Service creation in NestJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this NestJS service method call?

Consider this NestJS service method that returns a greeting message. What will be the output when getGreeting() is called?

NestJS

import { Injectable } from '@nestjs/common';

@Injectable()
export class GreetingService {
  getGreeting(): string {
    return `Hello, NestJS!`;
  }
}

const service = new GreetingService();
console.log(service.getGreeting());

A Hello, NestJS!

B undefined

C Error: Cannot read property 'getGreeting' of undefined

D Hello, World!

💡 Hint

Look at the return statement inside the getGreeting method.

The getGreeting method returns the string 'Hello, NestJS!'. When called, it outputs exactly that string.

📝 Syntax

intermediate

Which option correctly defines a NestJS service with dependency injection?

You want to create a NestJS service OrderService that depends on PaymentService. Which option correctly injects PaymentService into OrderService?

A

import { Injectable } from '@nestjs/common';

export class OrderService {
  constructor(private paymentService: PaymentService) {}
}

B

import { Injectable } from '@nestjs/common';

@Injectable()
export class OrderService {
  paymentService: PaymentService;
  constructor() {
    this.paymentService = new PaymentService();
  }
}

C

import { Injectable } from '@nestjs/common';

@Injectable()
export class OrderService {
  constructor(private paymentService: PaymentService) {}
}

D

import { Injectable } from '@nestjs/common';

@Injectable()
export class OrderService {
  constructor(paymentService: PaymentService) {
    this.paymentService = paymentService;
  }
}

💡 Hint

Look for the correct use of @Injectable and constructor parameter visibility.

Option C uses @Injectable decorator and declares the dependency in the constructor with 'private' keyword, which automatically assigns it as a class property and enables NestJS to inject it.

🔧 Debug

advanced

Why does this NestJS service fail to inject the dependency?

Given the following code, why will NestJS fail to inject UserRepository into UserService?

NestJS

import { Injectable } from '@nestjs/common';

@Injectable()
export class UserService {
  constructor(private userRepository: UserRepository) {}
}

@Injectable()
export class UserRepository {}

A UserService is missing the @Controller decorator.

B The constructor parameter should be public, not private.

C UserRepository must be imported from a module, not declared in the same file.

D UserRepository is not decorated with @Injectable(), so NestJS cannot inject it.

💡 Hint

Check if all injectable classes have the @Injectable decorator.

NestJS requires all injectable classes to have the @Injectable decorator to register them in the dependency injection system. UserRepository lacks this decorator, so injection fails.

❓ state_output

advanced

What is the value of count after calling increment() twice?

Consider this NestJS service that tracks a count. What will be the value of count after calling increment() two times?

NestJS

import { Injectable } from '@nestjs/common';

@Injectable()
export class CounterService {
  private count = 0;

  increment() {
    this.count++;
  }

  getCount() {
    return this.count;
  }
}

const service = new CounterService();
service.increment();
service.increment();
const result = service.getCount();

A 1

B 2

C 0

D undefined

💡 Hint

Each call to increment increases count by one.

The count starts at 0. Each increment call adds 1. After two calls, count is 2.

🧠 Conceptual

expert

Which statement best describes the role of services in NestJS?

In NestJS, what is the primary purpose of a service?

A To encapsulate business logic and provide reusable functionality.

B To style components and manage UI layout.

C To manage database connections directly without abstraction.

D To handle HTTP requests and define routes.

💡 Hint

Think about where the core logic of your app should live.

Services in NestJS are designed to hold business logic and reusable code, separate from controllers which handle HTTP requests.

Practice

1.

What is the main purpose of creating a service in a NestJS application?

easy

A. To handle HTTP requests directly

B. To style the user interface

C. To define database schemas

D. To hold the business logic separate from controllers

Solution

1. Step 1: Understand the role of services in NestJS

   Services are designed to contain business logic, keeping controllers clean and focused on handling requests.

2. Step 2: Differentiate services from controllers and other parts

   Controllers handle HTTP requests, schemas define data structure, and styling is unrelated to services.

