🧠 Node.js – Events, Event Loop & EventEmitter – Master Asynchronous Flow in Node.js

🧲 Introduction – What Are Events and EventEmitters in Node.js?

Node.js is built on non-blocking, asynchronous programming, and its event-driven architecture is the key to this model. The Event Loop allows Node.js to handle multiple operations without multi-threading, and the EventEmitter class lets developers implement their own custom event handling system.

🎯 In this guide, you’ll learn:

• How the Node.js event loop works
• What EventEmitter is and how to use it
• Real-world event-based examples
• Best practices for building event-driven applications

🔁 What Is the Event Loop in Node.js?

The Event Loop is the engine that keeps Node.js non-blocking. It listens for events and executes callback functions when those events are triggered.

✅ Key Concepts:

📌 The event loop allows Node.js to pause and resume tasks without blocking the main thread.

📣 What Is EventEmitter in Node.js?

The EventEmitter is a class in the events module that allows you to create, emit, and listen for custom events.

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

🧪 Example – Basic EventEmitter Usage

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

// Register an event listener
emitter.on('greet', (name) => {
  console.log(`Hello, ${name}!`);
});

// Emit the event
emitter.emit('greet', 'Node.js');

🧪 Output:

Hello, Node.js!

🧱 EventEmitter – Core Methods

📘 Example – once() for One-Time Events

const emitter = new EventEmitter();

emitter.once('login', (user) => {
  console.log(`${user} logged in`);
});

emitter.emit('login', 'Vaibhav');
emitter.emit('login', 'AnotherUser'); // No output

🧪 Output:

Vaibhav logged in

🔄 Event Flow – How It All Works Together

1. A module or function emits an event using emit().
2. The event loop places it in the queue.
3. Registered listener callbacks are executed.
4. Non-blocking code continues immediately.

🧪 Real Use Case – File Watcher with Events

const fs = require('fs');
const EventEmitter = require('events');
const watcher = new EventEmitter();

fs.watch('myFile.txt', () => {
  watcher.emit('fileChanged');
});

watcher.on('fileChanged', () => {
  console.log('File was changed!');
});

🧪 Output (when file changes):

File was changed!

🧰 Best Practices – EventEmitter & Loop

📌 Summary – Recap & Next Steps

Node.js uses an event-driven model where the Event Loop manages I/O and timers efficiently. The EventEmitter class helps you implement custom event-based architectures in your applications.

🔍 Key Takeaways:

• The Event Loop runs behind the scenes to process async events
• EventEmitter allows you to listen and react to events in code
• Use .on(), .once(), and .emit() to manage event flow
• Ideal for chat apps, file monitors, sockets, and real-time systems

⚙️ Real-world relevance:
Widely used in server apps, WebSocket communication, logging systems, and automation scripts.

❓FAQs – Node.js Events, Event Loop & EventEmitter

❓ Is Node.js single-threaded or multi-threaded?
✅ Node.js runs on a single thread, but uses the Event Loop to handle async operations non-blockingly.

❓ What’s the difference between setTimeout() and setImmediate()?
✅ setTimeout() schedules after the specified delay, while setImmediate() runs in the next loop phase after I/O.

❓ Can I create multiple EventEmitters?
✅ Yes. Each instance of EventEmitter is independent and can manage its own events.

❓ What happens if I emit an event with no listeners?
✅ Nothing happens. Node.js will silently ignore it unless it’s an 'error' event (which throws by default if unhandled).

❓ How many listeners can I add to an EventEmitter?
✅ By default, Node.js allows 10 listeners per event. Use emitter.setMaxListeners(n) to change the limit.