How do you apply Clean Architecture and separation of concerns in a Node.js application?

Short answer

Divide the system into concentric layers: Entities, Use Cases, Controllers/Presenters, and Infrastructure, with dependencies always pointing inward toward abstract business logic.

Explanation

The goal is to make core business logic (Domain & Use Cases) entirely independent of technical details such as the framework (Express/Fastify) or database type (PostgreSQL/MongoDB). Dependency Inversion achieves this: the Use Cases layer defines interfaces (Interfaces/Ports), and the Infrastructure layer implements and provides them through Dependency Injection.

Code example

// Use Case Independent of the database
export class RegisterUserUseCase {
  constructor(private readonly userRepo: UserRepositoryPort) {}
  async execute(dto: RegisterUserDto) {
    // Pure business logic
  }
}

Common mistakes

• Importing database models (such as Mongoose or TypeORM models) directly into the Use Cases layer.

Follow-up questions

• How do you implement Distributed Tracing in Node.js with the OpenTelemetry standard?

Sources

• Robert C. Martin — The Clean Architecture