For - https://medium.com/p/2801f6ab93b4

slack-events-listener.js

var slackEvents = require('slack-events-listener')('<verificationToken>', onSlackEvent);
var bodyParser = require('body-parser');
var app = require('express')();

function onSlackEvent(event, cb) {
  // do something. call cb with err if you want Slack to resend the message (your database might be down)
  writeToDatabase(event, cb);
}

// /slack_events should match whatever webhook you set in Slack
app.use('/slack_events', bodyParser.json(), slackEvents);