Serverless approach to deliver Emails to Slack using Mailgun & Google Cloud Functions

index.js

//Disclaimer: this code is a quick and dirty proof of concept and I am not a JavaScript developer
//see the full story on Medium: https://medium.com/@schafele/serverless-firefighting-deliver-emails-to-slack-using-mailgun-google-cloud-functions-a7e10475d1a2

const channel = "#yourchannel";
const username = "alarm";
const token = "your token";

const { WebClient } = require('@slack/client');
const web = new WebClient(token);

const sendSlackMessage = function(subject,msg) {
  return new Promise((resolve, reject) => {
    console.log('sending slack message...');
    web.chat.postMessage({
      channel: channel,
      username: username,
      "attachments": [
          {
            text: msg,
            fallback: subject,
            "author_name": "Operator",
            title: subject
          }
        ]
    }).then((res) => {
        console.log('Message sent: ', res.ts);
        resolve();
    }).catch(console.error);
  });
};

const sendSlackFiles = (uploads) => {
  return new Promise((resolve, reject) => {
    if (Object.keys(uploads).length > 0) {
      for (filename in uploads) {
        console.log('sending '+filename);
        web.files.upload({filename: 'Details', file: fs.createReadStream(uploads[filename]), channels: channel})
        .then((res) => {
          console.log('File uploaded: ', res.file.id);
          resolve();
        })
        .catch(console.error);
      }
    } else {
      resolve();
    }
  });
};

const path = require('path');
const os = require('os');
const fs = require('fs');
const Busboy = require('busboy');

exports.alarmEmail2slack = (req, res) => {
  if (req.method === 'POST') {

     const busboy = new Busboy({ headers: req.headers });
     const tmpdir = os.tmpdir();

     const fields = {};
     const uploads = {};

     busboy.on('field', (fieldname, val) => {
       fields[fieldname] = val;
       console.log(`Processed field ${fieldname}: ${val}.`);
     });

     busboy.on('file', (fieldname, file, filename) => {
       const filepath = path.join(tmpdir, filename);
       uploads[fieldname] = filepath;
       file.pipe(fs.createWriteStream(filepath));
     });

     busboy.on('finish', () => {
       console.log('input processed');

       if (fields.Subject !== undefined && fields.Subject.includes('secret subject')
       && fields.From !== undefined && fields.From === "secretemail@address.invalid") {
         const msg = (fields['body-plain'] !== undefined) ? fields['body-plain'] : 'No Msg';
         const subject = fields.Subject;
         Promise.all([ sendSlackMessage(subject, msg), sendSlackFiles(uploads)])
        .then(values => {

          //delete all uploaded files
          for (const name in uploads) {
             const file = uploads[name];
             fs.unlinkSync(file);
           }

           res.status(200).send('processed');

        });
      } else {
        res.status(200).send('nothing to process');
      }
     });

     busboy.end(req.rawBody); //this version works in production
     //req.pipe(busboy); //this version works in the emulator
   } else {
     res.status(405).end();
   }
};