TDD
Test-Driven Development (TDD) is a software development approach where you first write a failing test, then the minimal code to pass it, then refactor – the Red-Green-Refactor cycle.
Test-Driven Development reverses the usual order: instead of writing code and then adding tests, you write the test first. That may feel odd at first but leads to better design, fewer bugs and higher maintainability. TDD is not primarily a testing strategy but a design practice – the test forces you to think about desired behaviour before implementation.
What is TDD?
Test-Driven Development (TDD) is an iterative approach based on a short, repeating cycle: Red – write a failing test that describes the desired behaviour. Green – write the minimum code to make the test pass. Refactor – improve the code (remove duplication, improve readability) without changing behaviour. This cycle is repeated in small steps so the test suite grows and acts as a safety net for future changes. TDD was popularized by Kent Beck in Extreme Programming (XP) and is a core practice in agile development. It encourages clear API design because you look at the code first from the caller's perspective.
How does TDD work?
The developer writes a small, focused test for behaviour that does not exist yet. The test fails (Red). Then they write the simplest code that makes it pass (Green) – without worrying about elegance. In the third step (Refactor) they clean up: remove duplication, improve names and structure – the tests ensure nothing breaks. The cycle often takes only minutes and is repeated many times per day.
Practical Examples
A developer writes a test for a discount calculation: 'For 100 EUR and 10% discount the result should be 90 EUR' – then implements the calculation.
An API endpoint is developed with TDD: first an integration test that checks the expected HTTP responses.
A validation function for email is built step by step: each edge case (empty, missing @, special chars) is written as a test first.
Refactoring a class starts by capturing current behaviour with tests before changing structure.
A team uses TDD with pair programming: one person writes the test, the other the implementation (ping-pong TDD).
Typical Use Cases
Business-critical software where bugs are costly (finance, medical)
API development: Endpoints and contracts are defined by tests before implementation
Legacy modernization: Existing code is protected by tests before refactoring
Libraries and frameworks where reliable, well-tested interfaces matter
Agile teams using continuous integration and high test coverage as a quality standard
Advantages and Disadvantages
Advantages
- Fewer bugs: Issues are found immediately while writing, not later in QA or production
- Better design: TDD encourages loose coupling and clear responsibilities because code must be testable
- Living documentation: The test suite documents expected behaviour and stays up to date
- Refactoring safety: Changes are immediately validated by existing tests
- Higher long-term productivity: Less debugging and more confidence when changing code
Disadvantages
- Learning curve: TDD requires practice and a shift in thinking; at first it can feel slower
- Initial overhead: Development takes longer at the start because tests are written alongside code
- Not for everything: Exploratory prototypes and very UI-heavy work are harder with TDD
- Test maintenance: A large test suite must be maintained and can be costly to update when architecture changes
Frequently Asked Questions about TDD
What is the difference between TDD and BDD?
Do I need 100% test coverage with TDD?
How do I start with TDD when existing code has no tests?
Related Terms
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