tdd

TDD (Test-Driven Development) Rules

A skill that defines t-wada style test-driven development practices.

Core Principles

• Test First: Always write tests before implementation
• Red-Green-Refactor: Follow the cycle of Fail → Pass → Improve
• Baby Steps: Progress in small increments
• Express Intent: Clearly express code intent through tests

TDD Cycle

1. Red

Write a failing test first.

test "can add two numbers"
  calculator = new Calculator
  assert calculator.add(2, 3) == 5
// → test fails

Compiled languages: Create an empty function/class skeleton first so the code compiles. The test should fail at the assertion level, not at compilation.

2. Green

Write the minimal code to make the test pass.

class Calculator
  function add(a, b)
    return 5  // fake implementation

3. Refactor

Eliminate duplication and improve the code.

class Calculator
  function add(a, b)
    return a + b  // generalized via triangulation

Triangulation

// First test
test "2 + 3 = 5"
  assert calculator.add(2, 3) == 5
// Fake implementation: return 5

// Second test (triangulation)
test "3 + 4 = 7"
  assert calculator.add(3, 4) == 7
// → Generalization is required, leading to return a + b

Test Granularity

// ❌ Too coarse
test "user management feature"   // tests everything

// ✅ Appropriate
test "can create a user"
test "can update a user"

Clear Test Names

// ❌ Vague
test "user test"

// ✅ Clear
test "can create a user with a valid name"
test "throws an error when creating a user with an empty name"

Execution Steps

1. Create an empty function/class skeleton (required for compiled languages)
2. Write a test
3. Run ONLY the relevant test(s) to confirm the test fails (Red)
4. Implement the minimal code to make the test pass (Green)
5. Run ONLY the relevant test(s) to confirm the test passes
6. Refactor as needed (Refactor)