Singleton service behavior in Angular - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to provide a service as a singleton in Angular.

Angular

import { Injectable } from '@angular/core';

@Injectable({
  providedIn: '[1]'
})
export class DataService {
  data = 0;
}

Drag options to blanks, or click blank then click option'

Aplatform

Bany

Croot

Dmodule

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'any' or 'module' which create multiple instances.

Forgetting to add @Injectable decorator.

Using providedIn: 'root' makes the service a singleton across the entire app.

2fill in blank

medium

Complete the code to inject the singleton service into a component.

Angular

import { Component } from '@angular/core';
import { DataService } from './data.service';

@Component({
  selector: 'app-root',
  template: `<p>{{dataService.data}}</p>`
})
export class AppComponent {
  constructor(private [1]: DataService) {}
}

Drag options to blanks, or click blank then click option'

AdataService

Bservice

Cdata

Dds

Attempts:

3 left

💡 Hint

Common Mistakes

Using a generic name like 'service' which is less clear.

Not marking the constructor parameter as private.

The injected service variable name is usually descriptive like dataService.

3fill in blank

hard

Fix the error in the service provider to ensure singleton behavior.

Angular

@Injectable({
  providedIn: '[1]'
})
export class LoggerService {}

Drag options to blanks, or click blank then click option'

Aroot

Bcomponent

Cmodule

Dany

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'module' or 'any' which create multiple instances.

Omitting the @Injectable decorator.

Only providedIn: 'root' guarantees a singleton service across the app.

4fill in blank

hard

Fill both blanks to create a singleton service and inject it in a component.

Angular

import { Injectable } from '@angular/core';

@Injectable({
  providedIn: '[1]'
})
export class CounterService {
  count = 0;
}

import { Component } from '@angular/core';
import { CounterService } from './counter.service';

@Component({
  selector: 'app-counter',
  template: `<button (click)="increment()">Count: {{counterService.count}}</button>`
})
export class CounterComponent {
  constructor(private [2]: CounterService) {}

  increment() {
    this.counterService.count++;
  }
}

Drag options to blanks, or click blank then click option'

Aroot

Bany

CcounterService

Dservice

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'any' instead of 'root' for provider.

Using generic variable names like 'service' instead of descriptive ones.

Use providedIn: 'root' for singleton service and inject it as counterService in the component.

5fill in blank

hard

Fill all three blanks to create a singleton service, inject it, and update its value.

Angular

import { Injectable } from '@angular/core';

@Injectable({
  providedIn: '[1]'
})
export class MessageService {
  message = 'Hello';
}

import { Component } from '@angular/core';
import { MessageService } from './message.service';

@Component({
  selector: 'app-message',
  template: `<p>{{messageService.message}}</p><button (click)="[2]()">Change</button>`
})
export class MessageComponent {
  constructor(private [3]: MessageService) {}

  change() {
    this.messageService.message = 'Hi there!';
  }
}

Drag options to blanks, or click blank then click option'

Aroot

Bchange

CmessageService

Dupdate

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong provider scope.

Mismatching method name in template and class.

Using unclear variable names for injection.

Use providedIn: 'root' for singleton service, call change() on button click, and inject as messageService.

Practice

1. What does it mean when an Angular service is a singleton?

easy

A. There is only one shared instance of the service across the entire app.

B. A new instance of the service is created for every component.

C. The service can only be used in one component at a time.

D. The service is automatically destroyed after use.

Solution

1. Step 1: Understand singleton concept in Angular

   A singleton service means Angular creates one instance of the service and shares it everywhere.

2. Step 2: Compare options with singleton definition

   Only There is only one shared instance of the service across the entire app. correctly describes a single shared instance across the app.

