What is Observer Pattern?

The Observer pattern establishes a one-to-many dependency between objects. When the subject changes state, all registered observers are notified and updated automatically. This decouples the subject from its observers — the subject doesn't need to know what the observers do with the notification. Examples include event listeners in JavaScript (addEventListener), pub/sub messaging, reactive streams (RxJS), and MVC frameworks where views observe model changes. Variants include push (subject sends data with notification) and pull (observers query the subject after notification). The pattern is fundamental to reactive programming and event-driven systems.