Clean Architecture helps create maintainable, testable systems by enforcing separation of concerns through layered architecture. Let's implement it in TypeScript with clear file structure and code explanations.

Project Structure

1. Domain Layer - Business Core

Entity: Pure Business Object

Explanation

• Pure business logic with no external dependencies
• Enforces data validation rules
• Framework-agnostic implementation

2. Application Layer - Use Cases

Use Case: Business Process

Explanation

• Orchestrates business workflow
• Depends on abstract repository interface
• Contains zero infrastructure details

Repository Interface

Explanation

• Defines data access contract
• Implemented by infrastructure layer
• Enables dependency inversion

3. Infrastructure Layer - Implementation Details

MongoDB Implementation

Explanation

• Implements repository interface
• Contains database-specific code
• Easily swappable with other implementations

4. Presentation Layer - Delivery Mechanism

Express Controller

Explanation

• Handles HTTP-specific concerns
• Converts web requests to use case inputs
• Transforms outputs to HTTP responses

Express Routes

5. Composition Root - Dependency Wiring

Dependency Flow

Key Benefits

1. Independent Testability

   • Domain layer tests: Pure business logic
   • Application layer tests: Mock repositories
   • Infrastructure tests: Integration tests
   • Presentation tests: API contract tests

2. Technology Agnosticism

3. Long-term Maintainability

   • Business rules remain stable during tech stack changes
   • Clear boundaries reduce cognitive load

4. Team Scalability

   • Different teams can work on separate layers
   • Parallel development with contract-first approach

Testing Strategy

Domain Layer Test

Use Case Test

Conclusion

This implementation demonstrates how Clean Architecture:

1. Protects business rules from technical details
2. Enables technology decisions postponement
3. Facilitates independent component testing
4. Supports gradual complexity growth

The layered approach proves particularly valuable for applications expecting long-term evolution or potential technology migrations. While introducing initial complexity, it pays dividends in maintainability for mature projects.