Service-to-service injection in Angular - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to inject LoggerService into DataService.

Angular

import { Injectable } from '@angular/core';
import { LoggerService } from './logger.service';

@Injectable({
  providedIn: 'root'
})
export class DataService {
  constructor(private [1]: LoggerService) {}
}

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

Alogger

BLoggerService

ClogService

DdataLogger

Attempts:

3 left

💡 Hint

Common Mistakes

Using the service class name as the parameter name with uppercase (e.g., 'LoggerService') causes errors.

Using unrelated names that do not describe the service.

The constructor parameter name can be anything, but by convention, a lowercase descriptive name like 'logger' is used to hold the injected service instance.

2fill in blank

medium

Complete the code to inject DataService into AppComponent.

Angular

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

@Component({
  selector: 'app-root',
  template: `<h1>Welcome</h1>`
})
export class AppComponent {
  constructor(private [1]: DataService) {}
}

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

Aservice

BDataService

CdataService

Ddata

Attempts:

3 left

💡 Hint

Common Mistakes

Using uppercase names for constructor parameters.

Using generic names like 'service' that are not descriptive.

Using 'dataService' as the parameter name follows Angular style guide conventions for injected services.

3fill in blank

hard

Fix the error in the constructor injection of AuthService in ProfileService.

Angular

import { Injectable } from '@angular/core';
import { AuthService } from './auth.service';

@Injectable({
  providedIn: 'root'
})
export class ProfileService {
  constructor(private [1]: AuthService) {}
}

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

AauthService

Breadonly authService

Cpublic authService

Dprivate authService

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting the access modifier causes the service not to be accessible in the class.

Using only the parameter name without 'private' or 'public'.

To inject a service and make it a class property, the constructor parameter must have an access modifier like 'private' or 'public'.

4fill in blank

hard

Fill both blanks to inject HttpClient and LoggerService into ApiService.

Angular

import { Injectable } from '@angular/core';
import { HttpClient } from '@angular/common/http';
import { LoggerService } from './logger.service';

@Injectable({
  providedIn: 'root'
})
export class ApiService {
  constructor(private [1]: HttpClient, private [2]: LoggerService) {}
}

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

Ahttp

Blogger

ClogService

Dclient

Attempts:

3 left

💡 Hint

Common Mistakes

Using the same name for both parameters.

Using uppercase or unrelated names.

Common names for injected services are 'http' for HttpClient and 'logger' for LoggerService.

5fill in blank

hard

Fill all three blanks to inject UserService, AuthService, and LoggerService into DashboardService.

Angular

import { Injectable } from '@angular/core';
import { UserService } from './user.service';
import { AuthService } from './auth.service';
import { LoggerService } from './logger.service';

@Injectable({
  providedIn: 'root'
})
export class DashboardService {
  constructor(private [1]: UserService, private [2]: AuthService, private [3]: LoggerService) {}
}

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

AuserService

BauthService

Clogger

DdashboardLogger

Attempts:

3 left

💡 Hint

Common Mistakes

Using uppercase names for parameters.

Using the same name for different services.

Use descriptive camelCase names matching the service names: 'userService', 'authService', and 'logger'.

Practice

1. What is the main purpose of service-to-service injection in Angular?

easy

A. To allow one service to use another service's functionality easily

B. To create multiple instances of the same service

C. To directly manipulate the DOM inside a service

D. To replace components with services

Solution

1. Step 1: Understand service-to-service injection concept

   Service-to-service injection allows one service to use another service's methods or data by injecting it via the constructor.

2. Step 2: Identify the purpose

   This helps keep code clean and reusable by sharing logic between services without duplication.

3. Final Answer:

   To allow one service to use another service's functionality easily -> Option A

4. Quick Check:

   Service-to-service injection = sharing logic between services [OK]

Hint: Services inject other services to share logic easily [OK]

Common Mistakes:

• Thinking services create multiple instances by injection
• Confusing services with components for DOM manipulation
• Believing injection replaces components with services

2. Which syntax correctly injects a service named LoggerService into another service's constructor?

easy

A. constructor(LoggerService private logger) {}

B. constructor(private LoggerService logger) {}

C. constructor(private logger: LoggerService) {}

D. constructor(logger: LoggerService private) {}

Solution

1. Step 1: Recall Angular constructor injection syntax

   Angular uses TypeScript constructor parameters with access modifiers (like private) followed by the variable name and type.

