TDD

Test-Driven Development (TDD) is a software development approach in which tests are written before production code. The tests define the expected behavior, and the code is written to satisfy them. TDD emphasizes small, incremental steps and relies on a red-green-refactor cycle: write a failing test, implement minimal code to make it pass, and then refactor the code and tests for clarity and maintainability.

Process typically begins with a small automated unit test that specifies a simple function or behavior. The test is run and fails. The simplest possible implementation is then written to pass the test, after which the full test suite is run again and the code is refactored to improve structure while keeping tests green. Tests should be fast, reliable, and isolated to provide rapid feedback.

TDD is often used in agile and XP environments. Related practices include behavior-driven development (BDD) and acceptance test-driven development (ATDD), which extend the idea to collaboration and stakeholder expectations. The test pyramid is a common guideline, advocating many fast unit tests at the bottom and progressively fewer slower integration or UI tests at higher levels. Mocks or stubs are frequently used to isolate units during testing.

Benefits commonly attributed to TDD include improved design, clearer requirements, and better regression protection, as tests document intended behavior. Criticisms mention higher upfront effort, potential for brittle tests, and the need for discipline and tooling. TDD is not a universal solution and works best when paired with good design practices, appropriate tooling, and a well-supported team culture.

Historically, TDD emerged from the eXtreme Programming movement in the 1990s and has since become widely adopted across languages and industries.