server.js

// Imports the Node.js standard library for data transfer.
const http = require('http')

// Imports the methods from the bot.js file.
const bot = require('./bot')

/**
 * A function which handles server requests.
 *
 * Information about the classes and events utilized in
 * this function can be found here:
 * https://nodejs.org/api/http.html
 *
 * Information about the GroupMe API can be found here:
 * https://dev.groupme.com/tutorials/bots
 *
 * @param {http.IncomingMessage} request - Represents a request
 *                                         which was just made to
 *                                         the server.
 * @param {http.ServerResponse} response - Represents the response
 *                                         which will be sent
 *                                         back to the requester. 
 */
const requestListener = (request, response) => {
  /* Checks that the request made to the server is a POST request.
   * The GroupMe API specifies that messages will be sent in the
   * form of POST requests so if any request which is made to us
   * is not a POST request then it must have been made in error.
   */
  if (request.method === 'POST') {
    /* The request made to the server will be received in chunks
     * where each chunk is a string. To obtain the full request
     * as one string the data variable will be initially set to
     * the empty string and will be appended with each chunk
     * until it contains the full request string.
     */
    let data = '';

    /* The http.IncomingMessage object allows for registering
     * callbacks for certain events. One of those events is the
     * 'data' event. This event occurs when another chunk from
     * the request has just come in. When this happens our
     * callback function will be called with the new chunk
     * passed to our callback as a string parameter.
     */
    request.on('data', chunk => {
      /* Appends the new chunk of the request to our data
       * variable so that eventually the data variable will
       * contain the full request string.
       */
      data += chunk;
    })

    /* Another event of the http.IncomingMessage object is
     * the 'end' event. This event occurs when all of the chunks
     * of the request have come in. When this happens our callback
     * function will be called.
     */
    request.on('end', () => {
      /* The GroupMe API specifies that messages will be sent as
       * strings representing JSON objects. JavaScript has a
       * standard library function, JSON.parse, which takes a JSON
       * string as a parameter, and returns an object. The full
       * request string in the data variable is parsed into an
       * object here.
       */
      const message = JSON.parse(data)

      /* Responds to the request with a 200 status code, meaning
       * 'OK', and indicates that the request was successfully
       * received. It is considered good practice to do this.
       * Calling the end function on the response object indicates
       * that the response should be sent to the requester.
       */
      response.writeHead(200, {'Content-Type': 'text/plain'})
      response.write('OK')
      response.end()

      /* Passes the parsed message object to the messageListener
       * function from the bot.js file for handling.
       */
      bot.messageListener(message)      
    })
  } else {
    /* Responds to the invalid request with a 405 status code,
     * meaning 'Method Not Allowed', and indicates the only request
     * method allowed is POST. This is done using the
     * http.ServerResponse object which was passed to this function.
     * Calling the end function on the response object indicates that
     * the response should be sent to the requester.
     */
    response.writeHead(405, {'Content-Type': 'text/plain'})
    response.write('Only POST requests are allowed')
    response.end()
  }    
}

/* Creates an http.Server object from the requestListener function
 * defined above and instructs the server to listen on the proper
 * port. Heroku decides on what port to use and sets the PORT
 * environment variable prior to when the application begins to run.
 */
const server = http.createServer(requestListener)
server.listen(process.env.PORT)