Patterns #1 — Event Emitter / Pub-Sub

An Event Emitter (or Pub-Sub) is one of the most practical design patterns in JavaScript — it powers Node.js's EventEmitter, browser events, and virtually every state management library. This article implements on, off, emit, and once from scratch.

Prerequisites: JS Foundations #3 — Closures

1. Basic Event Emitter — on, emit

2. once() — Listen for Exactly One Event

3. removeAllListeners and listenerCount

4. Wildcard / Catch-All Events — "*"

Node.js EventEmitter doesn't support wildcards, but you can add it:

5. Max Listeners Warning (Node.js Convention)

Node warns when more than 10 listeners are attached to one event:

Key Takeaways

• on(event, cb): register a listener — returns this for chaining.
• emit(event, ...args): call all listeners for that event synchronously.
• off(event, cb): remove a specific listener by function reference.
• once(event, cb): wrap the callback, auto-remove after first emit.
• Wildcard "*": emit to both specific and catch-all listeners.
• Node.js EventEmitter is synchronous — listeners run in the order they were added.

Next: Patterns #2 — Observer Pattern & Reactive Primitives — build a basic Observable with subscribe/unsubscribe/notify.