Event Emitter

Overview

Event emitters are objects that trigger an event by sending a message when an action occurs (therefore, this is pub/sub).

• we can write code that listens to events from event emitters and execute some code in response.

The typical modern way in Javascript to perform some action after completion of another is with promises.

• a downside of promises is that they tightly couple the triggering action to the resulting action, making it difficult to modify the resulting action in the future. This is a general benefit that is offered by Pub-Sub patterns over more traditional call-and-response approaches.
  • expl: If we wanted to change the behavior of our application, we could adjust how the subscribers react to the events without having to change the publisher.

the emit() function returns true if there are listeners for the event. If there are no listeners for an event, it returns false.

Sometimes we’re only interested in listening to the first time an event was fired, as opposed to all the times it’s emitted. Node.js provides an alternative to on() for this case with the once() function.

• when the event is emitted and received by a listener that uses once(), Node.js automatically removes the listener and then executes the code in the callback function.

Creating Event Emitters

There are two common ways to create an event emitter in Node.js.

1. use an event emitter object directly
   • Do this if the events you want to emit are independent of your business objects or are a result of actions from many business objects
2. create an object that extends the event emitter object
   • Do this if the events you want to emit are an effect of an object's actions, in order to have access to its functions for convenience.
     • ex. if we are making a TicketManager class, we could have a method buy which performs some database interaction, but then emits the buy event. This is a good use-case for extending the EventEmitter

const EventEmitter = require("events");

class TicketManager extends EventEmitter {
    constructor(supply) {
        super();
        this.supply = supply;
    }
    
    buy(email, price) {
        this.supply--;
        this.emit("buy", email, price, Date.now());
    }
}

UE Resources

Event-emitters