Core Modules & Async›Pro· 30 min read

Events & the Event Loop

How Node stays fast: the event loop and the EventEmitter pattern that powers everything.

What you will learn

Understand the event loop at a high level
Use the EventEmitter
Connect this to async behaviour

Why Node is non-blocking

Node uses a single event loop that handles many operations without waiting. When a slow task (a file read, a database query) finishes, Node fires an event and runs your callback. This is why Node handles thousands of connections efficiently.

EventEmitter

Much of Node is built on events: one part of your code announces that something happened, and other parts that were listening react to it. It is like a doorbell — pressing it (emitting) makes anyone who is listening come to the door. You create this pattern yourself with the built-in events module.

Emit and listen for a custom event

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

emitter.on("greet", (name) => {
  console.log("Hello,", name);
});

emitter.emit("greet", "Asha");   // logs: Hello, Asha

Note: Output: Hello, Asha Step by step: new EventEmitter() makes an object that can send and receive events. emitter.on("greet", ...) listens for an event named "greet" and says what to do when it happens. emitter.emit("greet", "Asha") fires that event and passes "Asha" along — which arrives as the name in the listener, so it prints Hello, Asha. If nothing were listening for "greet", emitting it would simply do nothing.

Tip: Servers, streams and many Node objects are EventEmitters — when you write server.on("request", ...) you are using this exact pattern.

Q. What lets Node handle many operations without waiting around?

Answer: Node’s event loop processes operations asynchronously, running callbacks when tasks complete instead of blocking — keeping it fast.

✍️ Practice

Create an EventEmitter, listen for a custom event, and emit it with data.
Add two listeners for the same event.

🏠 Homework

Read about the event loop and write a 3-sentence summary in your own words.