How dependency injection works in Angular - Practice Exercises

Challenge - 5 Problems

🧠 Conceptual

intermediate

What is the role of the injector in Angular's dependency injection?

In Angular, the injector is a core part of dependency injection. What does it do?

A It creates and provides instances of services when requested by components or other services.

B It compiles Angular templates into JavaScript code.

C It manages routing between different pages in the application.

D It handles user input events like clicks and key presses.

💡 Hint

Think about who is responsible for giving components the services they need.

The injector is responsible for creating and delivering service instances to components or other services that ask for them. It manages the lifecycle and sharing of these instances.

❓ component_behavior

intermediate

What happens if a service is provided in a component's providers array?

Consider a service provided in the providers array of a component. How does Angular handle this service instance?

A Angular uses the same global instance of the service for the entire app.

B Angular creates a new instance of the service only for that component and its children.

C Angular throws an error because services cannot be provided in components.

D Angular creates a new instance of the service for every component in the app.

💡 Hint

Think about service scope and how providing in a component affects instance sharing.

When a service is provided in a component's providers array, Angular creates a new instance scoped to that component and its child components. Other parts of the app use different instances if they don't have their own providers.

📝 Syntax

advanced

Which code snippet correctly injects a service using Angular's constructor injection?

Choose the correct way to inject a service named DataService into a component using Angular's constructor injection.

Angular

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

@Component({
  selector: 'app-example',
  template: '<p>Example works!</p>'
})
export class ExampleComponent {
  constructor( /* injection here */ ) {}
}

A constructor(@Inject(DataService) private dataService: DataService) {}

B constructor(dataService: DataService) { this.dataService = dataService; }

C constructor(private dataService) {}

D constructor(private dataService: DataService) {}

💡 Hint

Remember how TypeScript and Angular use constructor parameters with access modifiers for injection.

Using private dataService: DataService in the constructor automatically creates and assigns the service instance to a private property. Option D misses the access modifier, so dataService is not stored as a property. Option D uses @Inject incorrectly without a parameter. Option D lacks the service type.

❓ lifecycle

advanced

When is a service instance created by Angular's injector?

At what point does Angular create an instance of a service provided in the root injector?

A When the first component or service requests it (lazy creation).

B Immediately when the app starts, before any component loads.

C Only when the service is manually instantiated in code.

D When the Angular compiler processes the service file.

💡 Hint

Think about when Angular actually needs the service instance.

Angular creates service instances lazily. This means the instance is created only when something first asks for it, not before. This helps save resources.

🔧 Debug

expert

Why does this Angular component fail to inject the service?

Given this component code, why does Angular throw an error about MyService not being provided?

Angular

import { Component } from '@angular/core';
import { MyService } from './my.service';

@Component({
  selector: 'app-test',
  template: '<p>Test works!</p>'
})
export class TestComponent {
  constructor(private myService: MyService) {}
}

A The component selector is missing from the module declarations.

B The constructor parameter is missing the <code>public</code> access modifier.

C The service <code>MyService</code> is not listed in any <code>providers</code> array or marked with <code>@Injectable({providedIn: 'root'})</code>.

D The service file path is incorrect in the import statement.

💡 Hint

Check how Angular knows where to get the service instance from.

Angular needs services to be provided either in a module or component providers array or declared with @Injectable({providedIn: 'root'}) to register them with the injector. Without this, Angular cannot create the service instance and throws an error.