index.js

"use strict";

// Imports dependencies and set up http server
const express = require("express"),
  bodyParser = require("body-parser"),
  app = express().use(bodyParser.json()); // creates express http server

// Creates the endpoint for our webhook

app.get("/", (req, res) => {
  res.send("Hello world");
});

app.post("/webhook", (req, res) => {
  let body = req.body;

  // Checks this is an event from a page subscription
  if (body.object === "page") {
    // Iterates over each entry - there may be multiple if batched
    body.entry.forEach(function(entry) {
      // Gets the message. entry.messaging is an array, but
      // will only ever contain one message, so we get index 0
      let webhook_event = entry.messaging[0];
      console.log(webhook_event);
    });

    // Returns a '200 OK' response to all requests
    res.status(200).send("EVENT_RECEIVED");
  } else {
    // Returns a '404 Not Found' if event is not from a page subscription
    res.sendStatus(404);
  }
});

// Adds support for GET requests to our webhook
app.get("/webhook", (req, res) => {
  // Your verify token. Should be a random string.
  let VERIFY_TOKEN = "EAAD2k7Jkpr8BAHUaypTu7O0Hb6wAGzxkDtuYje9UREw61VY7N6mggtzRBz6v4z4ciElUSdzDg2WSSDCNT7wkULhZBg4xkZBYdS33PO7cyxcstJx6G2aKiyjHVFW5hF8KplweIdUhCS6NrNROlJgfNp1o0399ZAfT2hZCjrPpggZDZD";

  // Parse the query params
  let mode = req.query["hub.mode"];
  let token = req.query["hub.verify_token"];
  let challenge = req.query["hub.challenge"];

  // Checks if a token and mode is in the query string of the request
  if (mode && token) {
    // Checks the mode and token sent is correct
    if (mode === "subscribe" && token === VERIFY_TOKEN) {
      // Responds with the challenge token from the request
      console.log("WEBHOOK_VERIFIED");
      res.status(200).send(challenge);
    } else {
      // Responds with '403 Forbidden' if verify tokens do not match
      res.sendStatus(403);
    }
  }
});

// Sets server port and logs message on success
app.listen(process.env.PORT || 1337, () => console.log(`webhook is listening`));