Dependency injection basics in NestJS - Mini Project: Build & Apply

Dependency injection basics

📖 Scenario: You are building a simple NestJS service that provides a greeting message. You want to use dependency injection to keep your code clean and testable.

🎯 Goal: Create a NestJS service called GreetingService that returns a greeting message. Then inject this service into a controller called GreetingController to return the greeting when a GET request is made.

📋 What You'll Learn

Create a service class named GreetingService with a method getGreeting() that returns the string 'Hello, NestJS!'.

Create a controller class named GreetingController.

Inject GreetingService into GreetingController using constructor injection.

Add a GET route /greet in GreetingController that returns the greeting from GreetingService.

💡 Why This Matters

🌍 Real World

Dependency injection helps keep code modular and testable by letting NestJS manage service creation and sharing.

💼 Career

Understanding dependency injection is essential for building scalable and maintainable backend applications with NestJS.

1

Create the GreetingService

Create a class called GreetingService with a method getGreeting() that returns the string 'Hello, NestJS!'.

NestJS

# Create the GreetingService class with getGreeting method
# Your code here

Need a hint?

Use the @Injectable() decorator to make the service injectable.

2

Create the GreetingController

Create a class called GreetingController and add the @Controller('greet') decorator to it.

NestJS

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

# Create the GreetingController class with @Controller('greet') decorator
# Your code here

Need a hint?

Use the @Controller('greet') decorator to define the route prefix.

3

Inject GreetingService into GreetingController

Add a constructor to GreetingController that injects GreetingService as a private readonly property named greetingService.

NestJS

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

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

@Controller('greet')
export class GreetingController {
  # Add constructor to inject GreetingService
  # Your code here
}

Need a hint?

Use constructor injection with private readonly greetingService: GreetingService.

4

Add GET route to return greeting

Add a method called getGreeting() in GreetingController decorated with @Get() that returns the result of this.greetingService.getGreeting().

NestJS

import { Injectable } from '@nestjs/common';
import { Controller, Get } from '@nestjs/common';

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

@Controller('greet')
export class GreetingController {
  constructor(private readonly greetingService: GreetingService) {}

  # Add a GET route method getGreeting() that returns greetingService.getGreeting()
  # Your code here
}

Need a hint?

Use the @Get() decorator to define the GET route handler.