Understanding and Avoiding Circular Dependencies in NestJS
NestJS is a powerful framework that allows developers to build scalable and maintainable applications. One of the key features of NestJS is its dependency injection system, which enables developers to manage dependencies between different components of an application. However, when not used carefully, this system can lead to circular dependencies, which can be difficult to resolve.
What are Circular Dependencies?
A circular dependency occurs when two or more components depend on each other, either directly or indirectly. This can create a cycle of dependencies that cannot be resolved by the dependency injection system.
How Does NestJS Dependency Injection Work?
In NestJS, dependencies are managed through a container that stores references to providers. Providers are classes that provide services or functionality to other components. When a component requests a dependency, the container checks if it has already been instantiated. If it has, the container returns the existing instance. If not, the container creates a new instance and stores it for future use.
How Do Circular Dependencies Arise?
Circular dependencies can arise when two or more components depend on each other. For example, suppose we have two services, UserService and FileService, that depend on each other.
“`
// user.service.ts
import { Injectable } from ‘@nestjs/common’;
import { FileService } from ‘./file.service’;
@Injectable()
export class UserService {
constructor(private readonly fileService: FileService) {}
async getUserProfilePicture(): Promise
return this.fileService.getUserProfilePicture();
}
}
// file.service.ts
import { Injectable } from ‘@nestjs/common’;
import { UserService } from ‘./user.service’;
@Injectable()
export class FileService {
constructor(private readonly userService: UserService) {}
async getUserProfilePicture(): Promise
return this.userService.getUserProfilePicture();
}
}
“`
In this example, UserService depends on FileService, which in turn depends on UserService. This creates a circular dependency that cannot be resolved by the dependency injection system.
Avoiding Circular Dependencies
To avoid circular dependencies, we can refactor our code to eliminate the cycle of dependencies. One way to do this is to introduce a third service that depends on both UserService and FileService.
“`
// profile-picture.service.ts
import { Injectable } from ‘@nestjs/common’;
import { UserService } from ‘./user.service’;
import { FileService } from ‘./file.service’;
@Injectable()
export class ProfilePictureService {
constructor(private readonly userService: UserService, private readonly fileService: FileService) {}
async getUserProfilePicture(): Promise
return this.fileService.getUserProfilePicture(this.userService.getUser());
}
}
“`
In this example, ProfilePictureService depends on both UserService and FileService, but neither of these services depends on ProfilePictureService. This breaks the cycle of dependencies and resolves the circular dependency.
Working Around Circular Dependencies
While it’s always best to avoid circular dependencies, there may be cases where they cannot be avoided. In such cases, NestJS provides a way to work around them using forward references.
A forward reference allows us to reference a class that has not yet been defined. We can use this feature to break the cycle of dependencies and resolve the circular dependency.
“`
// user.service.ts
import { Injectable } from ‘@nestjs/common’;
import { forwardRef } from ‘@nestjs/common’;
import { FileService } from ‘./file.service’;
@Injectable()
export class UserService {
constructor(@Inject(forwardRef(() => FileService)) private readonly fileService: FileService) {}
async getUserProfilePicture(): Promise
return this.fileService.getUserProfilePicture();
}
}
// file.service.ts
import { Injectable } from ‘@nestjs/common’;
import { forwardRef } from ‘@nestjs/common’;
import { UserService } from ‘./user.service’;
@Injectable()
export class FileService {
constructor(@Inject(forwardRef(() => UserService)) private readonly userService: UserService) {}
async getUserProfilePicture(): Promise
return this.userService.getUserProfilePicture();
}
}
“`
In this example, we use forward references to break the cycle of dependencies between UserService and FileService. This allows us to resolve the circular dependency and use both services in our application.