Observer
Defines a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.
Observer: Notify Without Knowing Who
Subjects announce changes; observers react — one-to-many, decoupled.
Observer in Production: Events and Reactive Streams
DOM events, reactive streams, and distributed event buses as observers.
Advanced Observer: Event Sourcing as Observer
Events as the source of truth, projections as observers.
Observer: Review & Mastery Quiz
Scenario questions on decoupling, streams, and sourcing.
Free Books & Guides
Refactoring.Guru — Observer
The definitive modern guide to the Observer pattern. Clear explanations, UML diagrams, real-world analogies, and code examples in multiple languages.
SourceMaking — Observer
A comprehensive companion guide covering the Observer pattern with detailed rules of thumb, criticisms, and robust code snippets.
GoF Design Patterns — Observer
The original Gang of Four book describing the pattern. Available in the "Design Patterns: Elements of Reusable Object-Oriented Software" on Wikipedia.
Official Documentation
Video Courses
freeCodeCamp — Design Patterns Course
A comprehensive multi-hour video course covering all major design patterns with implementations in JavaScript and TypeScript.
Design Patterns in Modern JavaScript Playlist
YouTube playlist covering creational, structural, and behavioral patterns with clear code walkthroughs.
Web Dev Simplified — Observer
Clear, concise video tutorials covering design patterns with practical examples.
Practice & Examples
Awesome Low-Level Design (GitHub)
A curated collection of free LLD resources, pattern implementations, and interview problems across multiple languages.
Design Patterns Implementation (GitHub)
The most popular open-source collection of design pattern implementations with detailed explanations.
Refactoring.Guru Examples in Observer
Code examples of the Observer pattern in TypeScript, Python, Java, and more.