What do you know about the EventEmitter class in Node.js?

EventEmitter is a class in Node.js that underpins many built-in modules (such as http, fs) and is used to implement the event-driven model.

Main features:

• Event subscription: Allows registering functions (handlers) that will be executed when a specific event occurs.
• Event emission: Enables calling (emitting) events, triggering registered handlers.
• Data passing: When emitting an event, any data can be passed to handlers.

Main methods:

• emitter.on(eventName, listener): Adds a handler listener for the event eventName.
• emitter.once(eventName, listener): Adds a one-time handler listener for the event eventName. It will be called only once upon the first occurrence of the event.
• emitter.emit(eventName[, ...args]): Calls all handlers registered for the event eventName, passing them the arguments ...args.
• emitter.removeListener(eventName, listener): Removes a specific handler listener for the event eventName.
• emitter.removeAllListeners([eventName]): Removes all handlers for the event eventName. If eventName is not specified, removes all handlers for all events.

Example usage:

const EventEmitter = require('events');

class MyEmitter extends EventEmitter {}

const myEmitter = new MyEmitter();

myEmitter.on('event', (a, b) => {
  console.log('Event occurred!', a, b);
});

myEmitter.emit('event', 'argument a', 'argument b'); // Will output: Event occurred! argument a argument b

EventEmitter implements the "Observer" pattern. This allows building loosely coupled systems where components interact through events without knowing about each other's internal implementation.