Event-Driven Programming in Node.js

Robert Lane

Event-Driven Programming is a logical pattern that we can choose to confine our programming within to avoid issues of complexity and collision. In this article we’re going to go over how Event-Driven Programming works and how we can make the best use of it in our Node.js projects.

Most developers are introduced to concepts of Event-Driven Programming early on in their study of programming yet they might not fully realize it until a bit later. You’ll find that the concept is rather ubiquitous. Check any major framework or software out there and odds are you’ll find evidence of Event-Driven Programming.

Overview

For the most recognizable example of Event-Driven Programming for people at any level of programming skill, we’ll turn to our old friend The Web Browser.

Every time you interact with a webpage through it’s user interface, an event is happening. When you click a button a click event is triggered. When you press a key a keydown event is triggered. These events have associated functions that, when triggered, are executed to make a change to the user interface in some way.

Event-Driven Programming makes use of the following concepts:

  • An Event Handler is a callback function that will be called when an event is triggered.
  • A Main Loop listens for event triggers and calls the associated event handler for that event.

EventEmitter

Node.js natively provides us with a useful module called EventEmitter that allows us to get started incorporating Event-Driven Programming in our project right away. Of course, creating our own version of EventEmitter wouldn’t be much of a challange, and in fact there are several modules published on npm such as EventEmitter2 and EventEmitter3 which promise a faster performance than the native EventEmitter.

Those are both worth checking out if your project needs to run faster than EventEmitter will allow. They are both built to allow for syntax that is almost identical to what we’ll use for EventEmitter so learning one will make it easy to work with all of them.

We access the EventEmitter class through the events module. Once imported we’ll need to create a new object from the class to start using it.

const EventEmitter = require('events').EventEmitter;
const myEventEmitter = new EventEmitter;

Now we can get started with Event-Driven Programming in Node.

Imagine we’re creating a chat room. We want to alert everyone when a new user joins the chat room. We’ll need an event listener for a userJoined event. First, we’ll write a function that will act as our event listener, then we can use EventEmitters on method to set the listener.

const EventEmitter = require('events').EventEmitter;
const chatRoomEvents = new EventEmitter;

function userJoined(username){
  // Assuming we already have a function to alert all users.
  alertAllUsers('User ' + username + ' has joined the chat.');
}

// Run the userJoined function when a 'userJoined' event is triggered.
chatRoomEvents.on('userJoined', userJoined);

The next step would be to make sure that our chat room triggers a userJoined event whenever someone logs in so that our event handler is called. EventEmitter has an emit method that we we use to trigger the event. We would want to trigger this event from within a login function inside of our chatroom module.

function login(username){
  chatRoomEvents.emit('userJoined', username);
}

We could expand further by creating events for when a user logs out, when a message is sent, when a message is received, or any other event we could possibly need for our chat room to be as dynamic as we want it.

Removing Listeners

There will likely come a time when you want to remove an event listener from an event. This could be for performance reasons (the event is no longer needed) or to avoid memory leaks (if an event listener references an object that is no longer needed, it won’t be able to be garbage-collected. This can lead to a build up of unnecessary objects).

To remove event listeners in EventEmitter we can use the removeListener or removeAllListeners method. It’s important to note that in the EventEmitter that comes built-in with Node you must pass a reference to the exact function you wish to remove when using the removeListener method. This means wherever you wish to remove the event, you’ll need to make sure the function is able to be referenced from that place in your code. For this reason it is often best to name your event handling functions and declaring them before you register the event listener, as opposed to leaving them anonymous.

In the following example, it would be a challenge to remove the listener for the message event from outside of the userJoined function due to the fact that it’s an anonymous function declared within a closure. In this case the only place we would be able to directly reference this method would be in the EventEmitter Object itself. This would be impractical if we ever had more than one listener registered to a single event as we would then have to figure out a way to decipher which of the listeners is our intended target.

const EventEmitter = require('events').EventEmitter;
const chatRoomEvents = new EventEmitter;

function userJoined(username){
  chatRoomEvents.on('message', function(message){
    document.write(message);
  })
}

chatRoomEvents.on('userJoined', userJoined);

All of that headache can be avoided if we rewrite the code like so:

const EventEmitter = require('events').EventEmitter;
const chatRoomEvents = new EventEmitter;

function displayMessage(message){
  document.write(message);
}

function userJoined(username){
  chatRoomEvents.on('message', displayMessage);
}

chatRoomEvents.on('userJoined', userJoined);

Now if we want to remove the displayMessage function from the message event’s list of handlers:

chatRoomEvents.removeListener('message', displayMessage);

Object Oriented Programming + Event-Driven Programming

The last thing I want to touch on here is the combination of the Object Oriented and Event-driven programming paradigms. These two make for a very valuable combination in a wide variety of situations and I think it can be beneficial to understand and conceptualize why.

The Object Oriented approach promotes the idea that all behavior of an individual unit (or object) be handled from code within that unit. Using this approach, applications are built with many different units that all speak to and interact with each other.

Imagine we’re building a mail application. We might have an object whose sole purpose is to process the incoming and outgoing mail messages for our client. This object would contain all of its own behavioral functions. We might have a sendMail function that delivers our mail to a server. We might also have a receiveMail function that tells the server to deliver us any new mail it has for us. We’ll call the object responsible for these server interactions our Mailbox.

const Mailbox = {
  sendMail: function(){
    // code to send mail.
  },
  receiveMail: function(){
    // check server for new mail.
  }
}

Now if we are building other units that want to make use of our sendMail function, they can access it through Mailbox.sendMail. This is a standard approach. But what happens when we later-on decide we also want to log the mail every time we send it? We now need to either modify our sendMail function to incorporate this behavior, or create another function that is triggered immediately after the sendMail function. In which case the object responsible for triggering the sendMail function must be sure to also trigger the log function. As our applications get more complex, and as new sequences of behavior are added, this could get somewhat out of hand.

This is where we can make use of Event-driven programming. By registering event listeners we can actually reverse the flow of communication between our objects. Rather than on object needing to reach inside another object to trigger a function, our objects can just emit events and whichever objects are listening to those event will process it in the way they have been told to. The source of an objects behavior is now entirely contained within itself, rather than needing to be accessed by external objects.

Let’s imagine we have a hungry alligator, represented by the gator class. Using a non-event-driven approach, the process of eating for our alligator looks like this:


class Food {
  constructor(name) {
    this.name = name;
  }

  becomeEaten() {
    return 'I have been eaten.';
  }
}

var bacon = new Food('bacon');

class gator {
  eat() {
    bacon.becomeEaten();
  }
}

In this example, our gator had to access the methods inside of Food in order to eat. This is a lot of work for our lazy gator so we’re going to make things easier for him.

const EventEmitter = require('events').EventEmitter;
const myGatorEvents = new EventEmitter;

class Food {
  constructor(name) {
    this.name = name;
    // Become eaten when gator emits 'gatorEat'
    myGatorEvents.on('gatorEat', this.becomeEaten);
  }

  becomeEaten(){
    return 'I have been eaten.';
  }
}

var bacon = new Food('bacon');

const gator = {
  eat() {
    myGatorEvents.emit('gatorEat');
  }
}

Now all our gator has to do is just say gatorEat and the EventEmitter takes care of the rest.

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