What is dependency injection?

Dependency Injection (DI) is a technique where a class receives its dependencies from an external source rather than creating them internally. Instead of class Service { private db = new Database(); }, the dependency is injected: class Service { constructor(private db: DatabaseInterface) {} }. There are three forms: Constructor injection (most common and recommended), setter injection (via setter methods), and interface injection. DI is the practical implementation of DIP — by injecting abstractions, you decouple the class from concrete implementations. DI containers (Spring in Java, Laravel's service container in PHP, NestJS IoC in Node.js) automate the wiring of dependencies across large applications.