In the previous section, you learned how to listen to those built-in events—things such as connection events and disconnection events. Those are fine, and they're a great starting place, but in this section, what we want to talk about is emitting and listening to custom events, and this is where Socket.io gets really interesting.
When you're able to emit and listen to custom events, you can send anything you want from the server to the client or from the client to the server. Now, to go through a quick example of how that'll look, we're going to use an example app, which will be an email application:
On the left, we have our server, which is starting up a Socket.io web server. On the right, we have our email app, which is showing a list of all our current emails. Now, one custom event that our app might require is a newEmail event:
The newEmail event is going to be emitted by the server when an email comes in. For example, if I sign up to a new service...