Design Patterns

Proven architectural patterns for building maintainable, extensible, and testable TypeScript codebases. All 22 Gang of Four patterns with practical implementations.

When to Apply

Reference these patterns when:

• Solving recurring architectural problems
• Refactoring tightly coupled code
• Building plugin/extension systems
• Making code more testable via dependency injection
• Reviewing PRs with architectural concerns
• Choosing between inheritance and composition

Pattern Categories

Creational Patterns

Create objects flexibly, hiding creation logic from consumers.

Pattern	Intent
Factory Method	Delegate object creation to subclasses
Abstract Factory	Create families of related objects without concrete types
Builder	Construct complex objects step-by-step
Prototype	Clone existing objects instead of building from scratch
Singleton	Ensure exactly one instance with global access

See references/CREATIONAL.md for implementations.

Structural Patterns

Compose classes and objects into larger, flexible structures.

Pattern	Intent
Adapter	Make incompatible interfaces work together
Bridge	Separate abstraction from implementation
Composite	Treat individual objects and compositions uniformly
Decorator	Attach responsibilities dynamically without subclassing
Facade	Simplify complex subsystem with a unified interface
Flyweight	Share common state to reduce memory across many objects
Proxy	Control access to an object through a substitute

See references/STRUCTURAL.md for implementations.

Behavioral Patterns

Manage algorithms, responsibilities, and communication between objects.

Pattern	Intent
Chain of Responsibility	Pass requests along a handler chain
Command	Encapsulate requests as objects for queuing/undo
Iterator	Traverse collections without exposing internals
Mediator	Centralize complex communication between objects
Memento	Capture and restore object state
Observer	Notify dependents automatically on state changes
State	Alter behavior when internal state changes
Strategy	Swap algorithms at runtime
Template Method	Define algorithm skeleton, let subclasses override steps
Visitor	Add operations to objects without modifying them

See references/BEHAVIORAL.md for implementations.

Pattern Selection Guide

Problem	Pattern(s)
Need to decouple object creation	Factory Method, Abstract Factory
Complex object with many optional fields	Builder
Expensive object creation, need copies	Prototype
Global shared resource (config, pool)	Singleton
Incompatible third-party interface	Adapter
Multiple dimensions of variation	Bridge
Tree structures (files, UI, org charts)	Composite
Add features without subclassing	Decorator
Simplify complex API surface	Facade
Thousands of similar objects, memory heavy	Flyweight
Lazy loading, access control, caching	Proxy
Flexible request processing pipeline	Chain of Responsibility
Undo/redo, task queues, macros	Command
Custom collection traversal	Iterator
Many-to-many object communication	Mediator
Snapshots, save/restore state	Memento
Event systems, reactive updates	Observer
Object behavior depends on its state	State
Swappable algorithms (sort, compress, etc)	Strategy
Algorithm with fixed steps, variable parts	Template Method
Operations across heterogeneous objects	Visitor

Best Practices

DO

• Choose patterns that solve actual problems you're facing now
• Prefer composition over inheritance
• Use dependency injection to decouple components
• Keep pattern implementations simple — avoid gold-plating
• Document why a pattern was chosen (not just which one)
• Consider testability when choosing patterns
• Combine patterns when appropriate (e.g., Strategy + Factory)

DON'T

• Apply patterns preemptively for hypothetical future needs
• Force a pattern where a simple function/object suffices
• Create unnecessary abstraction layers
• Use Singleton as a disguised global variable
• Choose inheritance when composition works better
• Ignore team familiarity — a simpler pattern everyone knows beats a "better" one nobody understands

SOLID Quick Reference

Principle	Summary	Related Patterns
Single Responsibility	One class, one reason to change	Strategy, Command, Observer
Open/Closed	Open for extension, closed for modification	Decorator, Strategy, Template Method
Liskov Substitution	Subtypes must be substitutable for base types	Factory Method, Abstract Factory
Interface Segregation	Prefer small, focused interfaces	Adapter, Facade
Dependency Inversion	Depend on abstractions, not concretions	All patterns using interfaces/abstract