/tdd - Test-Driven Development

Implement the feature using strict TDD methodology.

Target: $ARGUMENTS

Iron Law

╔══════════════════════════════════════════════════════════════╗
║  NO IMPLEMENTATION CODE WITHOUT A FAILING TEST FIRST         ║
╚══════════════════════════════════════════════════════════════╝

Workflow

Cycle Structure

For each behavior to implement:

┌───────┐      ┌───────┐      ┌──────────┐
│  RED  │ ───► │ GREEN │ ───► │ REFACTOR │
└───────┘      └───────┘      └──────────┘

Step 1: RED - Write Failing Test

1. Create/open test file
2. Write test for ONE behavior:

   it('should [expected behavior] when [condition]', () => {
     // Arrange
     const input = ...;
   
     // Act
     const result = functionUnderTest(input);
   
     // Assert
     expect(result).toBe(expected);
   });

3. Run test:

   npm test -- --testNamePattern="[test name]"

4. Verify FAILS - Capture output showing failure
5. BLOCKED if test passes (wrong test or code exists)

Step 2: GREEN - Minimal Implementation

1. Write minimum code to pass:
   • No extra features
   • No optimization
   • Just enough to pass
2. Run test:

   npm test -- --testNamePattern="[test name]"

3. Verify PASSES - Capture output showing success
4. BLOCKED if test fails (fix before continuing)

Step 3: REFACTOR - Improve

1. Improve code quality:
   • Remove duplication
   • Improve naming
   • Simplify logic
2. Run ALL tests:

   npm test

3. Verify ALL PASS
4. Commit cycle:

   git add -A && git commit -m "feat: [behavior] (TDD cycle N)"

Repeat

Continue cycles until feature is complete.

Verification Protocol

Every test run must be captured:

──── RED Phase ────
Test: should calculate discount for valid coupon
Running: npm test -- --testNamePattern="calculate discount"
Output:
  FAIL  src/discount.test.ts
  ✕ should calculate discount for valid coupon
    Expected: 90
    Received: undefined
Status: FAIL ✓ - Proceeding to GREEN

──── GREEN Phase ────
Running: npm test -- --testNamePattern="calculate discount"
Output:
  PASS  src/discount.test.ts
  ✓ should calculate discount for valid coupon
Status: PASS ✓ - Proceeding to REFACTOR

Output

──── /tdd ────
Feature: [name]

Cycle #1: [behavior]
  RED:      Test written, FAIL ✓
  GREEN:    Implemented, PASS ✓
  REFACTOR: Cleaned, PASS ✓

Cycle #2: [behavior]
  RED:      Test written, FAIL ✓
  GREEN:    Implemented, PASS ✓
  REFACTOR: Cleaned, PASS ✓

──── TDD Summary ────
Cycles completed: 2
Tests added: 2
All tests passing: ✓
Coverage: [if available]

Anti-Rationalization

Rules

1. Never implement before RED
2. Capture all test output - Proof, not trust
3. One behavior per test
4. Minimal GREEN - Just enough to pass
5. Always REFACTOR - Don't skip
6. Document each cycle