nestjs-dependency-injection

Dependency injection in NestJS uses providers and modules to wire dependencies automatically. Define services with @Injectable(), register them in a module's providers array, and inject them into constructors. NestJS handles instantiation and lifecycle management across your application.

NestJS supports four provider patterns: class providers (standard services), value providers (static values), factory providers (dynamic creation with useFactory), and async providers (for asynchronous setup). Each pattern suits different configuration and initialization scenarios.

Use NestJS modules to group related providers and declare exports. Import modules where their providers are needed, creating a clean dependency graph. The module system enforces boundaries and enables testable, reusable feature modules.

Yes. Factory providers with useFactory accept injected dependencies and return configured instances dynamically. This enables environment-specific setup where one service reads config and passes it to dependent providers during initialization.

NestJS provides singleton (default), transient, and request scopes. Singletons are shared across the app; transient creates new instances per injection; request scope ties instances to HTTP requests. Choose based on whether state should be shared or isolated.

Use string or symbol tokens as provider identifiers when the injected value isn't a class. Define the provider with provide: TOKEN and inject it by referencing the same token. This enables non-class dependencies and custom provider resolution.