Setting up mailgun in firebase functions

Question

I'm trying really hard to use mailgun on firebase functions by following this tutorial: https://www.automationfuel.com/firebase-functions-sending-emails/

I wish to be notified by e-mail when something is pushed to a specific node in my real-time database. The code looks so far like this:

const functions = require('firebase-functions')
const mailgun = require('mailgun-js')({
  apiKey: 'key-<######>',
  domain: '<mydomain>.com',
})

exports.sendEmail = functions.database
  .ref('someParent/someChild')
  .onWrite(event => {
    // only trigger for new users [event.data.previous.exists()]
    // do not trigger on delete [!event.data.exists()]
    if (!event.data.exists() || event.data.previous.exists()) {
      return
    }

    const post = event.data.val()
    console.log(post)
    const { email, message } = post

    const data = {
      from: 'mailgun@firebase.com <is this free choice???>',
      subject: 'Hello Word!',
      html: `<p>It's alive!!!</p>`,
      'h:Reply-To': mail,
      to: '<myOwn>@gmail.com',
    }

    mailgun.messages().send(data, function(error, body) {
      console.log(body)
    })
  })

It keeps failing with the following message in the log

Function returned undefined, expected Promise or value

It's not possible to see more than this, which makes it hard to debug..

---

(I do some transpiling from ES6 before deploying to firebase functions)

---

Update

I abandoned the concept of sending an email and went for some IoT instead.

But I think that this might work for you (or get you some of the way)

const functions = require('firebase-functions')
const mailgun = require('mailgun-js')({
  apiKey: 'key-<######>',
  domain: '<mydomain>.com',
})

exports.sendEmail = functions.database
  .ref('someParent/someChild')
  .onWrite(event => {
    // Do some checking of data if needed. 

    const post = event.data.val()
    console.log(post)
    const { email, message } = post

    const data = {
      from: 'mailgun@firebase.com <is this free choice???>',
      subject: 'Hello Word!',
      html: `<p>It's alive!!!</p>`,
      'h:Reply-To': mail,
      to: '<myOwn>@gmail.com',
    }

    return mailgun.messages().send(data, (error, body) => {
      console.log(body)
    })
  })