Service scope (root, module, component) in Angular

Introduction

Services in Angular help share data and logic. Service scope decides where the service lives and who can use it.

You want one shared service for the whole app.

You want a service only for a specific feature module.

You want a service unique to a single component and its children.

You want to control memory by limiting service lifetime.

You want different instances of a service in different parts of the app.

Syntax

Angular

@Injectable({ providedIn: 'root' })
@Injectable({ providedIn: SomeModule })
@Injectable({ providedIn: SomeComponent })

providedIn: 'root' means the service is a singleton for the whole app.

You can also provide services in modules or components to limit their scope.

Examples

This service is available everywhere in the app as a single instance.

Angular

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

This service is only available inside the AdminModule and its components.

Angular

@Injectable({ providedIn: AdminModule })
export class AdminService { }

This service instance is created only for SomeComponent and its child components.

Angular

@Injectable({ providedIn: SomeComponent })
export class LocalService { }

Sample Program

This example shows a service provided at root and another provided in a feature module. The FeatureComponent uses both services. The root service is shared app-wide, while the feature service is limited to the feature module.

Angular

import { Component, Injectable, NgModule } from '@angular/core';

@Injectable({ providedIn: 'root' })
export class RootService {
  value = 'root';
}

@Injectable()
export class FeatureService {
  value = 'feature';
}

@Component({
  selector: 'app-feature',
  template: `Feature: {{ featureService.value }} | Root: {{ rootService.value }}`
})
export class FeatureComponent {
  constructor(public featureService: FeatureService, public rootService: RootService) {}
}

@NgModule({
  declarations: [FeatureComponent],
  providers: [FeatureService],
  exports: [FeatureComponent]
})
export class FeatureModule {}

@Component({
  selector: 'app-root',
  template: `<app-feature></app-feature> Root: {{ rootService.value }}`
})
export class AppComponent {
  constructor(public rootService: RootService) {}
}

Important Notes

Services provided in 'root' are singletons and live as long as the app runs.

Providing a service in a module or component creates a new instance limited to that scope.

Use component scope to isolate service instances for specific UI parts.

Summary

Service scope controls where and how long a service instance lives.

Use 'root' for app-wide singletons, modules for feature-specific services, and components for isolated instances.

Choosing the right scope helps manage memory and service sharing effectively.

Practice

1. What does setting a service's providedIn property to 'root' mean in Angular?

easy

A. The service is a singleton shared across the entire application.

B. The service is created separately for each module.

C. The service is only available inside a specific component.

D. The service is not injectable anywhere.

Solution

1. Step 1: Understand providedIn: 'root' meaning

   This setting makes Angular create one instance of the service for the whole app.

2. Step 2: Compare with other scopes

   Unlike module or component scopes, 'root' shares the same instance everywhere.

3. Final Answer:

   The service is a singleton shared across the entire application. -> Option A

4. Quick Check:

   Service scope 'root' = singleton app-wide [OK]

Hint: Root scope means one service instance for whole app [OK]

Common Mistakes:

• Thinking 'root' creates multiple instances
• Confusing 'root' with component scope
• Assuming 'root' limits service to one module

2. Which of the following is the correct syntax to provide a service only in a specific Angular module?

easy

A. Add the service to the module's providers array

B. @Injectable({ providedIn: 'module' })

C. @Injectable({ providedIn: 'root' })

D. Add the service to the component's providers array

Solution

1. Step 1: Recall how to scope service to a module

   Angular does not support providedIn: 'module'. Instead, you add the service to the module's providers array.

2. Step 2: Check other options

   @Injectable({ providedIn: 'root' }) scopes service app-wide, Add the service to the component's providers array scopes to component, @Injectable({ providedIn: 'module' }) is invalid syntax.

3. Final Answer:

   Add the service to the module's providers array -> Option A

4. Quick Check:

   Module scope = add to module providers [OK]

Hint: Module scope means add service to module providers array [OK]

Common Mistakes:

• Using 'providedIn: module' which is invalid
• Confusing component providers with module providers
• Assuming 'root' scopes service to module only

3. Given this Angular component code:

@Component({
  selector: 'app-example',
  template: `
{{counter}}
`,
  providers: [CounterService]
})
export class ExampleComponent {
  counter = 0;
  constructor(private counterService: CounterService) {
    this.counter = this.counterService.increment();
  }
}

If two instances of app-example are rendered, what will be the value of counter in each?

medium

A. Both will show 0 because service is not provided.

B. Both will show the same incremented value shared globally.

C. Both will show 1 because each has its own service instance.

D. Both will show an error because service is missing.

Solution

1. Step 1: Understand component-level service scope

   Providing CounterService in providers inside the component creates a new instance per component.

2. Step 2: Analyze two component instances

   Each component instance gets its own service instance, so each counter starts fresh and increments once to 1.

3. Final Answer:

   Both will show 1 because each has its own service instance. -> Option C

4. Quick Check:

   Component scope = separate service instances per component [OK]

Hint: Component providers create separate service instances per component [OK]

Common Mistakes:

• Assuming service is shared globally despite component providers
• Thinking service is not provided at all
• Confusing component scope with root scope

4. You have a service provided in a feature module's providers array. However, when injecting it in a lazy-loaded module, Angular throws an error that the service is not found. What is the likely cause?

medium

A. The service is provided in the root injector, causing conflicts.

B. The lazy-loaded module has its own injector and does not see the feature module's providers.

C. The service is declared but not exported from the module.

D. The service class is missing the @Injectable decorator.

Solution

1. Step 1: Understand lazy-loaded module injectors

   Lazy-loaded modules have their own injectors and do not inherit providers from eagerly loaded modules.

2. Step 2: Identify provider visibility issue

   Since the service is provided in a feature module not loaded by the lazy module, the lazy module injector can't find it.

3. Final Answer:

   The lazy-loaded module has its own injector and does not see the feature module's providers. -> Option B

4. Quick Check:

   Lazy modules have separate injectors = missing providers [OK]

Hint: Lazy modules have isolated injectors; providers must be re-declared [OK]

Common Mistakes:

• Assuming root injector shares providers with lazy modules
• Forgetting to add @Injectable decorator
• Confusing export with provider visibility

5. You want to create a logging service that keeps a separate log for each component instance but also shares some global settings. Which service scope setup is best?

hard

A. Provide the service in root and use a static variable for logs.

B. Provide the service in root and use internal state to separate logs per component.

C. Provide the service only in a feature module's providers array.

D. Provide the service in each component's providers array and inject a global settings service from root.

Solution

1. Step 1: Separate logs per component instance

   Providing the logging service in each component's providers creates a new instance per component, isolating logs.

2. Step 2: Share global settings

   Injecting a global settings service provided in root allows sharing common configuration across all logging instances.

3. Final Answer:

   Provide the service in each component's providers array and inject a global settings service from root. -> Option D

4. Quick Check:

   Component scope for logs + root scope for settings = best mix [OK]

Hint: Use component scope for instance data, root scope for shared config [OK]

Common Mistakes:

• Using root scope for logs causes shared state issues
• Providing only in module loses per-component isolation
• Using static variables breaks instance separation