Glossary

Observer Pattern

Observer is a pattern where a subject object maintains a list of dependent observer objects and automatically notifies them when its state changes. It is the foundation of event systems.

Participants

Subject (Publisher) — maintains the observer list; provides subscribe / unsubscribe / notify methods
Observer (Subscriber) — implements an update() method called by the subject

Where it appears

DOM events in the browser (addEventListener)
Event systems in frameworks (EventEmitter in Node.js)
Reactive Programming (RxJS Observable)
In-application message bus

Observer vs Pub/Sub

In Observer, the subject knows its observers directly. In Pub/Sub there is a broker between them: publisher and subscriber are unaware of each other.