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

  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);
  };
});

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 intersted in it.