Node JS Events in Depth with Examples

Node JS runs code in an event-based model and uses events to drive tasks. An event is a signal that tells the program to run a task at a set time. The core module for events is called EventEmitter, and it gives a way to work with events.

Understand the EventEmitter in Node.js

EventEmitter is a built-in module that sends and receives named signals inside a Node.js app. It allows the code to fire custom actions and to react to them at once.

You create an EventEmitter object and define custom events with it, and then react to those events. You can set up a function that runs each time the event fires, and so control app flow.

The emit method sends out an event signal, and the on method listens to that signal. This model gives control over how parts of your code respond to events and data.

You can add many listeners to the same event, and you can also remove them later. The removeListener or off method stops a function from running when that event fires again.

You can link events so one fires after another and build a flow of actions.
This method is useful in tasks that need many steps and need an order between them.

You can catch errors in an EventEmitter by using a special error event.
This event lets the code react to faults and stop them from breaking the app.

Examples

Basic EventEmitter:

const EventEmitter = require('events');
const emitter = new EventEmitter();

emitter.on('start', () => {
  console.log('Start event fired');
});

emitter.emit('start');

This code creates an EventEmitter object and sets a listener for a “start” event and then fires it with emit to show a basic event flow.

Event with Data:

const EventEmitter = require('events');
const emitter = new EventEmitter();

emitter.on('message', (data) => {
  console.log('Message received:', data);
});

emitter.emit('message', 'Hello Node');

This code adds a listener for “message” and passes data with emit so the listener uses that data to run extra logic at run time.

Remove a Listener:

const EventEmitter = require('events');
const emitter = new EventEmitter();

function onConnect() {
  console.log('Connected');
}

emitter.on('connect', onConnect);
emitter.emit('connect');
emitter.off('connect', onConnect);
emitter.emit('connect');

This code attaches a listener, fires the event, then removes the listener, and fires the event again so nothing runs the second time.

Chain Events:

const EventEmitter = require('events');
const emitter = new EventEmitter();

emitter.on('step1', () => {
  console.log('Step 1');
  emitter.emit('step2');
});

emitter.on('step2', () => {
  console.log('Step 2');
});

emitter.emit('step1');

This code links two events so “step1” fires first and then triggers “step2” to build a sequence of actions inside the app.

Wrapping Up

You learned how Node JS handles events and how EventEmitter works and how to chain and remove events and how to handle errors.

Here is a quick recap:

• You can create an EventEmitter, add listeners with on, send events with emit, manage them with off, and handle errors with the error event.

FAQs

const EventEmitter = require('events');
const emitter = new EventEmitter();

emitter.on('greet', () => {
  console.log('Hello from Node JS Event!');
});

emitter.emit('greet');

const EventEmitter = require('events');
const emitter = new EventEmitter();

emitter.on('ping', () => {
  console.log('Ping received multiple times');
});

emitter.once('pong', () => {
  console.log('Pong received only once');
});

emitter.emit('ping');
emitter.emit('ping');
emitter.emit('pong');
emitter.emit('pong');

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