NestJS Circular Dependency Detected Error: How to Fix It

Understanding Circular Dependency in NestJS

A circular dependency occurs when two classes or modules in a Node.js application depend on each other. This can happen in NestJS when a service, module, or controller tries to inject a dependency which itself directly or indirectly depends on the requesting entity. This circular reference disrupts the dependency injection system and prevents NestJS from properly instantiating the services or components, causing a ‘Circular Dependency Detected’ error.

To illustrate, imagine two services: ServiceA requires ServiceB to function, and vice versa. On application startup, when NestJS tries to resolve these dependencies, it gets stuck in a loop, not being able to fully create an instance of either service because each one is waiting for the other to be instantiated. NestJS reports this as an error to prevent unexpected behavior in your application.

Fixing Approach: Refactoring Code

One of the primary solutions to resolve a circular dependency is to refactor your code so the services or modules do not directly depend on each other. This might involve creating a third service or module that abstracts the common functionality, which can then be shared without creating a circular reference. Another approach could be to use a design pattern such as the Publisher/Subscriber pattern, which can help decouple the classes and facilitate communication between them without direct references.

Fixing Approach: Forward References

In cases where refactoring is not feasible, NestJS provides a mechanism called forward references to circumvent circular dependencies. When you use the forwardRef() function provided by NestJS around the injection token and in the module’s imports and providers, you create a deferment of the dependency resolution process. By doing this, you allow all involved classes or modules to be defined before resolving the dependencies between them.

Applying Forward References to Services

To apply forward references when two services have a circular dependency, you inject a forward reference within the constructor of each service using the @Inject(forwardRef(() => OtherService)) decorator.

Here is how you can modify the services:

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

@Injectable()
class ServiceA {
  constructor(
    @Inject(forwardRef(() => ServiceB)) private serviceB: ServiceB,
  ) {}
  // ServiceA methods
}

@Injectable()
class ServiceB {
  constructor(
    @Inject(forwardRef(() => ServiceA)) private serviceA: ServiceA,
  ) {}
  // ServiceB methods
}

Applying Forward References to Modules

If the circular dependency is at the module level, use the forwardRef() function in the @Module() decorator’s imports array. Here’s a sample code snippet:

import { Module, forwardRef } from '@nestjs/common';
import { ModuleA } from './moduleA';
import { ModuleB } from './moduleB';

@Module({
  imports: [forwardRef(() => ModuleA), forwardRef(() => ModuleB)],
  // ... Other metadata
})
class AppModule {}

Complete Code Example with Circular Dependency Fixed

Consider the following simplified example, in which we have two services UserService and AuthService that depend on each other. The AuthService needs access to UserService to validate the user’s identity, and the UserService requires the AuthService to check the authorization status of the user. We will employ the forwardRef() function to resolve the circular dependency.

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

@Injectable()
class UserService {
  constructor(
    @Inject(forwardRef(() => AuthService)) private authService: AuthService,
  ) {}

  getUser() {
    // ...fetch user method
  }
}

@Injectable()
class AuthService {
  constructor(
    @Inject(forwardRef(() => UserService)) private userService: UserService,
  ) {}

  validateUser() {
    // ...validate user method
  }
}

@Module({
  providers: [UserService, forwardRef(() => AuthService)],
  exports: [UserService, AuthService]
})
class UserModule {}

@Module({
  providers: [AuthService, forwardRef(() => UserService)],
  exports: [AuthService, UserService]
})
class AuthModule {}

@Module({
  imports: [
    forwardRef(() => UserModule),
    forwardRef(() => AuthModule),
  ],
})
class AppModule {}

By introducing the changes above into your code, you should successfully resolve the circular dependency detected error in your NestJS project.