/angular-dependency-injection

name: angular-dependency-injection
description: Configure DI, inject() usage, and providers in Angular. Use when configuring Angular dependency injection, using inject(), or defining providers.
metadata:
  triggers:
    files:
    - '**/*.service.ts'
    keywords:
    - angular inject
    - providedIn
    - injection token
    - provideAppInitializer

Dependency Injection

**Priority: P0 (CRITICAL)**

Principles

• **`inject()` over Constructor**: Use **inject(MyService)** function in **class fields or constructor-equivalent** class positions for cleaner injection. It works in any **injection context** (class fields, factory functions, guards).
• **Tree Shaking**: Always use **@Injectable({ providedIn: 'root' })** for app-wide singletons unless specific scoping required.
• **Tokens**: Use **new InjectionToken<T>('description')** for configuration, primitives, or interface abstraction. Provide via: **{ provide: API_URL, useValue: 'https://api.example.com' }** in `app.config.ts`. Inject with: **inject(API_URL)**.

Guidelines

• **Providers**: Prefer **provide\*()** functions (e.g., **provideHttpClient()**) in `app.config.ts` providers array over importing NgModules.
• **Factories**: Use `useFactory` strictly when dependencies need runtime configuration.
• **App Initializer**: Use **provideAppInitializer(() => inject(ConfigService).load())** (Angular 19+) to run async code **before app bootstrap** — replaces old `APP_INITIALIZER` token pattern.
• **Route Providers**: Scope services to route tree using **providers: [MyService]** in **route config** ( routes array) instead of `providedIn: 'root'`. This creates instance destroyed when leaving route.
• **Multi Providers**: Use **{ provide: TOKEN, useClass: Impl, multi: true }** to **collects all multi providers** into array (e.g., **HTTP_INTERCEPTORS**, validators).

Anti-Patterns

• **No `providedIn: 'platform'`**: Use `'root'` scoping; reserve platform only for Micro Frontend sharing.
• **No `forwardRef`**: Refactor architecture to eliminate circular dependencies instead.

References

• [DI Patterns](references/di-patterns.md)