NestJS Basics - Modules in NestJS

Introduction to Modules in NestJS

In NestJS, Modules help structure the application, making it organized, scalable, and maintainable.

Understanding Dependency Injection and the Purpose of Modules

Dependency Injection (DI) is a design pattern used to implement IoC (Inversion of Control), allowing dependencies to be injected into a class rather than the class creating them itself. This improves code modularity and ease of testing.

In NestJS, DI is implemented through the use of decorators like @Injectable() and types that indicate what should be injected. Modules in NestJS group related components, like services and controllers, together. This modular approach keeps your code organized and helps manage dependencies effectively.

Creating a Basic NestJS Module

• defined using @Module() decorator which takes metadata object
• metadata object typically includes arrays of providers, controllers, and other imported modules

import { Module } from '@nestjs/common';
import { BooksController } from './books.controller';
import { BooksService } from './books.service';

@Module({
  controllers: [BooksController],
  providers: [BooksService],
})
export class BooksModule {}

Implementing the Book Ratings Service

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

const ratings = {
  '6abc1421-843f-4eb2-8693-5c1a319f1976': 4,
  'ac4ab8e0-29d6-4b47-bf6e-6737e1e8115b': 5,
  'cb4fce20-c17e-4ebb-ae27-cb04af5b530f': 3,
};

@Injectable()
export class BookRatingsService {
  getRatingForBook(id: string): number {
    return ratings[id] ?? 0;
  }
}

Integrating Book Ratings with the Books Service

import { Injectable } from '@nestjs/common';
import { BookRatingsService } from './bookRatings.service';
import { Book, books } from './books.data';

@Injectable()
export class BooksService {
  constructor(private readonly bookRatingsService: BookRatingsService) {}

  getAllBooks(): Book[] {
    return books.map((book) => ({
      ...book,
      rating: this.bookRatingsService.getRatingForBook(book.id),
    }))
  }
}

Configuring the Books Module

By registering both BooksService and BookRatingsService in the BooksModule, you can group related services that handle book-related logic together.

Modules defined in this way can be reused in different parts of a NestJS application. For example, if you later need to build an AuthorsModule, it could follow the same structure, allowing you to build a highly modular and maintainable system.

import { Module } from '@nestjs/common';
import { BooksController } from './books.controller';
import { BooksService } from './books.service';
import { BookRatingsService } from './bookRatings.service';

@Module({
  controllers: [BooksController],
  providers: [
    BooksService,
    BookRatingsService,
  ]
})
export class BooksModule {}

Integration into the Main Application

By integrating the BooksModule into the main AppModule, the BooksModule is now a part of the overall application.

• existing top-level AppModule continues to serve its initial purpose
• now also including everything in the BooksModule so both sets of routes are exposed
• any endpoints defined in BooksController will be accessible alongside any other routes defined in the AppController or any other modules that might be added in the future

import { Module } from '@nestjs/common';
import { AppController } from './app.controller';
import { AppService } from './app.service';
import { BooksModule } from './books/books.module';

@Module({
  imports: [BooksModule],
  controllers: [AppController],
  providers: [AppService],
})
export class AppModule {}