tdd-workflow

name	TDD Workflow
description	Use when practicing test-driven development, writing a failing test first, or coaching red-green-refactor. Triggers on 'TDD', 'red-green-refactor', 'test first', 'failing test', 'write a test'.

TDD Workflow

When to invoke

• Starting a new behavior or bug fix.
• Pairing / mobbing on unfamiliar code and wanting a safety net.
• When changes keep breaking things nobody expected.

The loop

RED → write the smallest failing test that expresses the next behavior
GREEN → write the smallest code that makes it pass
REFACTOR → improve design; tests stay green

Commit at each green. One behavior per cycle.

Rules

1. No production code without a failing test. No test, no change.
2. One failing test at a time. Never have two reds.
3. Smallest step that fails. If your first test is hard to write, the design is telling you something.
4. Test names describe behavior, not implementation: calculates_tax_for_tax_exempt_customer, not test_method1.
5. Given-When-Then / Arrange-Act-Assert structure in the test body.
6. Refactor phase is not optional - that's where most of the value lives.

Choosing the next test

Order tests to drive the design:

• Start with the simplest non-trivial case (the "0→1" or happy path with one input).
• Then add a single variation (a boundary, a branch, an error).
• Resist writing a giant test that covers everything.

Faking and stubbing

• Use a test double only when the real collaborator is slow, non-deterministic, or not yet written.
• Don't mock types you don't own - wrap them in a thin seam first.
• A test that mocks everything tests nothing.

When TDD is hard, it's usually the design

• Hard to construct the object under test → too many collaborators, violate SRP.
• Can't assert without reading three other objects → Law of Demeter / encapsulation issue.
• Have to mock the world → hidden coupling; introduce an abstraction.

Anti-patterns

• Writing the code, then the test (that's verification, not TDD).
• Skipping the refactor phase.
• Tests that duplicate the implementation (change detectors).
• Giant test fixtures shared across files - brittle.
• Asserting on implementation details (private methods, exact SQL string).

References

• Kent Beck - Test Driven Development: By Example
• GOOS - Growing Object-Oriented Software, Guided by Tests
• Martin Fowler - Mocks Aren't Stubs