Test-Driven Development

Write tests first, then implementation. No production code without a failing test.

Purpose

TDD ensures code correctness through disciplined test-first development. Tests written after implementation prove nothing - they pass immediately, providing no evidence the code works correctly. This skill enforces the RED-GREEN-REFACTOR cycle as a non-negotiable practice.

The Iron Law

NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST.

If you write code before the test, you must delete it and start over. The test drives the implementation, not the other way around.

The Cycle

RED: Write a Failing Test

Write ONE minimal test that demonstrates the required behavior:

1. Test the public interface, not internals
2. Never mock what you can use for real
3. Name the test to describe the behavior
4. Run the test - it MUST fail

Mandatory verification:

Run the test. Confirm it fails for the RIGHT reason:
- Missing function/method (expected)
- Wrong return value (expected)
- NOT: Syntax error
- NOT: Import error
- NOT: Test framework misconfiguration

If the test passes immediately, you've written it wrong or the feature already exists. Investigate before proceeding.

GREEN: Write Minimal Code

Write the SIMPLEST code that makes the test pass:

1. No extra features
2. No premature optimization
3. No "while I'm here" additions
4. Just enough to satisfy the test

Mandatory verification:

Run the test. Confirm:
- The new test passes
- All other tests still pass
- No new warnings or errors

REFACTOR: Improve Without Breaking

Improve code quality while keeping tests green:

1. Remove duplication
2. Improve names
3. Extract helpers
4. Simplify logic

After each change:

Run all tests. They must still pass.
If any test fails, revert the refactor.

Cycle Example

Requirement: Function that validates email addresses

RED:
  Write test: expect(isValidEmail("user@example.com")).toBe(true)
  Run test: FAIL - isValidEmail is not defined
  Correct failure reason: function doesn't exist yet

GREEN:
  Write: function isValidEmail(email) { return true; }
  Run test: PASS
  All tests pass

RED:
  Write test: expect(isValidEmail("invalid")).toBe(false)
  Run test: FAIL - Expected false, got true
  Correct failure reason: no validation logic yet

GREEN:
  Write: function isValidEmail(email) { return email.includes("@"); }
  Run test: PASS
  All tests pass

REFACTOR:
  Extract regex pattern to constant
  Run tests: PASS
  Continue improving...

Common Rationalizations (Reject All)

Integration with Implement Phase

When executing plan steps that involve code:

1. Before coding: Write failing test for the behavior
2. Verify RED: Test fails for the right reason
3. Write code: Minimal implementation
4. Verify GREEN: Test passes, no regressions
5. Refactor: Improve while green
6. Mark complete: Only after tests pass

If a plan step doesn't mention tests, add them anyway. TDD is not optional.

Test Quality Guidelines

Good Tests

• Describe behavior, not implementation
• Fail for one reason only
• Run fast (milliseconds)
• Don't depend on external state
• Read like specifications

Bad Tests

• Test private methods directly
• Require specific execution order
• Mock what you can use for real (indicates bad design)
• Pass without verifying anything meaningful
• Break when refactoring internals

Edge Cases and Boundaries

Test these explicitly:

• Empty inputs
• Null/undefined values
• Boundary values (0, -1, MAX_INT)
• Invalid inputs
• Error conditions
• Concurrent access (if applicable)

Anti-Patterns

Writing Code First

Wrong: Write feature, then write tests to cover it Right: Write test, watch it fail, then write feature

Testing After the Fact

Wrong: "I'll add tests in a follow-up PR" Right: Tests are part of the implementation, not separate

Skipping RED Verification

Wrong: Assume test will fail, write code immediately Right: Run test, observe failure, understand why

Over-Mocking

Wrong: Mock every dependency to "isolate" the unit Right: Never mock what you can use for real

Testing Implementation Details

Wrong: Test that internal method X calls internal method Y Right: Test that public interface produces correct results

Verification Checklist

Before marking a step complete:

• Test was written BEFORE production code
• Test failed for the correct reason (not syntax/import errors)
• Implementation is minimal (no extra features)
• All tests pass (new and existing)
• Code was refactored while green
• No untested production code was added