Service scope (root, module, component) in Angular - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

Service instance count with root scope

Consider an Angular service provided in root scope. If you inject this service into two different components, how many instances of the service will be created?

A One instance per module containing the components

B Two separate instances, one per component

C One single instance shared by both components

D A new instance every time the service is injected

💡 Hint

Think about the providedIn: 'root' meaning in Angular services.

Services provided in root scope are singleton across the entire app. So only one instance is created and shared.

❓ component_behavior

intermediate

Service instance behavior with module scope

If a service is provided in a feature module (not root), and two components from that module inject it, how many instances of the service exist?

A One instance shared by all components in the module

B One instance per component

C One instance per lazy loaded module only

D One instance per app regardless of module

💡 Hint

Consider how Angular creates service instances for modules.

Services provided in a module are singleton within that module. All components in that module share the same instance.

❓ state_output

advanced

Service instance count with component scope

Given a service provided in a component's providers array, what happens when two instances of that component are created?

A Angular throws an error due to multiple instances

B Both components share the same service instance

C Service instance is shared across all components in the module

D Each component instance gets its own separate service instance

💡 Hint

Think about how component-level providers work in Angular.

When a service is provided in a component, each component instance gets a new service instance. They do not share it.

📝 Syntax

advanced

Correct syntax for providing a service in a module

Which of the following is the correct way to provide a service in an Angular module?

A Add the service class to the <code>providers</code> array in the module decorator

B Add the service class to the <code>declarations</code> array in the module decorator

C Add the service class to the <code>imports</code> array in the module decorator

D Add the service class to the <code>bootstrap</code> array in the module decorator

💡 Hint

Remember where Angular expects services to be listed in a module.

Services must be listed in the providers array to be injectable in that module.

🔧 Debug

expert

Unexpected multiple service instances in lazy loaded modules

You have a service provided in root scope. However, when you lazy load two different modules that inject this service, you notice two separate instances are created. What is the most likely cause?

A The service is not decorated with <code>@Injectable</code>

B The service is also listed in the <code>providers</code> array of each lazy loaded module

C The service is provided only in <code>root</code> and should not create multiple instances

D Angular does not support singleton services with lazy loaded modules

💡 Hint

Check if the service is accidentally provided in multiple places.

If a service is provided in root but also listed in providers of lazy loaded modules, Angular creates separate instances per module.

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