| name | write-unit-test |
| description | Use when implementing any function, method, or module that contains logic — before or immediately after writing the code. |
| source | Kent Beck Test-Driven Development (2002), Google Testing Blog, xUnit pattern (Meszaros 2007) |
| tags | ["testing","tdd","developer","defect-reduction"] |
| stable | true |
Write Unit Test
Verify a single unit of logic in isolation with an automated, repeatable test.
Why This Is Best Practice
Adopted by: Google (mandated in Google Engineering Practices, 2014), Microsoft,
Meta, Stripe, Airbnb, and virtually every mature engineering organization. Kent Beck
formalized it with TDD in 2002; it has been mainstream for 20+ years without serious
challenge.
Impact: Unit tests reduce defect escape rate by 40–80% depending on coverage
(Capers Jones, Software Engineering Best Practices, 2010). Google reports that code
with unit test coverage has 50% fewer production incidents than untested code (Google
SRE Book, 2016). Teams practicing TDD deliver 15–35% fewer defects with no significant
productivity loss after initial learning (IBM Research, George & Williams 2004).
Why best: No other technique catches logic errors earlier or cheaper. Manual testing
is not repeatable; integration tests are too slow for rapid iteration; code review misses
subtle logic errors. Unit tests are the only practice that makes refactoring safe at speed.
Sources: Kent Beck TDD (2002), Google Engineering Practices, IBM Research George & Williams (2004)
Steps
- Write the test before writing the implementation (RED): define expected inputs and outputs.
- Run the test — it must fail. A test that passes before implementation tests nothing.
- Write the minimum code to make the test pass (GREEN).
- Refactor the implementation while keeping the test green (REFACTOR).
- Repeat for each distinct behavior.
Rules
- Test behavior, not implementation: test what a function does, not how it does it.
- Each test must be independent — no shared mutable state between tests.
- Name tests descriptively:
it('returns null when input is empty') not it('test1').
- Keep tests fast: unit tests must complete in milliseconds, not seconds.
Common Mistakes
- Testing the framework instead of application logic.
- Writing tests that only test the happy path — edge cases and error paths matter.
- Sharing state between tests so that test order affects results.
- Coupling tests to implementation details, causing them to break during refactoring.
