EventEmitter & event-driven design

Let’s do a deep dive into EventEmitter and event-driven design in Node.js. This is core to Node’s asynchronous architecture.

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1️⃣ What is Event-Driven Design?

• Event-driven programming = design pattern where code reacts to events rather than executing sequentially.

• Instead of directly calling a function, you emit events and listen for them.

• Enables decoupling, asynchronous handling, and non-blocking I/O.

Node.js itself is heavily event-driven (think: HTTP requests, streams, timers, I/O callbacks).

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2️⃣ EventEmitter Basics

• EventEmitter is a core Node.js class (require('events'))

• Allows registering listeners and emitting events

Import and Create Emitter

const EventEmitter = require("events");
const myEmitter = new EventEmitter();

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Listening to an Event

myEmitter.on("greet", (name) => {
  console.log(`Hello, ${name}!`);
});

• .on(eventName, listener) → register listener for an event

• Multiple listeners can listen to the same event

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Emitting an Event

myEmitter.emit("greet", "Tyson"); 
// Output: Hello, Tyson!

• .emit(eventName, ...args) triggers all listeners for that event

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3️⃣ One-Time Event Listeners

• .once → listener fires only once, then removed automatically

myEmitter.once("welcome", (user) => {
  console.log(`Welcome, ${user}`);
});
 
myEmitter.emit("welcome", "Tyson"); // fires
myEmitter.emit("welcome", "Alice"); // ignored

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4️⃣ Removing Listeners

const listener = (msg) => console.log(msg);
 
myEmitter.on("message", listener);
myEmitter.removeListener("message", listener);

• Prevents memory leaks in long-running apps

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5️⃣ Practical Example: Event-Driven HTTP Server

const EventEmitter = require("events");
const http = require("http");
 
const serverEvents = new EventEmitter();
 
serverEvents.on("requestReceived", (url) => {
  console.log(`Incoming request for ${url}`);
});
 
const server = http.createServer((req, res) => {
  serverEvents.emit("requestReceived", req.url);
  res.end("Hello, world!");
});
 
server.listen(3000, () => console.log("Server running on port 3000"));

• The HTTP server emits custom events

• You can handle logging, metrics, or analytics without cluttering the main request handler

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6️⃣ EventEmitter with Streams

• Node streams are built on EventEmitter

• Examples:

  • readable.on("data", chunk => ...)

  • writable.on("finish", () => ...)

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7️⃣ Key Takeaways

1. Node.js is event-driven at its core

2. EventEmitter allows decoupling logic and asynchronous handling

3. .on = register listeners, .emit = trigger events

4. .once = one-time listener

5. Removing listeners prevents memory leaks

6. Event-driven design is widely used in HTTP servers, streams, WebSockets, and microservices

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Next, we can go through child processes & clustering in Node.js, which is about scaling Node apps beyond a single thread.

Do you want to go there next?