Nodemailer: 4 mail over 8 are sent with empty body - node.js

Question

i'm building a management webapp for courses management. I have a mongodb with all my alumns , their email and their registration id. I wanna send the id to their mail. I have a html template and then with the fs.readfile and .replace() functions i replace the placeholders with the real infos i wanna send.
async function sendMail(mail,idreg){
var html_template;
const dataReg = "12/02/2019";
const oraReg = "16:00";
console.log(mail,idreg)
fs.readFile("./html_template/beefree-9o6bg29htfb.html",(err,data)=>{
html_template = data.toString().replace("XYZXYZXYZ",idreg).replace("XX/YY/ZZZZ",dataReg).replace("XX:YY",oraReg);
});
// Generate test SMTP service account from ethereal.email
// Only needed if you don't have a real mail account for testing
let account = await nodemailer.createTestAccount();
// create reusable transporter object using the default SMTP transport
let transporter = nodemailer.createTransport({
host: "smtp.gmail.com",
port: 465,
secure: true, // true for 465, false for other ports
auth: {
user: "mymail#gmail.com ", // generated ethereal user
pass: "mypw" // generated ethereal password
}
});
// setup email data with unicode symbols
let mailOptions = {
from: '"Cogestione Moscati" <mymail#gmail.com>', // sender address
to: mail, // list of receivers
subject: "Il tuo codice di registrazione per la Cogestione", // Subject line
html: html_template // html body
};
// send mail with defined transport object
let info = await transporter.sendMail(mailOptions)
console.log("Message sent: %s", info.messageId);
}
The problem is that i sent 8 times the same mail. 4 times i received correctly, 4 times i received with no body.

Answer 1

The code underneath your call to fs.readFile is not provided as part of the callback to that function. Basically, the file read is being kicked off and the rest of the code begins execution, resulting in a race to create html_template before the email is sent.
Two solutions:
Extend the callback to include the rest of the code. This would be the preferred method, as it is non-blocking and thus allows your application to continue serving other requests.
Use fs.readFileSync, the synchronous equivalent.
Also, since you are using async/await, you could also just let data = await fs.readFile("path/to/file");