how to make a Facebook messenger subscription with Node-Schedule

I'm building a chatbot which shows a school meal menu for students. I've already received a permission from facebook, so my bot is allowed to send a message to users at anytime. But the problem is how to make a subscription feature.

I use Node.js and MongoDB. My current method of enabling subscription is really inefficient. Here's some parts of my codes:

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

var isTimer = false; 
var schedule = require('node-schedule');

var morningTime; 
var lunchTime; 
var dinnerTime; 
var snackTime; 

var mongo = require("mongodb"); 
var MongoClient = require("mongodb").MongoClient; 
var url = 'mongodb://greenblatt:kinesis1717!@hafscluster-shard-00-00-cyacf.mongodb.net:27017,hafscluster-shard-00-01-cyacf.mongodb.net:27017,hafscluster-shard-00-02-cyacf.mongodb.net:27017/mbot?ssl=true&replicaSet=hafsCluster-shard-0&authSource=admin'

var app = express();
app.use(bodyParser.urlencoded({extended: false}));
app.use(bodyParser.json());
app.listen((process.env.PORT || 5000));

app.get("/", function (req, res) {
  res.send("Deployed!");
});


app.get("/webhook", function (req, res) {
  if (req.query["hub.verify_token"] === process.env.VERIFICATION_TOKEN) {
    console.log("Verified webhook");
    res.status(200).send(req.query["hub.challenge"]);
  } else {
    console.error("Verification failed. The tokens do not match.");
    res.sendStatus(403);
  }
});

app.post('/webhook', function (req, res) {
  var data = req.body;

  if (data.object === 'page') {

    data.entry.forEach(function(entry) {
      var pageID = entry.id;
      var timeOfEvent = entry.time;

      entry.messaging.forEach(function(event) {
        if (event.message) {
          receivedMessage(event);
        } else if(event.postback){
          receivedPostback(event);
        }
        else {
          console.log("Webhook received unknown event: ", event);
        }
      });
    });

    res.sendStatus(200);
  }
});

function setSubscribe(senderID){

  if(isTimer){

  } else{

    MongoClient.connect(url, function(err, db){
      if (err) throw err;

      db.collection('users', function(err, collection) {
          collection.find({"psid": senderID}).toArray(function(err, result) {

          if(!result[0]){
            db.collection("users").insertOne({psid: senderID,isSub:false}, function(err, res) {
            if (err) throw err;
            });
          }
            else if(result[0].isSub === true){

              morningTime = schedule.scheduleJob('00 08 * * *', function(){
                sendGenericMessage(senderID, "pmorning");
              });
              lunchTime = schedule.scheduleJob('00 13 * * *', function(){
                sendGenericMessage(senderID, "plunch");
              });
              dinnerTime = schedule.scheduleJob('00 18 * * *', function(){
                sendGenericMessage(senderID, "pdinner");
              });
              snackTime = schedule.scheduleJob('20 20 * * *', function(){
                sendSnack(senderID); 
              });

            }

          });
          db.close(); 
      });
    })
    isTimer = true; 
  }
}

function receivedMessage(event) {
  var senderID = event.sender.id;
  psid = senderID; 
  var recipientID = event.recipient.id;
  var timeOfMessage = event.timestamp;
  var message = event.message;

  console.log("Received message for user %d and page %d at %d with message:", 
    senderID, recipientID, timeOfMessage);
  console.log(JSON.stringify(message));

  var messageId = message.mid;
  var messageText = message.text;
  var messageAttachments = message.attachments;

  setSubscribe(senderID); 

  if (messageText) {
    switch (messageText) {
      case 'Check': 
        sendTextMessage(senderID, "psid: " + psid);
        sendTextMessage(senderID, "isTimer: " + isTimer);
        break; 

But whenever I revise my app.js(the code above) and push it, node-schedule stops and subscription doesn't work. 1. So how should I solve it? 2. Who should I check if the user subscribed? (there's a field 'isSub' in db)

via brain_team