Injecting services into components in Angular - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Angular component with injected service?

Consider this Angular standalone component that injects a service to get a greeting message. What will be displayed when this component renders?

Angular

import { Component, inject } from '@angular/core';
import { GreetingService } from './greeting.service';

@Component({
  selector: 'app-greet',
  standalone: true,
  template: `<p>{{ message }}</p>`
})
export class GreetComponent {
  private greetingService = inject(GreetingService);
  message = this.greetingService.getGreeting();
}

// GreetingService code:
// export class GreetingService {
//   getGreeting() { return 'Hello, Angular!'; }
// }

A Hello, World!

B Hello, Angular!

C Error: No provider for GreetingService

D undefined

💡 Hint

Think about what the service method returns and how the component uses it.

The component injects GreetingService and calls getGreeting(), which returns 'Hello, Angular!'. This string is assigned to message and displayed.

📝 Syntax

intermediate

Which option correctly injects a service in an Angular standalone component?

Choose the correct way to inject DataService into a standalone Angular component using the new inject() function.

A private dataService = inject(DataService);

B constructor(private dataService: DataService) {}

C inject(DataService) private dataService;

D const dataService = new DataService();

💡 Hint

Remember that inject() is used as a function call inside the class body, not in constructor parameters.

In Angular 17+, the recommended way in standalone components is to call inject(DataService) directly inside the class body to get the service instance.

🔧 Debug

advanced

Why does this Angular component fail with 'NullInjectorError'?

This component tries to inject UserService but throws NullInjectorError: No provider for UserService. What is the cause?

Angular

import { Component, inject } from '@angular/core';
import { UserService } from './user.service';

@Component({
  selector: 'app-user',
  standalone: true,
  template: `<p>User: {{ userName }}</p>`
})
export class UserComponent {
  private userService = inject(UserService);
  userName = this.userService.getUserName();
}

// UserService is NOT provided in any module or component providers array.

A The template syntax {{ userName }} is invalid.

B The inject() function is used incorrectly; it must be called in constructor.

C The component is missing the @Injectable decorator on UserService.

D UserService is not registered as a provider anywhere in the app.

💡 Hint

Think about Angular's dependency injection system and where services must be registered.

Angular throws NullInjectorError when a requested service is not provided in any injector. Since UserService is not registered as a provider, Angular cannot inject it.

❓ state_output

advanced

What is the value of count after clicking the button twice?

This Angular component injects a CounterService that tracks a count. What will be the displayed count after clicking the button two times?

Angular

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

@Component({
  selector: 'app-counter',
  standalone: true,
  template: `<button (click)="increment()">Increment</button><p>Count: {{ count }}</p>`
})
export class CounterComponent {
  private counterService = inject(CounterService);
  count = this.counterService.getCount();

  increment() {
    this.counterService.increment();
    this.count = this.counterService.getCount();
  }
}

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

A 2

B 0

C 1

D Error: count is undefined

💡 Hint

Each click calls increment() which increases the count by 1.

The service starts count at 0. Each button click calls increment(), increasing count by 1. After two clicks, count is 2.

🧠 Conceptual

expert

Which statement about Angular service injection is TRUE?

Choose the correct statement about injecting services into Angular standalone components.

A Services must always be provided in NgModules to be injectable.

B The inject() function can only be used inside constructors.

C Standalone components can inject services provided in their own providers array or globally.

D Injecting services requires manually creating instances with <code>new</code> keyword.

💡 Hint

Think about Angular's new standalone component features and service providers.

Standalone components can inject services that are provided globally or in their own providers array. The inject() function can be used inside the class body, not only constructors.

Practice

1. What is the main purpose of injecting a service into an Angular component?

easy

A. To make the component run faster by compiling it differently

B. To automatically style the component with CSS

C. To change the component's HTML template dynamically

D. To allow the component to use shared functionality or data from the service

Solution

1. Step 1: Understand what services do in Angular

   Services hold shared logic or data that multiple components can use.

2. Step 2: Understand why components inject services

   Injecting a service lets the component access that shared logic or data easily.

