eolas/zk/events.md

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Programming Languages	backend node-js

Node.js events module

In most cases you won't interact with the events module directly since other modules and third-party modules are abstractions on top of it. For instance the http module is using events under the hood to handle requests and responses.

Another way of putting this is to say that all events in Node inherit from the EventEmitter constructor, which is the class you instantiate to create a new event. At bottom everything in Node is an event with a callback, created via event emitters.

Because Node's runtime is event-driven, it is event-emitter cycles that are being processed by the Event Loop, although you may know them as fs or http (etc) events. The call stack that the Event Loop works through is just a series of event emissions and their associated callbacks.

Event Emitters

• All objects that emit events are instances of the EventEmitter class. This object exposes an eventEmitter.on() function that allows one or more functions to be attached to named events emitted by the object.
• These functions are listeners of the emitter.

Basic syntax

const EventEmitter = require("events"); // import the module

// Raise an event
const emitter = new EventEmitter("messageLogged");

// Register a listener
emitter.on("messagedLogged", function () {
  console.log("The listener was called.");
});

• If we ran this file, we would see The listener was called logged to the console.
• Without a listener (similar to a subscriber in Angular) nothing happens.
• When the emission occurs the emitter works synchronously through each listener function that is attached to it.

Event arguments

• Typically we would not just emit a string, we would attach an object to the emitter to pass more useful data. This data is called an Event Argument.
• Refactoring the previous example:

// Raise an event
const emitter = new EventEmitter("messageLogged", function (eventArg) {
  console.log("Listener called", eventArg);
});

// Register a listener
emitter.on("messagedLogged", { id: 1, url: "http://www.example.com" });

Extending the EventEmitter class

• It's not best practice to call the EventEmitter class directly in app.js. If we want to use the capabilities of the class we should create our own module that extends EventEmitter, inheriting its functionality with specific additional features that we want to add.
• So, refactoring the previous example:

// File: Logger.js

const EventEmitter = require("events");

class Logger extends EventEmitter {
  log(message) {
    console.log(message);
    this.emit("messageLogged", { id: 1, url: "http://www.example.com" });
  }
}

The this in the log method refers to the properties and methods of EventEmitter which we have extended.

• We also need to refactor our listener code within app.js so that it calls the extended class rather than the EventEmitter class directly:

// File app.js

const Logger = require('./Logger')
const logger = new Logger()

logger.on('messageLogged', function(eventArg){
	console.log('Listener called', eventArg)
}

logger.log('message')