Service-based state management in Angular - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

How does Angular service state persist across components?

Consider an Angular service that holds a counter value. If two different components inject this service and update the counter, what will be the final counter value seen by both components?

Assume the service starts with counter = 0. Component A increments it by 1, then Component B increments it by 2.

Angular

export class CounterService {
  counter = 0;
  increment(value: number) {
    this.counter += value;
  }
}

// Component A calls: counterService.increment(1);
// Component B calls: counterService.increment(2);

A Component A sees 1, Component B sees 2.

B Both components see counter value 0 because services are recreated.

C Both components see counter value 3.

D Component A sees 2, Component B sees 3.

💡 Hint

Think about how Angular injects services by default and whether the service instance is shared.

Angular services provided in root are singletons. Both components share the same instance, so increments accumulate.

📝 Syntax

intermediate

Identify the correct way to inject a service in Angular standalone component

Which of the following is the correct syntax to inject a service named DataService into a standalone Angular component using the new Angular 17+ inject() function?

A const dataService = new DataService();

B const dataService = inject(DataService);

C constructor(private dataService: DataService) {}

D const dataService = useInject(DataService);

💡 Hint

Standalone components can use the inject() function instead of constructor injection.

The inject() function is the new recommended way to get services in standalone components without constructors.

❓ state_output

advanced

What is the output after updating service state asynchronously?

Given this Angular service and component code, what will be logged to the console?

export class MessageService {
  message = 'Hello';
  updateMessage(newMsg: string) {
    setTimeout(() => {
      this.message = newMsg;
    }, 1000);
  }
}

@Component({
  standalone: true,
  selector: 'app-msg',
  template: `{{msgService.message}}`
})
export class MsgComponent {
  constructor(public msgService: MessageService) {}
  ngOnInit() {
    this.msgService.updateMessage('Hi');
    console.log(this.msgService.message);
  }
}

A Logs 'Hello' immediately, template updates to 'Hi' after 1 second.

B Logs 'Hi' immediately, template shows 'Hi' immediately.

C Logs 'Hello' and template never updates.

D Logs 'Hi' immediately, template updates to 'Hello' after 1 second.

💡 Hint

Consider when the setTimeout callback runs compared to the console.log.

The console.log runs before the timeout changes the message, so it logs the old value. The template updates after the message changes.

🔧 Debug

advanced

Why does the Angular service state reset on navigation?

An Angular service holds user data but resets to initial state when navigating between routes. What is the most likely cause?

A Angular services always reset on route changes by design.

B The service is provided in root, so it resets on navigation.

C The component recreates the service manually on each navigation.

D The service is provided in a lazy-loaded module, causing multiple instances.

💡 Hint

Think about how Angular creates service instances with lazy-loaded modules.

Services provided in lazy-loaded modules create new instances per module load, causing state reset on navigation.

🧠 Conceptual

expert

What is the main benefit of using service-based state management in Angular?

Why do Angular developers often use services to manage shared state instead of component inputs or outputs?

A Services provide a single source of truth and persistent state across components.

B Services automatically update the UI without change detection.

C Services force components to reload on every state change.

D Services prevent any state changes unless the app reloads.

💡 Hint

Consider how shared state should behave across multiple components.

Services act as a centralized place to hold and manage state, making it easier to share and persist data across components.