Building a simple PubSub system in JavaScript

In a recent project building a web push service I wanted to have my UI respond to application level events (semantically if you will) because there were a couple of components that require information from the system but are not dependent with each other and I wanted them to be able to manage themselves independently of the 'business logic'.

I looked around at lots of different tools to help me, but because I frequently have a heavy case of NIH syndrome and the fact that I think people can implement their own infrastructural elements pretty quickly, I decided to quickly knock-up a simple client-side PubSub service — it worked pretty well for my needs.

I debated whether I should use a custom DOM Event and use the existing infrastructure that the DOM already provides to developers — the ability to events and consuming events using addEventListener — but the only problem was that you have to hang the event handler off a DOM Element or the window because you can't have a model that inherits or mixes in EventTarget.

Thought: having EventTarget as an object would help obviate the need for creating custom PubSub systems.

With this constraint in mind, a will to code something up, and a propensity for not minding bugs that I create myself, I sketched out a rough plan:

/* When a user is added, do something useful (like update UI) */
EventManager.subscribe('useradded', function(user) {
  console.log(user)
});

/* The UI submits the data, lets publish the event. */
form.onsubmit(function(e) {
  e.preventDefault();

  // do something with user fields

  EventManager.publish('useradded', user);
})

All of this isn't new. Redux and many other systems already do this and in many cases they also help you manage state. In my head, I don't really have state that needs a model that is septate to the state already in the browser.

The implementation is pretty simple to implement and the abstraction is quite useful for me at least.

var EventManager = new (function() {
  var events = {};

  this.publish = function(name, data) {
    var handlers = events[name];
    if(!!handlers === false) return;
    handlers.forEach(function(handler) {
      handler.call(this, data);
    });
  };

  this.subscribe = function(name, handler) {
    var handlers = events[name];
    if(!!handlers === false) {
      handlers = events[name] = [];
    }
    handlers.push(handler);
  };

  this.unsubscribe = function(name, handler) {
    var handlers = events[name];
    if(!!handlers === false) return;

    var handlerIdx = handlers.indexOf(handler);
    handlers.splice(handlerIdx);
  };
});

Edit: Removed the use of promise.

And there we are. A simple pubsub system that is likely full of bugs, but I like it. :) I've put it on github if you are interested in it.

About Me: Paul Kinlan

I lead the Chrome Developer Relations team at Google.

We want people to have the best experience possible on the web without having to install a native app or produce content in a walled garden.

Our team tries to make it easier for developers to build on the web by supporting every Chrome release, creating great content to support developers on web.dev, contributing to MDN, helping to improve browser compatibility, and some of the best developer tools like Lighthouse, Workbox, Squoosh to name just a few.