What is dependency injection in Node.js?

Dependency injection (DI) is a design pattern where a module's dependencies are provided (injected) from the outside rather than created internally. This decouples components, making them individually testable and interchangeable. Without DI: class UserService { constructor() { this.db = new Database(); } } — UserService is tightly coupled to a specific Database implementation, making it hard to test or swap. With DI: class UserService { constructor(db) { this.db = db; } } — the database is injected, so tests can pass a mock. In plain Node.js/Express, manual DI is common: create dependencies in one place and pass them to service constructors. DI containers (IoC containers) automate this: InversifyJS (decorator-based DI container), Awilix (simple, lightweight), NestJS (has DI built-in as a core feature, similar to Angular's DI system). DI is fundamental to writing SOLID code, particularly the Dependency Inversion Principle.