Service creation in NestJS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a service in NestJS?

A service in NestJS is a class that handles business logic and data management. It is used to keep controllers simple and focused on handling requests.

beginner

How do you create a service using the NestJS CLI?

You run the command <code>nest generate service service-name</code> or <code>nest g s service-name</code>. This creates a service file with a class decorated by <code>@Injectable()</code>.

beginner

What does the @Injectable() decorator do in a NestJS service?

The <code>@Injectable()</code> decorator marks the class as a provider that can be injected into other classes, enabling dependency injection.

intermediate

Why should business logic be placed in services instead of controllers?

Placing business logic in services keeps controllers clean and focused on handling HTTP requests and responses. It also makes the logic reusable and easier to test.

beginner

How do you inject a service into a controller in NestJS?

You inject a service by adding it as a parameter in the controller's constructor. NestJS automatically provides the instance if the service is registered in the module.

Which decorator is used to mark a class as a service provider in NestJS?

A @Injectable()

B @Controller()

C @Service()

D @Module()

The @Injectable() decorator marks a class as a provider that can be injected into other classes.

What command creates a new service using the NestJS CLI?

A nest new service service-name

B nest create service service-name

C nest generate service service-name

D nest build service service-name

The command 'nest generate service service-name' creates a new service file with the proper structure.

Where should you put business logic in a NestJS application?

A In controllers

B In services

C In modules

D In main.ts

Business logic belongs in services to keep controllers simple and reusable.

How do you make a service available to a controller?

A Inject it via the controller's constructor

B Call it directly without injection

C Import it in the controller file

D Declare it in main.ts

Services are injected into controllers through the constructor for dependency injection.

What is the main benefit of using services in NestJS?

A Automatically creates routes

B Makes the app slower

C Removes the need for controllers

D Separates business logic from request handling

Services separate business logic from controllers, improving code organization and testability.

Explain how to create and use a service in NestJS from scratch.

Think about the steps from creating the service file to using it in a controller.

Why is dependency injection important when working with services in NestJS?

Consider how services are provided to other parts of the app.

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