This is a series of posts for my CS3216 project, you can find the original post here.

Easy to over-enginner things, try and make things abstract/generic.

The process I’ve adopted for final project is to start with the most hacky way to do things — pretend this code is read-only. Besides, the module is so frantic that you barely have time to refine your code…

We are writing a chat app, and our choice of backend framework is Socket.io. It works by attaching event handlers to events, for example:

io.on('connect', socket => {
  socket.on('add', data => { console.log(data); });

This means that when the client (browser) sends an add message to the backend, the data associated with the event will be logged.

An example client call is:

io.emit('add', { hi: 'there'});

For certain events, such as adding a message, we needed to have a roomId. It is easy to add in this check:

io.on('connect', socket => {
  socket.on('add', data => {
    if (!data.roomId) { console.error('No Room Id'); }
    else { /* something else */ }

However, this gets unwieldy fast, because there are multiple events that will require a roomId, such as adding a reaction, sending typing and stop typing indicators, reporting a user, etc.

As I started to implement more events, I found myself copy-pasting code, specifically the checks for data.roomId.

Copy-pasting is a smell that we have reusable code — DRY.

So I began to think of ways to extract this logic into a common, reusable piece of code.

1 way is to extract this into a common function:

function ensureRoomId(data) {
  if (!data.roomId) { throw Exception('No room id'); }

But this doesn’t really save much code, it’s still 1 line of code, and now I have to handle an exception.

I eventually settled on a more functional way.

function ensureRoomId(socket, fn) {
  return function(data) {
    if (!data.roomId) {
      socket.emit('error_room_id', 'No room id specified');
    } else {

socket.on('add', ensureRoomId(socket, data => {
  // data is guaranteed to have roomId!

Now every event handler that requires a roomId can wrap itself with ensureRoomId, and as a result, we get a single place to define a error_room_id error! This helps a lot of consistency of error code and messages.

A little ugliness there is that we have to pass in socket into ensureRoomId because it needs to be able to emit an event to that socket but I find that pretty bearable for now (and don’t have a good way to solve it).