a user changes his mailing list subscription.
exports.sendVerificationEmail = functions
  .auth
  .user()
  .onCreate((user) => {
    const email = user.email; // The email of the user.
    const displayName = user.displayName; // The display name of the user.
    const actionCodeSettings = {
      // URL you want to redirect back to. The domain (www.example.com) for
      // this URL must be whitelisted in the Firebase Console.
      url: '<YOUR URL>',
      // This must be true for email link sign-in.
      handleCodeInApp: false,
      iOS: {
        bundleId: '<YOUR BUNDLE ID>'
      },
      android: {
        packageName: '<YOUR PACKAGE NAME>',
        installApp: false,
        minimumVersion: '12'
      },
      dynamicLinkDomain: '<DYNAMIC LINK>'
    };
    const mailOptions = {
      from: '"New app Corp" <noreply@firebase.com>',
      to: email,
    };

    admin.auth().generateEmailVerificationLink(email, actionCodeSettings)
      .then(async (link) => {
        // Construct email verification template, embed the link and send
        // using custom SMTP server.
        try {
          // Building Email message.
          mailOptions.subject = 'Thanks and Welcome!';
          mailOptions.text = 'Thanks you for register to our app. This is the LINK TO VERIFY: \n' + link ;
          await mailTransport.sendMail(mailOptions);
          console.log(`New subscription confirmation email sent to:`, email);
        } catch(error) {
          console.error('There was an error while sending the email:', error);
        }
        return '200';
      })
      .catch((error) => {
        console.error(error);
        return '200';
      });
  });