3. Final Answer:

   To hold the business logic separate from controllers -> Option D

4. Quick Check:

   Service purpose = business logic separation [OK]

Hint: Services = business logic holders, not request handlers [OK]

Common Mistakes:

• Confusing services with controllers
• Thinking services handle HTTP requests
• Assuming services define database schemas

2.

Which decorator is used to make a class injectable as a service in NestJS?

easy

A. @Injectable()

B. @Module()

C. @Controller()

D. @Service()

Solution

1. Step 1: Identify the decorator for service injection

   In NestJS, @Injectable() marks a class as a provider that can be injected.

2. Step 2: Differentiate from other decorators

   @Controller() is for controllers, @Module() for modules, and @Service() does not exist in NestJS.

3. Final Answer:

   @Injectable() -> Option A

4. Quick Check:

   Service decorator = @Injectable() [OK]

Hint: Remember: Injectable means ready to inject [OK]

Common Mistakes:

• Using @Controller() instead of @Injectable()
• Confusing @Module() with service decorator
• Assuming @Service() exists in NestJS

3.

Given the following service code, what will be the output when getHello() is called?

import { Injectable } from '@nestjs/common';

@Injectable()
export class HelloService {
  getHello(): string {
    return 'Hello, NestJS!';
  }
}

medium

A. "Hello, NestJS!"

B. undefined

C. Error: Method not found

D. "Hello World!"

Solution

1. Step 1: Analyze the getHello method

   The method returns the string 'Hello, NestJS!'.

2. Step 2: Determine the output of calling getHello()

   Calling getHello() returns exactly the string defined, no errors or undefined.

3. Final Answer:

   "Hello, NestJS!" -> Option A

4. Quick Check:

   Method return = "Hello, NestJS!" [OK]

Hint: Read method return value carefully [OK]

Common Mistakes:

• Assuming method returns undefined
• Confusing output with error messages
• Mixing up string content

4.

Identify the error in this service code snippet:

import { Injectable } from '@nestjs/common';

@Injectable()
export class MathService {
  add(a: number, b: number) {
    return a + b
  }
}

medium

A. Missing parentheses in method declaration

B. Missing return type annotation for add method

C. No error, code is correct

D. Missing semicolon after return statement

Solution

1. Step 1: Check syntax correctness

   The method add correctly takes two numbers and returns their sum. Semicolons are optional in TypeScript.

2. Step 2: Verify method declaration and return type

   Return type annotation is optional; TypeScript infers it. Parentheses are present and correct.

3. Final Answer:

   No error, code is correct -> Option C

4. Quick Check:

   Code syntax = correct [OK]

Hint: Semicolons optional; return types inferred [OK]

Common Mistakes:

• Thinking semicolons are mandatory
• Expecting explicit return type always
• Misreading method syntax

5.

You want to create a service that provides a list of users and inject it into a controller. Which of the following is the correct way to create and inject the service?

1. Create service with @Injectable()
2. Define a method getUsers() returning an array
3. Inject service in controller constructor
4. Use service method inside controller method

hard

A. Create service without @Injectable(), inject with @Inject() decorator

B. Create service with @Injectable(), inject via constructor parameter

C. Create service with @Controller(), inject via property decorator

D. Create service with @Module(), inject manually inside methods

Solution

1. Step 1: Create service with @Injectable()

   This decorator makes the service injectable and reusable.

2. Step 2: Inject service via constructor parameter in controller

   Constructor injection is the NestJS standard for dependency injection.

3. Step 3: Use service methods inside controller methods

   This keeps code modular and clean.

4. Final Answer:

   Create service with @Injectable(), inject via constructor parameter -> Option B

5. Quick Check:

   Service injection = @Injectable() + constructor [OK]

Hint: Use @Injectable() and constructor injection [OK]

Common Mistakes:

• Omitting @Injectable() on service
• Using @Controller() on service
• Injecting service incorrectly with property decorators