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What is the Module pattern in Node.js, and how does it enable encapsulation? 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 a layered (n-tier) architecture in a Node.js/Express application, and what are its typical layers? 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?
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How is the Factory pattern commonly applied in Node.js applications, and what problem does it solve?

Intermediate

Answer

A factory function or factory class centralizes object creation logic behind a single function, hiding the specific class or configuration details from the calling code -- useful when object construction is conditional (different implementations based on environment or input) or complex enough that scattering 'new ClassName()' calls throughout the codebase would create tight coupling to specific implementations.
function createLogger(env) {
  if (env === 'production') return new JsonLogger();
  if (env === 'test') return new SilentLogger();
  return new ConsoleLogger();
}

const logger = createLogger(process.env.NODE_ENV);
Real-world example A multi-tenant SaaS application uses a factory function to construct the correct database adapter (PostgresAdapter, MySQLAdapter, or a mock TestAdapter) based on each tenant's configuration, so the rest of the application code interacts only with a common adapter interface without knowing which concrete class it received.

Common follow-ups: How does the Factory pattern relate to the Dependency Injection pattern -- are they complementary or competing approaches?;When does introducing a factory add unnecessary indirection for a genuinely simple case?

Design Patterns in JavaScript;Dependency Injection & IoC Principles
All 15 Architecture & Design Patterns questions