angular-dependency-injection

Dependency injection in Angular uses hierarchical injectors and providers to manage service instances. Create a service with @Injectable(), define a provider in NgModule or component, then inject it via constructor parameters. This approach improves testability and decouples components from service implementations.

Angular offers four main provider patterns: useClass creates new instances, useValue provides fixed objects, useFactory enables dynamic creation via functions, and useExisting aliases existing providers. Choose based on whether you need instance control, static values, computed dependencies, or provider aliasing.

Angular supports root, module, component, and element injectors with hierarchical resolution. Providers at root scope are singletons across the app, module scope shares instances within a module, and component scope isolates instances per component. Configure scope via @Injectable({providedIn: 'root'}) or NgModule provider declarations.

InjectionTokens provide type-safe identifiers for non-class dependencies like configuration objects or strings. Create tokens with new InjectionToken(), configure values via providers, and inject them using @Inject() decorator. Tokens prevent naming collisions and enable type-checked dependency configuration.

Yes, dependency injection enables modular refactoring by extracting logic into services, configuring providers at appropriate scopes, and injecting dependencies into components. This improves maintainability, testability, and allows feature modules to manage their own service instances independently.

Constructor injection works for typed service classes through automatic type-based resolution. InjectionToken injection handles non-class dependencies and enables explicit configuration of primitive values, objects, or functions. Use tokens when class-based injection cannot express your dependency.