juvin mail send

// should be outside, need to do it just once not everytime
transporter.verify(function (error, success) {
  if (error) {
    console.log('Failed to connect to SMTP server');
    console.log(error);
  } else {
    console.log('Connected to SMTP server!');
  }
});

// promise wrapper to send email, so that we can use this function easily in async method easily later on:
const sendEmailPromise = (mailOptions) => new Promise((resolve, reject) => {
  transporter.sendMail(mailOptions, async function (err, data) {
    // not rejecting in any scenario, if error is there just add err: True to result
    // this is because we are going to use it in Promise.all and if even one fails, Promise.all throws error which we dont want
    // attaching originalOptions just to log it later on if needed
    if (err) {
      resolve({ error: true, err: err, originalOptions: mailOptions });
    }
    else {
      resolve({ ...data, originalOptions: mailOptions });
    }
  });
});


exports.mailSend = catchAsyncErrors(async (req, res, next) => {
  const { list, buildId } = req.body;
  var rejected = 0; // should not be const, const value cannot change so ++rejected would not work actually
  const build = await Build.findById(buildId);
  if (!build) {
    return next(new ErrorHandler("Build not found", 404));
  }
  var mail_queue = [];
  list.forEach((item, index) => {
    const mailOptions = {
      // from: process.env.EMAIL,
      to: item.email,
      subject: "Invitation to Test",
      html: `<h1>You have Been Invited to Test an Application</h1><p>Click <a href='${req.protocol}://${process.env.REQ_HOST}/invites/${buildId}'>here</a> or visit your profile on <a href="${req.protocol}://${process.env.REQ_HOST}/profile">${req.protocol}://${process.env.REQ_HOST}/profile</a> to view.</p>`,
    };
    mail_queue.push(sendEmailPromise(mailOptions));
  });
  // now that we have pushed each email in array we actually need to wait for each to be finished, this is where we use Promise.all
  const mailResults = await Promise.all(mail_queue);
  // now we loop through mailResults and just see if err: true OR .rejected.length > 0 as you were checking
  mailResults.forEach((result, index) => {
    if (result.err || (result.rejected && result.rejected.length > 0)) {
      console.log("Failed to send eamil for", result.originalOptions);
      console.log(result);
      console.log(); // blank line for clarity in logs
      rejected++;
    }
    else {
      build.invites.push({ tester: item._id, status: "pending" });
    }
  });
  await build.save(); // save build
  if (rejected > 0)
    return next(
      new ErrorHandler(
        "A few testers didn't get invited. Check the invited list.",
        417
      )
    );
  res.status(200).json({
    success: true,
  });
});