Nodemailer not working when deploying to Heroku - node.js

Question

I am using nodemailer to send email in my server using express. Everything worked perfectly in the localhost but when I deploy it on Heroku, it does not work anymore, look like it not support nodemailer on Heroku (that is what I have researched). This is my code, would you please help me out to deal with it. Thank you so much and have a good day
This is sending single mail
exports.send_mail = (req, res, next) => {
var {subjectTo, mailList, content} = req.body;
var {attachURL} = req;
var transporter = nodemailer.createTransport({
service: 'gmail',
secure: false,
port: 465,
auth: {
user: process.env.EMAIL,
pass: process.env.PASSWORD,
},
});
var mailOptions = {
from: 'sale.shopeeholic#gmail.com',
to: mailList,
cc: mailList,
subject: subjectTo,
text: `${content} \n Attached files: ${attachURL}`,
};
transporter.sendMail(mailOptions, (err, info) => {
if (err) {
console.log(err);
return res.status(400).json({err});
} else {
return res.status(200).json({message: `Mail sent to ${mailList}`});
}
});
};
This is sending merge mail/multiple mail
exports.merge_mail = (req, res, next) => {
console.log('merge mail begin');
const mailOptionList = req.body;
// {mails, mailContent, mailTitle}
var counter = 0;
var transporter = nodemailer.createTransport({
service: 'gmail',
auth: {
user: process.env.EMAIL,
pass: process.env.PASSWORD,
},
});
for (var i = 0; i < mailOptionList.length; i++) {
var mailOptions = {
from: 'sale.shopeeholic#gmail.com',
to: mailOptionList[i].mails.join(','),
cc: mailOptionList[i].mails.join(','),
subject: mailOptionList[i].mailTitle,
text: mailOptionList[i].mailContent,
};
transporter.sendMail(mailOptions, (err, info) => {
if (err) {
console.log(err);
return res
.status(400)
.json({err, message: `trouble in sending mail at index ${i}`});
} else {
console.log(`mail sent to ${JSON.stringify(mailOptionList[i].mails)}`);
counter++;
}
});
console.log(`mail sent to ${JSON.stringify(mailOptionList[i].mails)}`);
counter++;
console.log(counter);
}
if (counter === mailOptionList.length) {
return res.status(200).json({message: 'mail sent all'});
}

Answer 1

Probably process.env.EMAIL and process.env.PASSWORD are undefined. You have to set env variables in Heroku. Here's how:
https://devcenter.heroku.com/articles/config-vars