“In computer programming, event-driven programming is a programming paradigm in which the flow of the program is determined by
In simpler terms, events are actions that are performed with the use of computers and computer resources. An event is fired when you shut down your PC, open a browser, click the like button of a webpage, connect to a printer, etc.
NodeJs applications also fire events. For example:
I believe you now have an idea of what events are.
const EventEmitter = require("events");
const myEventEmitter = new EventEmitter();
In the code above, we required the events module that allows us to create an event emittermyEventEmitter from the constructor function EventEmitter().
Note: An event emitter emits or fires up an event.
Now, let’s register an event. We can do this by using .on or addListener. Registering an event takes two arguments: the event’s name and the function to be called when the event is fired.
Then, we use the emit() method to fire the events. You can see this in action below:
const EventEmitter = require("events");const myEventEmitter = new EventEmitter();myEventEmitter.on("open", ()=>{console.log("Event open emitted!");})myEventEmitter.addListener("close", ()=>{console.log("Event close emitted!");})myEventEmitter.emit("open");myEventEmitter.emit("close");
You can call the same event, but with different functions, to be executed. Note that all the event functions must be performed at one call. Therefore, the following will fire twice and call the event functions twice:
const { EventEmitter } = require('events')const eventEmitter = new EventEmitter()eventEmitter.on('my-event', () => {console.log('my-event fired with first function')})eventEmitter.on("my-event", ()=>{console.log('my-event fired with second function')})// HERE WE FIRE THE EVENTSeventEmitter.emit('my-event')eventEmitter.emit('my-event')
We make an event only fire once with the once method or use the removeListener to remove the listener.
A function will only be called the first time an event is fired – the second emit() method won’t be called. This is shown in the code below.
const { EventEmitter } = require('events')const eventEmitter = new EventEmitter()eventEmitter.once('my-event', () => { console.log('my-event fired') })eventEmitter.emit('my-event')eventEmitter.emit('my-event') // The event here will not be fired!
const EventEmitter = require("events");const myEventEmitter = new EventEmitter();const func1 = ()=>{console.log("Function 1 called!")};const func2 = ()=>{console.log("Function 2 called!")}myEventEmitter.on("connect", func1);myEventEmitter.on("connect", func2);myEventEmitter.emit("connect");myEventEmitter.removeListener("connect", func1);myEventEmitter.emit("connect");
This takes no argument. It is used to remove all events that are registered to an event emitter.
const EventEmitter = require("events");const myEventEmitter = new EventEmitter();myEventEmitter.on("load", ()=>{console.log("Web page loaded!")})myEventEmitter.on("close", ()=>{console.log("Web page closed!")})myEventEmitter.emit("load");myEventEmitter.removeAllListeners();myEventEmitter.emit("close"); //This will not fire or run!
We have seen how to create an event emitter, register an event, and fire up an event using the events built-in module of NodeJs. We also learnt how to fire an event once with the once() or removeListener() method. Lastly, we were able to remove all events attached to an emitter using removeAllListeners().