index.js

var express = require('express');
var app = express();
var bodyParser = require('body-parser');
var request = require('request');

app.use(express.static('static'));

// register body parser to express js in order to get the POST req.body data
app.use(bodyParser.json());

// replace wth your FB verification token
var token = process.env.FB_TOKEN

// GET /webhook will validate the request token and grant access to the FB application
app.get('/webhook/', function (req, res) {
  console.log('/webhook GET called')
  if (req.query['hub.verify_token'] === token) {
    console.log('webook challenge succeeded');
    res.send(req.query['hub.challenge']);
  }
  else{
    console.log('webook challenge failed')
    res.send('Error, wrong validation token');
  }
});

// POST /webhook will listen to the messages that are sent to the FB Page/App
app.post('/webhook/', function (req, res) {
  console.log('/webhook POST called')
  var messaging_events = req.body.entry[0].messaging;
  for (i = 0; i < messaging_events.length; i++) {
    var event = req.body.entry[0].messaging[i];
    var sender = event.sender.id;
    if (event.message && event.message.text) {
      text = event.message.text;
      console.log("Received msg : "+text);
      sendTextMessage(sender, "HODOR");
    }
  }
  res.sendStatus(200);
});

var port = process.env.PORT || 3010;

app.listen(port, function () {
  console.log('Example app listening on port %s!', port);
});

//This function will send back the messages to the sender
function sendTextMessage(sender, text) {
  var messageData = {
    text:text
  }
  request({
    url: 'https://graph.facebook.com/v2.6/me/messages',
    qs: {access_token:token},
    method: 'POST',
    json: {
      recipient: {id:sender},
      message: messageData
    }
  }, function(error, response, body) {
    if (error) {
      console.log('Error sending message: ', error);
    } else if (response.body.error) {
      console.log('Error: ', response.body.error);
    }
    else{
      console.log("Sent msg : "+text);
    }
  });
}