Test-Driven Development

Follow the RED-GREEN-REFACTOR cycle to implement a feature or bug fix with tests written first.

Steps

1. Identify the feature or fix to implement. Clarify requirements and expected behavior before writing any code.

2. Write a failing test (RED phase):

   • Create or update the test file for the target module.
   • Write a test that describes the expected behavior.
   • Include edge cases: null inputs, empty collections, boundary values.
   • Do not write any implementation code yet.

3. Run the test suite and verify the new test fails:

   • Use the project's test runner (jest, vitest, pytest, go test, etc.).
   • Confirm the failure is for the expected reason (not a syntax error or import issue).
   • If the test passes without implementation, the test is not testing new behavior — revise it.

   npm test
   pytest --cov
   go test -cover ./...
   

4. Write minimal implementation (GREEN phase):

   • Write only enough code to make the failing test pass.
   • Do not add extra features, optimizations, or abstractions.
   • Use immutable patterns — never mutate inputs.

5. Run the test suite again and verify the test passes:

   • All new tests must pass.
   • All existing tests must still pass.
   • If any test fails, fix the implementation (not the test, unless the test is wrong).

6. Refactor (IMPROVE phase):

   • Clean up the implementation while keeping all tests green.
   • Extract helpers if there is duplication.
   • Improve naming and readability.
   • Run tests after every change to ensure nothing breaks.

7. Check coverage:

   • Run tests with coverage reporting enabled.
   • Verify the new code has at least 80% coverage.
   • Add additional tests if coverage is below threshold.

If Tests Fail Unexpectedly

• Read the error message carefully before changing code.
• Check test isolation — tests should not depend on each other.
• Verify mocks and stubs are set up correctly.
• Fix the root cause, not the symptom.

Output

Passing test suite with new tests covering the implemented feature and at least 80% code coverage.