Explain the Observer pattern and its use cases.

Observer pattern defines one-to-many dependency where subject notifies observers of state changes. Components: Subject (maintains observers, sends notifications), Observer (interface with update method), ConcreteObservers (implement reaction to changes). Use cases: event handling, MVC (model notifies views), pub/sub systems, reactive programming. Push vs Pull: subject sends data or observers query subject. Modern implementations: RxJS, event emitters, message queues. Avoid memory leaks by properly unsubscribing.