Clean Architecture Layers: Dependency Rule Points Inward

Business logic imports database drivers, HTTP libraries, and UI framework code. Swapping a framework or database requires touching core business rules. Testing business logic requires running real infrastructure.

Organize code into concentric layers: Entities (innermost) > Use Cases > Interface Adapters > Frameworks & Drivers (outermost). The Dependency Rule: source code dependencies only point inward — outer layers may depend on inner layers, never the reverse.

src/
  domain/           # Innermost: entities, value objects, domain services
    entities/
      order.ts      # No imports from outside domain/
    ports/          # Interfaces owned by domain
      order.repository.ts

  application/      # Use cases — depends on domain only
    use-cases/
      place-order.ts  # imports from domain, never from infrastructure

  infrastructure/   # Implements domain ports
    repositories/
      prisma-order.repository.ts  # implements domain/ports/order.repository
    http/
      order.controller.ts  # imports from application use cases

  main.ts           # Composition root — wires everything together

// domain/use-cases/place-order.ts  — NO framework imports
import type { OrderRepository } from '../ports/order.repository';
export class PlaceOrderUseCase {
  constructor(private repo: OrderRepository) {}
  async execute(dto: PlaceOrderDto) { /* pure business logic */ }
}

The dependency rule ensures the domain is independently deployable and testable. The application core has no knowledge of databases, HTTP, or UI frameworks, so it can be tested with in-memory stubs at full speed.