Events & EventEmitter

A lot of Node's own APIs — HTTP servers, streams, file watchers — are built around one core idea: an object that emits named events, and other code that listens for them. The events module exposes that same pattern for your own code.

Creating and using an EventEmitter

const EventEmitter = require("events");

const emitter = new EventEmitter();

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

emitter.emit("greet", "Priya");
emitter.emit("greet", "Sam");

Hello, Priya!
Hello, Sam!

.on() registers a listener for a named event; .emit() fires that event, calling every listener registered for it, in the order they were added, and passing along any extra arguments.

Multiple listeners on the same event

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

emitter.on("order", (item) => console.log(`Logging order: ${item}`));
emitter.on("order", (item) => console.log(`Sending confirmation email for: ${item}`));

emitter.emit("order", "Wireless Mouse");

Logging order: Wireless Mouse
Sending confirmation email for: Wireless Mouse

This is the same shape you'll see all over Node's built-in modules — an HTTP server emits "request" for every incoming request, for instance, and http.createServer(callback) is really just a shortcut for registering that callback as a listener on "request".

Listening only once

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

emitter.once("ready", () => console.log("Ready fired!"));

emitter.emit("ready");
emitter.emit("ready");

Ready fired!

.once() auto-removes the listener after it fires the first time — the second emit("ready") has no listeners left to call.