Tdd

Skill: tdd

When to Use This Skill

Use this skill when:

• User wants to add a feature or fix a bug
• User says "write tests" or "use TDD"
• User wants test coverage for existing code

The Rule

Write a failing test before writing any implementation code.

If you cannot write the test first, stop and explain why before proceeding.

The Loop

1. Red — Write the smallest failing test that defines the desired behavior. Run it. Confirm it fails for the right reason — not a compile error, not a missing import.
2. Green — Write the minimum code to make it pass. No more.
3. Refactor — Clean up. Tests must still pass after every change.

Each cycle should take minutes, not hours. If a cycle is taking too long, the unit is too large — break it down.

What to Test

• Test behavior, not implementation. Test what the code does, not how it does it.
• If you're testing a private method, the design is wrong — extract it into its own unit.
• One assertion per test when possible. Multiple assertions hide which one failed.

Test Names

Write test names as sentences that describe behavior:

• it('returns null when user is not found') not it('test getUserById')
• it('throws when input is empty') not it('empty input test')

When TDD Feels Impossible

If writing the test first feels impossible, it usually signals a design problem:

• Unit is too large — break it down
• Code has hidden side effects — isolate them with dependency injection
• Wrong layer — move the test up (integration) or down (unit)

Never skip the failing test step. A test written after the code has never been seen to fail — you cannot trust it actually tests anything.

Frameworks

Stack	Runner	Notes
Node/TS	Vitest (prefer) or Jest	Vitest is faster, ESM-native
React	Vitest + Testing Library	Test user behavior, not DOM structure
Python	pytest	Use fixtures for setup
Go	testing (built-in)	Table-driven tests for multiple cases