Service creation in NestJS

Introduction

Services help organize your code by handling business logic separately from controllers. This keeps your app clean and easy to manage.

When you want to separate data handling from request handling in your app.

When you need to reuse logic across multiple parts of your application.

When you want to keep your controller code simple and focused on routing.

When you want to inject dependencies easily for testing or modularity.

Syntax

NestJS

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

@Injectable()
export class MyService {
  myMethod() {
    // business logic here
  }
}

Use the @Injectable() decorator to make the class available for dependency injection.

Services are usually plain classes with methods that contain your app's logic.

Examples

This service returns a simple greeting string.

NestJS

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

@Injectable()
export class GreetingService {
  getGreeting() {
    return 'Hello, world!';
  }
}

This service provides a method to add two numbers.

NestJS

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

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

Sample Program

This service defines a method to multiply two numbers. You can inject and use it in controllers or other services.

NestJS

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

@Injectable()
export class CalculatorService {
  multiply(a: number, b: number): number {
    return a * b;
  }
}

// Usage example (in a controller or another service):
// const calculator = new CalculatorService();
// console.log(calculator.multiply(3, 4));

Important Notes

Always decorate your service class with @Injectable() so NestJS can manage it.

Services can be injected into controllers or other services via constructor injection.

Keep services focused on business logic, not on handling HTTP requests directly.

Summary

Services hold your app's business logic separately from controllers.

Use @Injectable() to make services injectable and reusable.

Inject services into controllers to keep code clean and modular.

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