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→Fakes, mocks and integration tests: removed content covered in test double, mock, fake, integration test and unit test articles that wasn't specific to TDD, replacing with summaries and links. Also reduced prescriptive phrasing of unit testing being the only testing done under TDD, but keeping note that it is still the primary test level. |
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{{Software development process}}
'''Test-driven development''' ('''TDD''') is a way of writing [[source code|code]] that involves writing an [[test automation|automated]] [[unit testing|unit-level]] [[Test case (software)|test case]] that fails, then writing just enough code to make the test pass, then [[refactoring]] both the test code and the production code, then repeating with another new test case.
Alternative approaches to writing automated tests is to write all of the production code before starting on the test code or to write all of the test code before starting on the production code. With TDD, both are written together, therefore shortening debugging time necessities.<ref>{{Cite journal |last1=Parsa |first1=Saeed |last2=Zakeri-Nasrabadi |first2=Morteza |last3=Turhan |first3=Burak |date=2025-01-01 |title=Testability-driven development: An improvement to the TDD efficiency |url=https://www.sciencedirect.com/science/article/pii/S0920548924000461 |journal=Computer Standards & Interfaces |volume=91 |pages=103877 |doi=10.1016/j.csi.2024.103877 |issn=0920-5489|url-access=subscription }}</ref>
TDD is related to the test-first programming concepts of [[extreme programming]], begun in 1999,<ref name="Cworld92">{{cite web |url=http://www.computerworld.com/softwaretopics/software/appdev/story/0,10801,66192,00.html |title=Extreme Programming |author=Lee Copeland |date=December 2001 |publisher=Computerworld |access-date=January 11, 2011 |archive-url=https://web.archive.org/web/20110605060209/http://www.computerworld.com/s/article/66192/Extreme_Programming?taxonomyId=063 |archive-date=June 5, 2011 |url-status=dead }}</ref> but more recently has created more general interest in its own right.<ref name=Newkirk>Newkirk, JW and Vorontsov, AA. ''Test-Driven Development in Microsoft .NET'', Microsoft Press, 2004.</ref>
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* Dependencies between test cases. A test suite where test cases are dependent upon each other is brittle and complex. Execution order should not be presumed. Basic refactoring of the initial test cases or structure of the UUT causes a spiral of increasingly pervasive impacts in associated tests.
* Interdependent tests. Interdependent tests can cause cascading false negatives. A failure in an early test case breaks a later test case even if no actual fault exists in the UUT, increasing defect analysis and debug efforts.
* Testing precise execution
* Building "all-knowing oracles". An oracle that inspects more than necessary is more expensive and brittle over time. This very common error is dangerous because it causes a subtle but pervasive time sink across the complex project.<ref name="pathfindersolns.com">{{YouTube| id=0BWSms3J40Y| title=Test-Driven Development (TDD) for Complex Systems Introduction}} by Pathfinder Solutions</ref>{{Clarify|reason=needs better explanation, what is an all-knowing oracle? needs better tone, more factual|date=February 2022}}
* Testing implementation details.
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Creating and managing the [[Software architecture|architecture]] of test software within a complex system is just as important as the core product architecture. Test drivers interact with the UUT, [[test double]]s and the unit test framework.<ref name="Pathfinder Solutions" />
== Advantages and Disadvantages
=== Advantages ===
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# '''Increased Code Volume''': Implementing TDD can result in a larger codebase as tests add to the total amount of code written.
# '''False Security from Tests''': A large number of passing tests can sometimes give a misleading sense of security regarding the code's robustness.<ref>{{Cite journal |last1=Parsa |first1=Saeed |last2=Zakeri-Nasrabadi |first2=Morteza |last3=Turhan |first3=Burak |date=2025-01-01 |title=Testability-driven development: An improvement to the TDD efficiency |url=https://www.sciencedirect.com/science/article/pii/S0920548924000461 |journal=Computer Standards & Interfaces |volume=91 |pages=103877 |doi=10.1016/j.csi.2024.103877 |issn=0920-5489|url-access=subscription }}</ref>
# '''Maintenance Overheads''': Maintaining a large suite of tests can add overhead to the development process.
# '''Time-Consuming Test Processes''': Writing and maintaining tests can be time-consuming.
# '''Testing Environment Set-Up''': TDD requires setting up and maintaining a suitable testing environment.
# '''Learning Curve''': It takes time and effort to become proficient in TDD practices.
# '''Overcomplication''':
# '''Neglect of Overall Design''': Focusing too narrowly on passing tests can sometimes lead to neglect of the bigger picture in software design.
=== Benefits ===
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* [[Self-testing code]]
* [[Software testing]]
* [[Transformation Priority Premise]]
* [[Unit testing]]
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