How to check if 'nodemailer' sent email successfully?

Question

Node.js

When a user registers, I send him a notification by e-mail. For this I use the 'nodemailer' module. How can I make app.js check the condition when calling mailer: If the mail was successfully sent, do res.send('Registration was successful') ?
App.js code:

const express  = require('express');
var app = express();
const mailer  = require('../app/module/sendmail');
const port     = process.env.PORT || 8000;

app.post('/signup, function (req, res) {
  mailer('[email protected]', 'passwd');
})

app.listen(port);
console.log('NodeJS слушает порт: ' + port);

Below is the Sendmail.js module code:

const nodemailer = require('nodemailer');
const morgan = require("morgan");

var transporter = nodemailer.createTransport({
  service: 'gmail',
  auth: {
    user: process.env.GMAIL_USER,               
    pass: process.env.GMAIL_PASS
  }
});

module.exports = function(email, passwd){
var mailOptions = {
  from: process.env.GMAIL_USER,
  to: email,
  subject: 'Благодарим за регистрацию',
  html: `<p>Вы успешно зарегистрированы!</p>
         <p>Ваш логин: <b>${email}</b><br>
         Пароль: <b>${passwd}</b></p>`
};

transporter.sendMail(mailOptions, function(error, info){
  if(error){
    console.log('Ошибка отправки на почту извещения');
  } else {
    console.log('Обработанно');
  }
});
}

The validation should work in app.js, not in the included module Sendmail.js.

Answer 1

Easy option: declare transporter inside the express handler.
The correct option is to promissify sendMail, i.e. wrap in a function that returns a promise that can be subscribed to from the outside.

function sendMail(mailOptions) {
  return new Promise((resolve, reject) =>
    transporter.sendMail(mailOptions, (error, info) => {
      if(error){
        reject(error);
      } else {
        resolve(info);
      }
    });
  });
}

Using Promises:

sendMail(mailOptions)
  .then(info => console.log(info))
  .catch(error => console.error(error));

or like this:

try {
  const info = await sendMail(mailOptions);
  console.log(info);
} catch (error) {
  console.error(error);
}

Answer 2

well, in `module.exports = function(email, passwd)` pass another callback, which is called upon sending the message....
Better yet, switch to promises....

Answer 3

It is best not to rely on such verification. The letter can leave safely, but still be destroyed somewhere along the way. This fact cannot be verified.