What is TDD (Test-Driven Development)?

What is TDD?

TDD (Test-Driven Development) treats tests as part of design, not only as a final verification step. A developer writes a small test for the expected behavior, confirms that it fails, and then adds only enough production code to make it pass.

The Red, Green, Refactor Cycle

The practical loop has three steps:

1. Red: Add a failing test for behavior that does not exist yet.
2. Green: Write the simplest implementation that passes the test.
3. Refactor: Improve names, boundaries, and duplication while preserving behavior.

This loop gives useful design feedback, especially in services and business logic with clear rules. If a test is hard to write, the code often has too many responsibilities or dependencies that are not well separated.

Where Is It Used?

TDD is most commonly practiced at the unit test level, although integration tests can support it. BDD uses a similar discipline but expresses scenarios closer to user behavior and acceptance criteria.

It is valuable for pricing rules, financial calculations, stock reservation, and API validation where hidden regressions are costly. Exploratory UI prototypes or frequently changing product discovery work may need a lighter testing strategy.