NestJS has gained immense popularity among developers looking to build robust and maintainable backend applications. Its modular architecture and dependency injection system allow for cleaner code and separation of concerns. Two core concepts that contribute significantly to this design are Services and Providers. Let's dive into these concepts and see how they fit into the NestJS ecosystem.

What are Services in NestJS?

In NestJS, a Service is a class that encapsulates business logic and can be used throughout your application. Services are typically responsible for tasks like:

• Interfacing with databases
• Communicating with external APIs
• Handling complex logic that spans multiple components

By organizing code into services, you enhance maintainability and testability. Services can be injected into controllers or other services using NestJS's powerful dependency injection system.

Creating a Service

To create a service in NestJS, you can use the Nest CLI or manually create a simple service class. For example, let’s create a service that manages user-related functionality.

Using Nest CLI

nest generate service user

This command generates a user.service.ts file inside the user directory.

Manual Creation

If you prefer doing it manually, you can create a file named user.service.ts as follows:

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

@Injectable()
export class UserService {
  private readonly users = [];

  create(user: any) {
    this.users.push(user);
    return user;
  }

  findAll() {
    return this.users;
  }
}

In this example, the UserService class is decorated with the @Injectable() decorator, making it a service that can be injected into other components. It provides two methods: create() to add a new user and findAll() to retrieve all users.

What are Providers in NestJS?

In the NestJS paradigm, Providers are objects that can be injected as dependencies. A provider can be a service, repository, factory, or even a value. Providers are the fundamental building blocks that facilitate the dependency injection pattern in NestJS.

When you define a class with the @Injectable() decorator, it becomes a provider. This means it can be injected into other components, such as controllers or other services.

Custom Provider Example

Sometimes, you may need custom providers for specific cases—like when creating a provider that returns a fixed configuration object:

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

export const APP_CONFIG = 'APP_CONFIG';

@Injectable()
export class AppConfig {
  getConfig() {
    return {
      appName: 'My NestJS App',
      version: '1.0.0',
    };
  }
}

To make this available as a provider, you can use the providers array in a module:

import { Module } from '@nestjs/common';
import { AppConfig, APP_CONFIG } from './app.config';

@Module({
  providers: [
    {
      provide: APP_CONFIG,
      useClass: AppConfig,
    },
  ],
  exports: [APP_CONFIG],
})
export class AppModule {}

In this setup, AppConfig provides application configuration, and you make it accessible to other modules by exporting it.

Dependency Injection: How It All Comes Together

With services and providers, you can build the dependency graph in your applications effectively. For instance, you can inject the UserService into a controller like this:

import { Controller, Get } from '@nestjs/common';
import { UserService } from './user.service';

@Controller('users')
export class UserController {
  constructor(private readonly userService: UserService) {}

  @Get()
  findAll() {
    return this.userService.findAll();
  }
}

In this example, you pass an instance of UserService to UserController via the constructor. NestJS handles the instantiation and injection for you.

Scoping of Providers

Providers can have different scopes, affecting their lifecycle and visibility. By default, providers are singleton, but you can change their scope if needed. For example, if you want a new instance of a service for every request, you can use the @Scope decorator.

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

@Injectable({ scope: Scope.REQUEST })
export class RequestScopedService {
  // Implementation here
}

In this case, a new instance of RequestScopedService will be created for every incoming request, allowing for stateful behavior specific to that request.

Conclusion

Understanding Services and Providers in NestJS is crucial for leveraging the full power of this framework. By structuring your application with well-defined services and implementing provider patterns, you can build scalable, maintainable, and testable back-end systems. With these building blocks, you're on your way to creating robust applications that follow best practices in modern back-end development. Happy coding!