Event Module in Node.js

Key Concepts:

• EventEmitter: This class is the core of the events module, allowing you to create, listen for, and trigger events.
• emit(): Emits an event, triggering all the listeners attached to that event.
• on(): Attaches a listener function to a specific event.

Usage of the Event Module

Here’s a basic example demonstrating how to use the events module:

1. Import the EventEmitter class:

   const EventEmitter = require('events');
   

2. Create an instance of EventEmitter:

   const eventEmitter = new EventEmitter();
   

3. Register an event listener using on():

   eventEmitter.on('greet', () => {
       console.log('Hello, world!');
   });
   

4. Trigger an event using emit():

   eventEmitter.emit('greet');
   // Outputs: Hello, world!
   

Example: Passing Data with Events

You can also pass data when emitting an event:

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

// Register listener with arguments
eventEmitter.on('greet', (name) => {
    console.log(`Hello, ${name}!`);
});

// Emit the event with a parameter
eventEmitter.emit('greet', 'John');
// Outputs: Hello, John!

Removing Event Listeners

You can remove event listeners using removeListener() or removeAllListeners():

const greet = () => {
    console.log('Hello, world!');
};

eventEmitter.on('greet', greet);

// Remove the listener
eventEmitter.removeListener('greet', greet);

EventEmitter Methods:

• on(event, listener): Attaches a listener for the specified event.
• emit(event, [arg1], [arg2], [...]): Emits the specified event, calling all attached listeners.
• once(event, listener): Attaches a listener that is called at most once.
• removeListener(event, listener): Removes the specified listener.
• removeAllListeners([event]): Removes all listeners for the event.
• listeners(event): Returns an array of listeners for the event.

Example: Using once()

The once() method attaches a listener that is called only once:

eventEmitter.once('greetOnce', () => {
    console.log('This will only be logged once');
});

eventEmitter.emit('greetOnce');  // Outputs: This will only be logged once
eventEmitter.emit('greetOnce');  // No output

Inheriting from EventEmitter

In Node.js, it's common to extend the EventEmitter class to make custom classes emit events:

const EventEmitter = require('events');

class MyEmitter extends EventEmitter {}

const myEmitter = new MyEmitter();

// Register event listener
myEmitter.on('event', () => {
    console.log('An event occurred!');
});

// Emit the event
myEmitter.emit('event');
// Outputs: An event occurred!

Real-World Usage

The events module is widely used in Node.js for handling things like:

• Streams: Both readable and writable streams are event emitters.
• HTTP Requests: The request and response objects in HTTP are also event emitters.
• Timers: You can listen to setTimeout, setInterval, etc., via events.

Thank you