Test-Driven Development

Test-Driven Development (TDD) is a methodology where you write tests before implementation to produce robust, maintainable software. It improves code quality by enforcing testable designs and reducing bugs through structured refactoring.

To start writing tests before implementation, follow the TDD workflow by building vertical slices and using tracer bullets. This approach ensures you design for testability from the outset and avoid horizontal slicing anti-patterns.

You should avoid testing implementation details because it creates brittle tests that break during refactoring. Focusing on result-oriented functions and dependency injection ensures your tests validate behavior rather than internal mechanics.

The best way to design testable code is using dependency injection to create deep modules and result-oriented functions. This allows you to isolate components during testing and supports the vertical slicing approach.

Use vertical slices in TDD when building new features end-to-end, as it delivers functional tracer bullets through all layers. Horizontal slicing is an anti-pattern that delays integration and defers testing of component interactions.