Understanding Dependency Injection in NestJS with a Book Service Example

Dependency Injection (DI) in NestJS simplifies the process of managing dependencies between services and classes. Let’s explore it using a BookService example to demonstrate how DI works in NestJS. 🚀

🎯 Scenario

We want to build an applic…


This content originally appeared on DEV Community and was authored by Abhinav

Dependency Injection (DI) in NestJS simplifies the process of managing dependencies between services and classes. Let’s explore it using a BookService example to demonstrate how DI works in NestJS. 🚀

🎯 Scenario

We want to build an application that:

  1. Retrieves a list of books from a database. 📖
  2. Formats the books for display. ✨

We’ll use the following services:

  1. BookService: Handles book-related logic. 🛠️
  2. DatabaseService: Handles database interactions. 💾

🛠️ Step 1: Creating the DatabaseService

The DatabaseService simulates retrieving data from a database.

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

@Injectable()
export class DatabaseService {
  private books = [
    { id: 1, title: 'The Great Gatsby', author: 'F. Scott Fitzgerald' },
    { id: 2, title: '1984', author: 'George Orwell' },
    { id: 3, title: 'To Kill a Mockingbird', author: 'Harper Lee' },
  ];

  getAllBooks() {
    return this.books;
  }
}

🛠️ Step 2: Creating the BookService

The BookService depends on the DatabaseService to retrieve books and then formats the output.

import { Injectable } from '@nestjs/common';
import { DatabaseService } from './database.service';

@Injectable()
export class BookService {
  constructor(private readonly databaseService: DatabaseService) {}

  getFormattedBooks() {
    const books = this.databaseService.getAllBooks();
    return books.map((book) => `${book.title} by ${book.author}`);
  }
}

Here, the DatabaseService is injected into the BookService through the constructor. 🔗

📦 Step 3: Registering Services in a Module

Both services need to be registered as providers in a module. 🗂️

import { Module } from '@nestjs/common';
import { BookService } from './book.service';
import { DatabaseService } from './database.service';

@Module({
  providers: [BookService, DatabaseService],
  exports: [BookService], // Exporting BookService for use in other modules
})
export class BookModule {}

🌐 Step 4: Using the BookService in a Controller

We can now use the BookService in a controller to expose the functionality through an API.

import { Controller, Get } from '@nestjs/common';
import { BookService } from './book.service';

@Controller('books')
export class BookController {
  constructor(private readonly bookService: BookService) {}

  @Get()
  getBooks() {
    return this.bookService.getFormattedBooks();
  }
}

📦 Step 5: Adding the Controller to the Module

Update the BookModule to include the controller. 📑

import { Module } from '@nestjs/common';
import { BookService } from './book.service';
import { DatabaseService } from './database.service';
import { BookController } from './book.controller';

@Module({
  providers: [BookService, DatabaseService],
  controllers: [BookController],
})
export class BookModule {}

🛠️ How Dependency Injection Works Here

  1. DatabaseService:

    • Declared as a provider using the @Injectable() decorator. 🛠️
    • Registered in the module. ✅
    • Injected into the BookService. 🔗

  2. BookService:

    • Declared as a provider using the @Injectable() decorator. 🛠️
    • Injected into the BookController. 🔗

  3. IoC Container:

    • NestJS automatically resolves and injects the dependencies (DatabaseService → BookService → BookController). 🤖

✨ Example Output

When you make a GET request to /books, the response would look like:

[
  "The Great Gatsby by F. Scott Fitzgerald",
  "1984 by George Orwell",
  "To Kill a Mockingbird by Harper Lee"
]

🎉 Advantages of DI in NestJS

  1. Loose Coupling: BookService doesn’t need to know how DatabaseService works. 🔗
  2. Reusability: Services like DatabaseService can be reused across the app. ♻️
  3. Testing: Mock services can be injected during testing for isolated unit tests. 🧪

🚀 Summary

Dependency Injection in NestJS automates the creation and wiring of dependencies. By using decorators like @Injectable(), @Module(), and constructor injection, we can build scalable and maintainable applications. 🛠️

With this approach, we’ve created a clean and reusable architecture for the BookService example, demonstrating the power and simplicity of DI in NestJS. 📚✨


This content originally appeared on DEV Community and was authored by Abhinav


Print Share Comment Cite Upload Translate Updates
APA

Abhinav | Sciencx (2025-01-28T17:29:33+00:00) Understanding Dependency Injection in NestJS with a Book Service Example. Retrieved from https://www.scien.cx/2025/01/28/understanding-dependency-injection-in-nestjs-with-a-book-service-example/

MLA
" » Understanding Dependency Injection in NestJS with a Book Service Example." Abhinav | Sciencx - Tuesday January 28, 2025, https://www.scien.cx/2025/01/28/understanding-dependency-injection-in-nestjs-with-a-book-service-example/
HARVARD
Abhinav | Sciencx Tuesday January 28, 2025 » Understanding Dependency Injection in NestJS with a Book Service Example., viewed ,<https://www.scien.cx/2025/01/28/understanding-dependency-injection-in-nestjs-with-a-book-service-example/>
VANCOUVER
Abhinav | Sciencx - » Understanding Dependency Injection in NestJS with a Book Service Example. [Internet]. [Accessed ]. Available from: https://www.scien.cx/2025/01/28/understanding-dependency-injection-in-nestjs-with-a-book-service-example/
CHICAGO
" » Understanding Dependency Injection in NestJS with a Book Service Example." Abhinav | Sciencx - Accessed . https://www.scien.cx/2025/01/28/understanding-dependency-injection-in-nestjs-with-a-book-service-example/
IEEE
" » Understanding Dependency Injection in NestJS with a Book Service Example." Abhinav | Sciencx [Online]. Available: https://www.scien.cx/2025/01/28/understanding-dependency-injection-in-nestjs-with-a-book-service-example/. [Accessed: ]
rf:citation
» Understanding Dependency Injection in NestJS with a Book Service Example | Abhinav | Sciencx | https://www.scien.cx/2025/01/28/understanding-dependency-injection-in-nestjs-with-a-book-service-example/ |

Please log in to upload a file.



There are no updates yet.
Click the Upload button above to add an update.

You must be logged in to translate posts. Please log in or register.

Related Posts