Design Patterns

Quick reference for the 23 Gang of Four design patterns organized by category.

Pattern Selection Guide

Problem	Consider
Complex object creation	Factory Method, Abstract Factory, Builder
Clone existing objects	Prototype
Single shared instance	Singleton
Incompatible interfaces	Adapter
Separate abstraction from implementation	Bridge
Tree structures	Composite
Add behavior dynamically	Decorator
Simplify complex subsystems	Facade
Many similar objects (memory)	Flyweight
Control access/lazy loading	Proxy
Sequential handlers	Chain of Responsibility
Encapsulate requests as objects	Command
Traverse collections	Iterator
Reduce coupling between components	Mediator
Save/restore state (undo)	Memento
Event notification	Observer
Object behavior varies by state	State
Interchangeable algorithms	Strategy
Algorithm skeleton with variable steps	Template Method
Operations on object structures	Visitor

Pattern Categories

Creational Patterns

Object creation mechanisms. See references/creational.md.

Pattern	Intent
Factory Method	Defer instantiation to subclasses
Abstract Factory	Create families of related objects
Builder	Construct complex objects step-by-step
Prototype	Clone existing objects
Singleton	Ensure single instance with global access

Structural Patterns

Object composition and relationships. See references/structural.md.

Pattern	Intent
Adapter	Convert interface to another expected interface
Bridge	Separate abstraction from implementation
Composite	Treat individual objects and compositions uniformly
Decorator	Add responsibilities dynamically
Facade	Simplified interface to complex subsystem
Flyweight	Share state to support many fine-grained objects
Proxy	Placeholder controlling access to another object

Behavioral Patterns

Object interaction and responsibility distribution. See references/behavioral.md.

Pattern	Intent
Chain of Responsibility	Pass request along handler chain
Command	Encapsulate request as object
Iterator	Sequential access without exposing representation
Mediator	Centralize complex communications
Memento	Capture and restore object state
Observer	Notify dependents of state changes
State	Alter behavior when state changes
Strategy	Interchangeable algorithm family
Template Method	Define algorithm skeleton, defer steps to subclasses
Visitor	Add operations without modifying classes

Quick Implementation Examples

Factory Method (TypeScript)

typescript

interface Product { operation(): string }

abstract class Creator {
  abstract createProduct(): Product
  doSomething(): string {
    const product = this.createProduct()
    return product.operation()
  }
}

class ConcreteCreator extends Creator {
  createProduct(): Product { return new ConcreteProduct() }
}

Strategy (TypeScript)

typescript

interface Strategy { execute(data: string[]): string[] }

class Context {
  constructor(private strategy: Strategy) {}
  setStrategy(strategy: Strategy) { this.strategy = strategy }
  doWork(data: string[]): string[] { return this.strategy.execute(data) }
}

// Usage: context.setStrategy(new SortAscending())

Observer (TypeScript)

typescript

interface Observer { update(state: any): void }

class Subject {
  private observers: Observer[] = []
  attach(o: Observer) { this.observers.push(o) }
  detach(o: Observer) { this.observers = this.observers.filter(x => x !== o) }
  notify(state: any) { this.observers.forEach(o => o.update(state)) }
}

Decorator (TypeScript)

typescript

interface Component { operation(): string }

class ConcreteComponent implements Component {
  operation() { return 'base' }
}

class Decorator implements Component {
  constructor(protected component: Component) {}
  operation() { return this.component.operation() }
}

class ConcreteDecorator extends Decorator {
  operation() { return `decorated(${super.operation()})` }
}

Resources

For detailed explanations, participants, and when-to-use guidance:

•references/creational.md - Factory Method, Abstract Factory, Builder, Prototype, Singleton
•references/structural.md - Adapter, Bridge, Composite, Decorator, Facade, Flyweight, Proxy
•references/behavioral.md - Chain of Responsibility, Command, Iterator, Mediator, Memento, Observer, State, Strategy, Template Method, Visitor