How does Test Driven Development promote good design?

Test Driven Development (TDD) promotes good design primarily through its encouragement of the separation of concerns in code. This approach involves writing tests before the actual code, pushing developers to think carefully about the functionality they are about to implement. As a result, TDD guides developers to create smaller, more focused units of code that each handle a specific responsibility. This separation makes the codebase easier to understand, maintain, and test.

When developers write tests first, they are often compelled to clarify the purpose and behavior of the code they are about to write. This leads to a design where components are loosely coupled and highly cohesive, ultimately resulting in a more modular and flexible architecture. Good design principles such as SOLID (Single Responsibility, Open/Closed, Liskov Substitution, Interface Segregation, and Dependency Inversion) find a natural application within the TDD workflow, reinforcing the idea that code should be designed with clear boundaries and specific roles.

In this way, TDD serves as a starting point for developing a clear, well-structured codebase that can adapt and evolve over time while still remaining testable and maintainable.