How many instances are created for services in Angular?

The number of instances created for a service in Angular depends on how the service is provided and its scope. Angular's dependency injection system manages service instantiation. Here's a breakdown:

1. Provided in `root` (Singleton):

- When a service is provided in the `root` injector (using `providedIn: 'root'` in the `@Injectable` decorator), Angular creates a single instance of the service. This instance is shared across the entire application. This is the most common and recommended way to provide services that need to be shared globally.

- Example:

@Injectable({
  providedIn: 'root'
})
export class MyService {
  ...
}

2. Provided in a Module:

- When a service is provided in a specific module (using the `providers` array in the `@NgModule` decorator), Angular creates a single instance of the service for that module and its components. If the same service is provided in multiple modules, each module will have its own instance.

- Example:

@NgModule({
  providers: [MyService],
  ...
})
export class MyModule { }

3. Provided in a Component:

- When a service is provided in a component (using the `providers` array in the `@Component` decorator), Angular creates a new instance of the service for each instance of that component. This is useful when you need a service to have a unique state for each component.

- Example:

@Component({
  selector: 'app-my-component',
  providers: [MyService],
  ...
})
export class MyComponent { }

4. Provided with `useClass`, `useValue`, `useFactory`, or `useExisting`:

- These options in the `providers` array allow you to customize how a service is instantiated. They can lead to different instance behaviors depending on the specific configuration. For example, `useValue` provides a specific value, while `useFactory` allows for more complex instantiation logic.

Key Points:

- Singleton Scope: Services provided in `root` are singletons, meaning only one instance is created for the entire application.

- Hierarchical Injection: Angular's dependency injection is hierarchical. If a service is provided in multiple places, the closest provider in the component tree is used.

- Lazy Loading: When using lazy-loaded modules, each lazy-loaded module gets its own injector, and services provided in those modules are scoped to that module.

In summary, the number of service instances depends on where the service is provided. Using `providedIn: 'root'` is the most common way to create a single, shared instance, while providing services in components creates a new instance for each component.

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