Is dependency injection in NestJS dependent on the features of TypeScript, for instance?

Question

Is dependency injection in NestJS dependent on the features of TypeScript, for instance?

When implementing the dependency injection pattern in NestJS, such as shown in this example from the official documentation:

import { Controller, Get, Post, Body } from '@nestjs/common';
import { CreateCatDto } from './dto/create-cat.dto';
import { CatsService } from './cats.service';
import { Cat } from './interfaces/cat.interface';

@Controller('cats')
export class CatsController {
  constructor(private catsService: CatsService) {}

  // Other methods...
}

How does this mechanism function? It seems that TypeScript is transpiled into plain JavaScript, potentially losing the type information for the CatsService class. How does NestJS manage to correctly inject the appropriate class in this scenario?

typescript dependency-injection nestjs

Answer 1

Answer №1

When it comes to dependency injection, Typescript doesn't necessarily play a crucial role. In fact, even in NestJS, you can achieve dependency injection using plain JavaScript with the help of Babel as a transpiler. I have delved into this topic further here, explaining how decorators work with typescript, and Semyon Volkov has demonstrated how these decorators are transpiled. Therefore, while TypeScript is not a strict requirement for dependency injection, Nest's preferred approach does rely on it.

In your scenario, Nest examines the type specified in `design:paramtypes` and identifies the corresponding class such as `CatsService`, allowing it to properly match and inject a `CatsService` instance where needed. Behind the scenes, there are additional mechanisms in place to manage module access, but this provides an overview of how dependency injection works.

Answer 2

When it comes to dependency injection, Typescript doesn't necessarily play a crucial role. In fact, even in NestJS, you can achieve dependency injection using plain JavaScript with the help of Babel as a transpiler. I have delved into this topic further here, explaining how decorators work with typescript, and Semyon Volkov has demonstrated how these decorators are transpiled. Therefore, while TypeScript is not a strict requirement for dependency injection, Nest's preferred approach does rely on it.

In your scenario, Nest examines the type specified in `design:paramtypes` and identifies the corresponding class such as `CatsService`, allowing it to properly match and inject a `CatsService` instance where needed. Behind the scenes, there are additional mechanisms in place to manage module access, but this provides an overview of how dependency injection works.

Answer 3

Answer №2

When we launch a Nest app, a Dependency Injection (DI) container is automatically generated for us, regardless of whether we are using TypeScript or not. DI operates as an object that manages the relationships between different classes.

Nest will analyze all classes, excluding controllers, that we have created and register them with the DI container. Each class is mapped to its required dependencies within the container:

   class1 ---> dependencies
   class2 ---> dependencies
   class3 ---> may not have any dependencies

When it comes time to create a controller, we instruct the DI container to generate it along with the necessary dependencies. The container examines the constructor arguments to determine the required dependencies and recursively creates any services or dependencies needed.

The DI container instantiates the necessary dependencies and stores them internally so that they can be reused by other classes in the future. This ensures that the Controller can be created and returned using the pre-existing dependencies.

This process eliminates the need for manual creation of services, as the DI container handles this task for us. Dependency injection simplifies code reuse and facilitates easier testing of the application.

Answer 4

When we launch a Nest app, a Dependency Injection (DI) container is automatically generated for us, regardless of whether we are using TypeScript or not. DI operates as an object that manages the relationships between different classes.

Nest will analyze all classes, excluding controllers, that we have created and register them with the DI container. Each class is mapped to its required dependencies within the container:

   class1 ---> dependencies
   class2 ---> dependencies
   class3 ---> may not have any dependencies

When it comes time to create a controller, we instruct the DI container to generate it along with the necessary dependencies. The container examines the constructor arguments to determine the required dependencies and recursively creates any services or dependencies needed.

The DI container instantiates the necessary dependencies and stores them internally so that they can be reused by other classes in the future. This ensures that the Controller can be created and returned using the pre-existing dependencies.

This process eliminates the need for manual creation of services, as the DI container handles this task for us. Dependency injection simplifies code reuse and facilitates easier testing of the application.

Answer 5

Answer №3

Check out this illustration. Here it is:

@Injectable()
export default class RequestService {
  constructor(
    @InjectRepository(RequestEntity)
    private requestEntityRepository: Repository<RequestEntity>,
  ) {}

After transpilation, it becomes:

let RequestService = class RequestService {
    constructor(requestEntityRepository) {
        this.requestEntityRepository = requestEntityRepository;
    }
...

RequestService = __decorate([
    common_1.Injectable(),
    __param(0, typeorm_1.InjectRepository(RequestEntity_1.default)),
    __metadata("design:paramtypes", [typeorm_2.Repository])
], RequestService);
exports.default = RequestService;

Answer 6

Check out this illustration. Here it is:

@Injectable()
export default class RequestService {
  constructor(
    @InjectRepository(RequestEntity)
    private requestEntityRepository: Repository<RequestEntity>,
  ) {}

After transpilation, it becomes:

let RequestService = class RequestService {
    constructor(requestEntityRepository) {
        this.requestEntityRepository = requestEntityRepository;
    }
...

RequestService = __decorate([
    common_1.Injectable(),
    __param(0, typeorm_1.InjectRepository(RequestEntity_1.default)),
    __metadata("design:paramtypes", [typeorm_2.Repository])
], RequestService);
exports.default = RequestService;

Is dependency injection in NestJS dependent on the features of TypeScript, for instance?

Answer №1

Answer №2

Answer №3

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