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Node.js - Events 

Prashant Surya  |   Nov 19, 2019  |  Views: 15

Description:

What are Events in Node.js


Events are the actions, by which we can handle our callback methods synchronously.

Events can be created, fired, and listen for any action name.

Objects in Node.js also emit events which are the instances of events.EventEmitter.

Example:
fs.readStream emits an event when a file is opened.

var fs = require('fs');
var rs = fs.createReadStream('./sample.txt');
rs.on('open', function () {
  console.log('File is open');
});


What is EventEmitter in Node.js?


EventEmitter creates the instance of an event.

If EventEmitter instance faces any error, it emits an 'error' event.


// Import events module
var events = require('events');
// Creates an eventEmitter object
var eventEmitter = new events.EventEmitter();

EventEmitter properties
  • addListener - add a new listener to the end of event listeners array
  • on -  to bind a function with event
  • once - adds one time listener, after fired once, its been removed
  • removeListener - removes a listener from the listener array
  • removeAllListeners - removes all listeners,
  • setMaxListeners - to limit the listeners in listeners array - default is 10
  • listeners - array of listeners of the event
  • emit - execute each of the listeners in order, with the supplied arguments


Example of EventListener and its methods:


Step 1. Create a demo file named events.js and paste the following code:

var events = require('events');
var eventEmitter = new events.EventEmitter();


// create a new listener function
var newListner1 = function newListner1() {
   console.log('newListner1 is executed.');
}


// create a another new listener function
var newListner2 = function newListner2() {
   console.log('newListner2 is executed.');
}


// Bind the yourEventName event with the listner1 function
eventEmitter.addListener('yourEventName', newListner1);



// Bind the yourEventName event with the newListner2 function
eventEmitter.on('yourEventName', newListner2);

var eventListeners = require('events').EventEmitter.listenerCount
   (eventEmitter,'yourEventName');

console.log(eventListeners + " Listner(s) listening to yourEventName event");


// Fire the yourEventName event 
eventEmitter.emit('yourEventName');

// Remove the binding of newListner1 function
eventEmitter.removeListener('yourEventName',  newListner1);
console.log("Now newListner1 will not listen");


// Fire the yourEventName event 
eventEmitter.emit('yourEventName');


//Now check the new count after removing
eventListeners = require('events').EventEmitter.listenerCount(eventEmitter,'yourEventName');
console.log(eventListeners + " Listner(s) listening to yourEventName event");

Note: yourEventName will be the name of your event, for which you want to the add listeners.


Step 2. Run this file with node.js
node events.js

Step 3. Verify Output:

2 Listner(s) listening to yourEventName event
newListner1 is executed.
newListner2 is executed.
Now newListner1 will not listen
newListner2 executed.
1 Listner(s) listening to yourEventName event


Comments:
Content Contributor
Prashant Surya

Prashant Surya

Software developer at Uniserve Data Technologies Pvt. Ltd.  |   Noida, Uttar Pradesh, India

I am a software developer, freelancer and trainer working on technologies likes. MEAN stack, MERN Stack, ASP.NET full stack, WebRTC, socket.io ,PWA, SQL, JavaScript, jQuery, HTML, CSS, SEO, responsive UI/UX designing... having 7 years of experience in development in multiple domains like, eCommerce, RTA, capital markets, education, real estate...


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