typescript-design-patterns

name	typescript-design-patterns
description	Design patterns for TypeScript—Factory, Strategy, Builder, Decorator, Singleton, Mixin, and more with problem-oriented and GoF triggers.
triggers	["design patterns","typescript design patterns","Strategy pattern","Factory pattern","Abstract Factory","Builder pattern","Singleton pattern","Decorator pattern","Proxy pattern","Mixin pattern","Observer pattern","Flyweight pattern","Mediator pattern","Companion Object","swap algorithms at runtime","create families of related objects","construct a complex object step-by-step","add cross-cutting behavior","share behavior across classes","pair a type and factory","decouple components via a hub","reuse instances to reduce memory"]

TypeScript Design Patterns

Quick Pattern Selector

Problem	Pattern
One shared instance (DB, config, logger)	Singleton
Swap algorithms at runtime	Strategy
Create families of related objects	Abstract Factory
Create objects without naming the concrete class	Factory
Construct complex objects step-by-step	Builder
Pair a type and utility object under one name	Companion Object
Notify subscribers on state change	Observer
Add cross-cutting behavior non-invasively	Decorator (TS 5+ standard)
Intercept/validate/log property access	Proxy
Share behavior across unrelated classes	Mixin (class-expression pattern)
Reuse instances to reduce memory	Flyweight
Decouple via a central hub	Mediator

→ Full pattern examples with trade-offs: references/patterns.md

---

Response Format

For every pattern question, provide:

1. What it is — one sentence
2. When to use / NOT to use — concrete conditions
3. Minimal TypeScript 5+ example — runnable
4. Trade-offs — gains vs. costs
5. ⚠️ Caveat — flag any TS-version or API-specific nuance

---

Pattern Summaries

Factory — Create objects without exposing concrete classes. Pair type and factory via companion object. → references/patterns.md

Strategy — Encapsulate interchangeable algorithms; swap at runtime without changing caller. → references/patterns.md

Abstract Factory — Interface for compatible product families. Client depends on factory, never concretes. → references/patterns.md

Builder — Fluent step-by-step construction with validation and immutable product. Covers Step Builder for compile-time field enforcement. → references/patterns.md

Companion Object Pattern — Bind same name to type and utility object. One import covers both. → references/patterns.md

Real Mixins ⚠️ — Use class-expression pattern only; legacy applyMixins is outdated. (is-a → inheritance; has-a → delegation; can-do → mixin.) → references/patterns.md

Decorator (TS 5.0+ Standard) ⚠️ — Cross-cutting concerns declaratively. Standard API needs no flag. Frameworks like Angular/NestJS still require "experimentalDecorators": true (APIs not interoperable). → references/patterns.md