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What is the Singleton pattern, and what are the specific risks of using it in a Node.js application? What is the Middleware (Chain of Responsibility) pattern as implemented by Express, and how does the 'next()' function drive it? What is Dependency Injection, and how is it typically implemented in Node.js given the language has no built-in DI container? What is the Strategy pattern, and how would you apply it to support multiple payment providers in a Node.js checkout service? What is the Repository pattern, and how does it help decouple business logic from a specific database technology? What is the Observer pattern, and how does Node.js's EventEmitter class implement it natively? What is Domain-Driven Design (DDD), and how do its concepts of entities, value objects, and aggregates apply to a Node.js codebase? What is a Hexagonal Architecture (Ports and Adapters), and what specific benefit does it provide for a Node.js API? What is a Circuit Breaker pattern, and why is it important when a Node.js service calls an unreliable downstream dependency? What is the CQRS (Command Query Responsibility Segregation) pattern, and when is it appropriate to introduce in a Node.js system?
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What is a layered (n-tier) architecture in a Node.js/Express application, and what are its typical layers?

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Answer

A layered architecture separates an application into distinct responsibilities, commonly: the routing/controller layer (handling HTTP requests and responses), the service/business-logic layer (implementing the actual application rules), the data-access/repository layer (talking to the database), and the model layer (data shape definitions) -- each layer depends only on the layer below it, which improves testability (business logic can be tested without an HTTP server or a real database) and makes it easier to swap implementations, like changing databases without touching business logic.
// controller
app.post('/users', (req, res) => userService.createUser(req.body).then(u => res.json(u)));

// service (business logic)
async function createUser(data) {
  validateUser(data);
  return userRepository.save(data);
}

// repository (data access)
async function save(userData) { return db.collection('users').insertOne(userData); }
Real-world example A growing Express API refactors from having database queries directly inside route handlers to a proper repository layer, which immediately makes it possible to unit test the business logic against an in-memory fake repository instead of requiring a real database connection for every test.

Common follow-ups: How does this layered approach compare to a more feature-based ('vertical slice') folder structure?;At what point does strict layering start to add more overhead than value for a small project?

Express & Middleware;Testing with Jest Mocha & the Node Test Runner
All 15 Architecture & Design Patterns questions