EventEmitter

EventEmitter is the base class behind HTTP, streams, and the bulk of Node's async APIs.

4 min read Level 1/5 #nodejs#events#emitter

What you'll learn

Emit and listen to custom events
Use `once` for single-fire
Convert events to promises

EventEmitter is the base class for Node’s pub/sub pattern. HTTP servers, streams, child processes — they all extend it.

A Custom Emitter

import { EventEmitter } from "node:events";

const bus = new EventEmitter();

bus.on("user-created", (user) => {
  console.log("welcome,", user.name);
});

bus.emit("user-created", { id: 1, name: "Ada" });
// welcome, Ada

on(event, fn) — subscribe
emit(event, ...args) — fire
off(event, fn) — unsubscribe

`once`

Listen for the next occurrence, then auto-remove:

bus.once("ready", () => console.log("first time only"));
bus.emit("ready");   // logs
bus.emit("ready");   // nothing — listener already removed

Multiple Listeners

You can subscribe many functions to one event:

bus.on("order", (o) => savePaymentTrail(o));
bus.on("order", (o) => sendEmail(o));
bus.on("order", (o) => updateDashboard(o));

bus.emit("order", { id: 42 });   // all three run

By default a max of 10 listeners per event triggers a warning. Bump it if needed:

bus.setMaxListeners(100);

Events → Promises

To await an event:

import { once } from "node:events";

const [user] = await once(bus, "user-created");
console.log("got", user);

once returns a promise that resolves with the event’s arguments as an array — perfect for “wait for a signal” patterns.

Real-World Use

import { createServer } from "node:http";

const server = createServer();

server.on("request",  (req, res) => res.end("hello"));
server.on("listening", () => console.log("up"));
server.on("error",     (e) => console.error("boom", e));

server.listen(3000);

Every Node API you’ll use has events under the hood.

The process Object →