3. Final Answer:

   To allow the component to use shared functionality or data from the service -> Option D

4. Quick Check:

   Service injection = shared code access [OK]

Hint: Services share code; injection lets components use it [OK]

Common Mistakes:

• Thinking injection changes component style
• Believing injection speeds up compilation
• Confusing service injection with template changes

2. Which is the correct way to inject a service named DataService into an Angular standalone component using the new inject() function?

easy

A. const dataService = inject(DataService); inside the component class body

B. constructor(private dataService: DataService) {} inside the component class

C. import { DataService } from './data.service'; outside the component

D. @Inject(DataService) dataService; inside the component class

Solution

1. Step 1: Recall the new inject() function usage

   In Angular 17+, inject() is called inside the component class body to get the service instance.

2. Step 2: Identify the correct syntax

   Using const dataService = inject(DataService); inside the class body is the correct pattern.

3. Final Answer:

   const dataService = inject(DataService); inside the component class body -> Option A

4. Quick Check:

   inject() function usage = const dataService = inject(DataService); inside the component class body [OK]

Hint: Use inject() inside class body for standalone components [OK]

Common Mistakes:

• Using constructor injection instead of inject() in standalone components
• Trying to use @Inject decorator incorrectly
• Confusing import statements with injection

3. Given this Angular component code snippet, what will be logged to the console?

import { Component, inject } from '@angular/core';
import { LoggerService } from './logger.service';

@Component({
  selector: 'app-sample',
  standalone: true,
  template: `
Check console
`,
  providers: [LoggerService]
})
export class SampleComponent {
  private logger = inject(LoggerService);

  constructor() {
    this.logger.log('Hello from SampleComponent');
  }
}

medium

A. Logs 'Hello from SampleComponent' to the console

B. Throws an error because LoggerService is not provided

C. Logs nothing because log() is not called

D. Logs 'undefined' because logger is not initialized

Solution

1. Step 1: Check service injection and provider

   The component provides LoggerService, so inject(LoggerService) works correctly.

2. Step 2: Check constructor behavior

   The constructor calls this.logger.log() with the message, so it logs to console.

3. Final Answer:

   Logs 'Hello from SampleComponent' to the console -> Option A

4. Quick Check:

   Service provided + log called = console output [OK]

Hint: Service must be provided to inject and use it [OK]

Common Mistakes:

• Forgetting to add service to providers array
• Assuming inject() fails without constructor
• Thinking log() is not called

4. Identify the error in this Angular component code that tries to inject AuthService:

import { Component, inject } from '@angular/core';
import { AuthService } from './auth.service';

@Component({
  selector: 'app-login',
  standalone: true,
  template: `
Login
`
})
export class LoginComponent {
  private auth = inject(AuthService);

  constructor() {
    console.log(this.auth.isLoggedIn());
  }
}

medium

A. The selector name is invalid

B. AuthService is not listed in providers, so injection will fail

C. The constructor must receive AuthService as a parameter

D. inject() cannot be used inside the class body

Solution

1. Step 1: Check if AuthService is provided

   The component does not list AuthService in providers or globally, so Angular cannot create it.

2. Step 2: Understand inject() requirements

   inject() needs the service to be provided either in providers or root; otherwise, it throws an error.

3. Final Answer:

   AuthService is not listed in providers, so injection will fail -> Option B

4. Quick Check:

   Missing provider causes injection error [OK]

Hint: Always provide service before injecting it [OK]

Common Mistakes:

• Assuming inject() works without provider
• Confusing constructor injection with inject() function
• Ignoring providers array

5. You want to inject a UserService into two standalone components, ProfileComponent and SettingsComponent, so they share the same instance of the service. How should you provide UserService to achieve this?

hard

A. Use inject() without providing UserService anywhere

B. Add UserService to the providers array of both components separately

C. Provide UserService in the root injector using @Injectable({ providedIn: 'root' })

D. Create a new instance of UserService manually in each component

Solution

1. Step 1: Understand service scope and sharing

   Providing a service in root makes Angular create one shared instance app-wide.

2. Step 2: Compare with providing in components

   Providing in each component creates separate instances, so no sharing occurs.

3. Final Answer:

   Provide UserService in the root injector using @Injectable({ providedIn: 'root' }) -> Option C

4. Quick Check:

   Root provider = shared singleton service [OK]

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

Common Mistakes:

• Providing service in each component causing multiple instances
• Forgetting to provide service at all
• Manually creating service instances instead of injecting