A collection of common design patterns implemented in Node.js with TypeScript. Each pattern lives in its own service folder with a real-world example, making it easy to understand and extend.

# Install dependencies
npm install

# Run all pattern demos
npm run dev

# Build
npm run build

# Run compiled output
npm start

src/
├── index.ts                          # Entry point — runs all demos
└── services/
    ├── strategy/                     # Strategy Pattern
    ├── factory/                      # Factory Pattern
    ├── facade/                       # Facade Pattern
    ├── iterator/                     # Iterator Pattern
    ├── singleton/                    # Singleton Pattern
    └── observer/                     # Observer Pattern

Category: Behavioural
Folder: src/services/strategy/

Defines a family of algorithms, encapsulates each one, and makes them interchangeable at runtime.

Example: Payment processing with swappable payment methods.

const service = new PaymentService(new CreditCardPayment("4111111111111234"));
service.checkout(99.99);

service.setStrategy(new AfterpayPayment(4));
service.checkout(199.99);

Category: Creational
Folder: src/services/factory/

Provides an interface for creating objects without specifying their concrete classes.

Example: Creating different notification channels (Email, SMS, Push).

const notification = NotificationFactory.create("email");
notification.send("[email protected]", "Hello!");

Category: Structural
Folder: src/services/facade/

Provides a simplified interface to a complex subsystem of classes.

Example: File conversion that coordinates downloading, converting, and saving.

const converter = new FileConvertFacade();
const result = converter.convertFile(
  "https://example.com/files/report.docx",
  "pdf",
  "/Users/output"
);

Category: Behavioural
Folder: src/services/iterator/

Provides a way to access elements of a collection sequentially without exposing the underlying representation. Uses generic iterators that work with any type.

Example: Iterating over a file collection with optional extension filtering.

const files = new FileCollection();
files.add({ name: "report.pdf", size: 2048, extension: "pdf" });
files.add({ name: "photo.png", size: 5120, extension: "png" });

const allIterator = files.createIterator();
while (allIterator.hasNext()) {
  console.log(allIterator.next().name);
}

const pdfIterator = files.createExtensionIterator("pdf");

Category: Creational
Folder: src/services/singleton/

Ensures a class has only one instance and provides a global point of access to it.

Example: Application configuration that is shared across the entire app.

const config1 = AppConfig.getInstance();
const config2 = AppConfig.getInstance();

config1 === config2; // true — same instance

config1.set("port", 8080);
config2.get("port"); // 8080 — shared state

Category: Behavioural
Folder: src/services/observer/

Defines a one-to-many dependency so that when one object changes state, all its dependents are notified automatically.

Example: Event notification system with Email, Log, and Slack observers.

const eventBus = new EventBus();

eventBus.subscribe("user:signup", new EmailObserver());
eventBus.subscribe("user:signup", new SlackObserver());

eventBus.notify("user:signup", { userId: "u-1", email: "[email protected]" });

• Runtime: Node.js
• Language: TypeScript
• Dev runner: tsx

This project is open-sourced software licensed under the MIT license.