2. Step 2: Match correct syntax

   The correct form is constructor(private logger: LoggerService) {}. Other options have incorrect order or missing keywords.

3. Final Answer:

   constructor(private logger: LoggerService) {} -> Option C

4. Quick Check:

   Access modifier + variable name + type = correct injection [OK]

Hint: Use 'private varName: ServiceType' in constructor [OK]

Common Mistakes:

• Placing 'private' after the variable name
• Swapping variable name and type order
• Omitting access modifier like private or public

3. Given these services, what will DataService.getData() return?

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

@Injectable({ providedIn: 'root' })
export class LoggerService {
  log() { return 'Logged'; }
}

@Injectable({ providedIn: 'root' })
export class DataService {
  constructor(private logger: LoggerService) {}
  getData() {
    return this.logger.log() + ' Data';
  }
}

medium

A. Error: logger is undefined

B. 'Data Logged'

C. 'Logged'

D. 'Logged Data'

Solution

1. Step 1: Understand service injection and method calls

   DataService injects LoggerService and calls its log() method, which returns 'Logged'.

2. Step 2: Analyze getData() return value

   getData() returns the string from logger.log() plus ' Data', so the result is 'Logged Data'.

3. Final Answer:

   'Logged Data' -> Option D

4. Quick Check:

   Injected method output + ' Data' = 'Logged Data' [OK]

Hint: Injected service method output concatenated with string [OK]

Common Mistakes:

• Assuming logger is undefined due to missing initialization
• Mixing order of concatenated strings
• Forgetting to call the log() method

4. What is wrong with this service-to-service injection code?

@Injectable({ providedIn: 'root' })
export class UserService {
  constructor(private authService: AuthService) {}
}

// AuthService is not decorated with @Injectable

medium

A. UserService must be provided in a module, not root

B. AuthService must be decorated with @Injectable to be injected

C. Constructor should not have access modifier

D. UserService should not inject any service

Solution

1. Step 1: Check injection requirements

   Angular requires any service to be decorated with @Injectable to be injectable into other services or components.

2. Step 2: Identify missing decorator

   AuthService lacks @Injectable, so Angular cannot inject it into UserService.

3. Final Answer:

   AuthService must be decorated with @Injectable to be injected -> Option B

4. Quick Check:

   @Injectable missing on injected service = error [OK]

Hint: Always add @Injectable to services used for injection [OK]

Common Mistakes:

• Thinking access modifier in constructor is wrong
• Believing services can't be provided in root
• Assuming services can't inject other services

5. You have two services: ApiService and CacheService. You want ApiService to use CacheService to store data before returning it. Which is the best way to implement this service-to-service injection pattern?

hard

A. Inject CacheService into ApiService constructor and call cache methods inside ApiService

B. Inject ApiService into CacheService and call API methods inside CacheService

C. Create a third service that duplicates both ApiService and CacheService methods

D. Use global variables to share data between ApiService and CacheService

Solution

1. Step 1: Understand service responsibilities

   ApiService handles API calls, CacheService handles data storage. ApiService should use CacheService to store data.

2. Step 2: Apply service-to-service injection

   Inject CacheService into ApiService's constructor, then call CacheService methods inside ApiService to store data before returning it.

3. Step 3: Avoid bad patterns

   Injecting ApiService into CacheService or duplicating methods breaks separation of concerns. Using global variables is bad practice.

4. Final Answer:

   Inject CacheService into ApiService constructor and call cache methods inside ApiService -> Option A

5. Quick Check:

   Inject dependent service into main service for clean logic [OK]

Hint: Inject helper service into main service, not vice versa [OK]

Common Mistakes:

• Injecting main service into helper service causing circular dependency
• Duplicating logic instead of reusing services
• Using global variables instead of injection