3. Final Answer:

   There is only one shared instance of the service across the entire app. -> Option A

4. Quick Check:

   Singleton = one shared instance [OK]

Hint: Singleton means one instance shared app-wide [OK]

Common Mistakes:

• Thinking each component gets its own service instance
• Confusing singleton with service destruction
• Believing singleton limits service usage to one component

2. Which is the correct way to make an Angular service a singleton using the @Injectable decorator?

easy

A. @Injectable({ scope: 'singleton' })

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

C. @Injectable({ singleton: true })

D. @Injectable({ providedIn: 'component' })

Solution

1. Step 1: Recall Angular service singleton syntax

   Angular uses providedIn: 'root' to provide a singleton service at the app root level.

2. Step 2: Check each option's validity

   Only @Injectable({ providedIn: 'root' }) uses the correct syntax. Others are invalid or do not exist in Angular.

3. Final Answer:

   @Injectable({ providedIn: 'root' }) -> Option B

4. Quick Check:

   Singleton service = providedIn: 'root' [OK]

Hint: Use providedIn: 'root' for singleton services [OK]

Common Mistakes:

• Using non-existent properties like singleton or scope
• Confusing component-level providers with root-level
• Omitting the @Injectable decorator

3. Given this Angular service and two components using it:

export class CounterService {
  count = 0;
  increment() { this.count++; }
  getCount() { return this.count; }
}

Both components call increment() once. What will getCount() return when called from either component?

medium

A. 0

B. 1

C. 2

D. Undefined

Solution

1. Step 1: Understand singleton service shared state

   Since the service is singleton, both components share the same count variable.

2. Step 2: Calculate increments from both components

   Each component calls increment() once, so total increments = 2.

3. Final Answer:

   2 -> Option C

4. Quick Check:

   Shared state increments twice = 2 [OK]

Hint: Singleton shares state; increments add up [OK]

Common Mistakes:

• Assuming each component has separate service instance
• Thinking count resets per component
• Ignoring shared service state

4. You created a service with @Injectable() but forgot to add providedIn: 'root'. What problem will occur when multiple components inject this service?

medium

A. Angular will throw a compile-time error.

B. The service will not be injectable anywhere.

C. The service will be singleton by default anyway.

D. Angular will create separate instances for each component, breaking singleton behavior.

Solution

1. Step 1: Understand default service provider behavior

   Without providedIn: 'root', Angular does not provide the service globally.

2. Step 2: Effect on multiple injections

   Each component that provides the service separately will get its own instance, so no singleton.

3. Final Answer:

   Angular will create separate instances for each component, breaking singleton behavior. -> Option D

4. Quick Check:

   Missing providedIn: 'root' = multiple instances [OK]

Hint: No providedIn: 'root' means no singleton [OK]

Common Mistakes:

• Assuming singleton is default without providedIn
• Expecting compile errors for missing providedIn
• Thinking service becomes unusable

5. You want to share user login status across multiple components using a singleton service. Which approach correctly ensures the service is singleton and updates are reflected everywhere?

hard

A. Create service with @Injectable({ providedIn: 'root' }) and use a BehaviorSubject to hold login status.

B. Create service without providedIn and add it to each component's providers array.

C. Create service with @Injectable({ providedIn: 'root' }) but store login status in a local variable inside each component.

D. Create service with @Injectable() and manually instantiate it in each component constructor.

Solution

1. Step 1: Ensure singleton service provision

   Using providedIn: 'root' makes the service singleton app-wide.

2. Step 2: Use BehaviorSubject for reactive shared state

   BehaviorSubject allows components to subscribe and get updates when login status changes.

3. Step 3: Evaluate other options

   Options B, C, and D break singleton or do not share state properly.

4. Final Answer:

   Create service with @Injectable({ providedIn: 'root' }) and use a BehaviorSubject to hold login status. -> Option A

5. Quick Check:

   Singleton + reactive state = shared updates [OK]

Hint: Use providedIn: 'root' and BehaviorSubject for shared reactive state [OK]

Common Mistakes:

• Adding service to component providers breaking singleton
• Storing shared data only inside components
• Manually creating service instances instead of DI