Dependency Injection

Dependency Injection

Idea: Don’t create dependencies inside a class; pass them in (constructor, param, or setter). Caller or a container provides the concrete implementation.

Why: Testable (inject mocks), swappable (different impl per env), loose coupling, single responsibility.

• Constructor injection: most common. Dependencies in the constructor; class doesn’t know concrete types if you use interfaces/abstracts.
• Interfaces: depend on IUserRepository, not PostgresUserRepository. Tests inject a fake.
• Containers: NestJS, Spring, Inversify, etc. Register “interface → implementation”, resolve at runtime. Use for big apps; avoid for tiny scripts.

Good: Inject all deps; keep constructors simple (no heavy logic). Prefer constructor over property/setter injection so required deps are obvious.

Anti-patterns: Service locator (class pulls from a global “get me X”), hidden singletons inside the class, circular deps (A → B → A).

When to use: Non-trivial app, need tests with mocks, multiple implementations. Skip for one-off scripts.