Design Patterns in JavaScript (Node.js)

This book is a practical guide to applying classic and modern design patterns in the JavaScript and Node.js ecosystem. Unlike abstract examples in Java or C++, each pattern here is examined through the lens of real backend development tasks: handling HTTP requests, working with databases, creating middleware, managing event streams, and building microservices.

The book covers three groups of patterns:

• Creational: Singleton (managing application configuration), Factory (creating complex objects), Builder (building database queries), Prototype (cloning objects for performance).
• Structural: Adapter (integrating third-party APIs), Decorator (extending middleware functionality), Proxy (caching and lazy loading), Composite (building a handler tree).
• Behavioral: Observer (EventEmitter), Strategy (choosing a data processing algorithm), Command (task queues), Chain of Responsibility (Express/Koa middleware chains).

Each chapter follows a consistent structure:

• A real production problem (e.g., "how to avoid duplicate Redis connections")
• An antipattern — the incorrect solution that is often encountered
• The correct solution using the pattern in Node.js
• A comparison of performance and code readability
• Examples in TypeScript (with optional types) and plain JavaScript

Who this book is for:

• JavaScript developers transitioning from frontend to Node.js
• Backend engineers looking to structure code and eliminate "spaghetti"
• Tech leads and architects designing scalable systems
• Developers preparing for technical interviews

What you will learn:

• How to transform chaotic callback code into a maintainable architecture
• Why Singleton is not always evil if its lifecycle is managed correctly
• How to implement the Observer pattern using EventEmitter and streams
• When to use Factory and when to use Builder for creating objects
• How to build a flexible middleware system without Express/Koa

The book is written for practitioners: minimum theory, maximum working examples. All patterns are tested on Node.js 18+ and are compatible with modern frameworks (Express, Fastify, NestJS). At the end of each chapter, there are exercises to reinforce the material and links to a repository with ready-made code.

Dive into the world of patterns to make your code predictable, testable, and ready for increased load!