What I do

I guide developers through the complete Test Driven Development (TDD) workflow by:

•Explain TDD Principles: Teach the core red-green-refactor cycle and its purpose
•Provide Language-Specific Guidance: Offer TDD workflows for Python/pytest, JavaScript/Jest/Vitest, Next.js, and other frameworks
•Define Test Structure: Show how to structure tests for TDD (failing tests first, minimal implementation, refactoring)
•Guide Through Workflow Steps: Walk through Write Test → Run Test (Red) → Write Minimal Code → Run Test (Green) → Refactor cycle
•Provide Examples: Show concrete TDD examples with before/after code comparisons
•Integrate with Test Generation: Work seamlessly with test-generator-framework, python-pytest-creator, and nextjs-unit-test-creator
•Enforce TDD Best Practices: Promote test isolation, clear test names, and incremental development

When to use me

Use this skill when:

•You want to adopt Test Driven Development methodology for a new feature
•You need guidance on TDD workflow for a specific language or framework
•You're teaching or mentoring developers on TDD practices
•You want to refactor existing code using TDD techniques
•You need to verify your test coverage follows TDD principles
•You're setting up testing standards for a new project
•You want to ensure tests are written before implementation code

Use this before using test generation skills like:

•test-generator-framework for generic test patterns
•python-pytest-creator for Python-specific TDD
•nextjs-unit-test-creator for Next.js TDD

Prerequisites

•Basic understanding of unit testing concepts
•Access to testing framework for your language (pytest, Jest, Vitest, etc.)
•Knowledge of the programming language you're working with
•Familiarity with project structure and build tools
•Optional: Existing test generation skills for your language

TDD Principles

The Red-Green-Refactor Cycle

TDD follows a continuous cycle:

code

┌─────────────────────────────────────────┐
│  1. Write a failing test (RED)      │
└─────────────────────────────────────────┘
                 ↓
┌─────────────────────────────────────────┐
│  2. Run test and see it fail (RED)   │
└─────────────────────────────────────────┘
                 ↓
┌─────────────────────────────────────────┐
│  3. Write minimal code to pass (GREEN) │
└─────────────────────────────────────────┘
                 ↓
┌─────────────────────────────────────────┐
│  4. Run test and see it pass (GREEN)  │
└─────────────────────────────────────────┘
                 ↓
┌─────────────────────────────────────────┐
│  5. Refactor code (REFACTOR)         │
└─────────────────────────────────────────┘
                 ↓
         (Return to step 1)

Core TDD Rules

•Never write production code without a failing test
•Write the minimal amount of code to make the test pass
•Refactor only after all tests pass
•Keep tests small and focused
•Write one test at a time
•Test behavior, not implementation details

Steps

Step 1: Choose Your Feature

Identify the feature or function you want to implement:

Example: "User login functionality"

Break down into behaviors:

•Validate email format
•Check if user exists
•Verify password matches
•Return authentication token on success
•Return error message on failure

Step 2: Write First Failing Test (RED)

Start with the simplest possible test case:

Python/pytest Example:

python

def test_login_with_valid_credentials():
    """User should be able to login with valid credentials."""
    response = auth.login("user@example.com", "password123")
    assert response.success is True
    assert "token" in response.data

JavaScript/Jest Example:

javascript

test('user can login with valid credentials', () => {
  const response = auth.login('user@example.com', 'password123');
  expect(response.success).toBe(true);
  expect(response.data).toHaveProperty('token');
});

Verify the test fails:

bash

# Python
pytest tests/test_auth.py -v

# JavaScript
npm test auth.test.js

Expected output: Test should fail with an error like AttributeError: module 'auth' has no attribute 'login' or auth.login is not a function

Step 3: Write Minimal Production Code (GREEN)

Write just enough code to make the test pass, nothing more:

Python/pytest Example:

python

# auth.py
class AuthResponse:
    def __init__(self, success, data):
        self.success = success
        self.data = data

def login(email, password):
    return AuthResponse(success=True, data={"token": "dummy_token"})

JavaScript/Jest Example:

javascript

// auth.js
const login = (email, password) => {
  return {
    success: true,
    data: { token: 'dummy_token' }
  };
};

module.exports = { login };

Verify the test passes:

bash

# Python
pytest tests/test_auth.py -v

# JavaScript
npm test auth.test.js

Expected output: Test should pass (1 passed)

Step 4: Refactor (REFACTOR)

Now that the test passes, improve the code quality without changing behavior:

Refactoring guidelines:

•Extract constants
•Improve variable names
•Remove code duplication
•Improve readability
•Add input validation

Python Refactored Example:

python

# auth.py
DUMMY_TOKEN = "dummy_token"

class AuthResponse:
    def __init__(self, success, data):
        self.success = success
        self.data = data

def login(email, password):
    if not email or not password:
        raise ValueError("Email and password are required")
    return AuthResponse(success=True, data={"token": DUMMY_TOKEN})

JavaScript Refactored Example:

javascript

// auth.js
const DUMMY_TOKEN = 'dummy_token';

const login = (email, password) => {
  if (!email || !password) {
    throw new Error('Email and password are required');
  }
  return {
    success: true,
    data: { token: DUMMY_TOKEN }
  };
};

module.exports = { login };

Verify tests still pass:

bash

# Python
pytest tests/test_auth.py -v

# JavaScript
npm test auth.test.js

Step 5: Add Next Test Case

Return to Step 2 for the next behavior:

Example: Test invalid credentials

python

def test_login_with_invalid_credentials():
    """User should not be able to login with invalid credentials."""
    response = auth.login("user@example.com", "wrong_password")
    assert response.success is False
    assert "error" in response.data

Write minimal code to pass, then refactor. Continue until all behaviors are tested.

Language-Specific TDD Workflows

Python TDD with pytest

Setup:

bash

# Install pytest
pip install pytest pytest-cov

# Create tests directory
mkdir -p tests

# Create __init__.py
touch tests/__init__.py

TDD Workflow:

bash

# 1. Create test file
touch tests/test_feature.py

# 2. Write failing test
vim tests/test_feature.py

# 3. Run test (should fail)
pytest tests/test_feature.py -v

# 4. Write minimal production code
vim feature.py

# 5. Run test (should pass)
pytest tests/test_feature.py -v

# 6. Refactor code
vim feature.py

# 7. Run tests again (should still pass)
pytest tests/test_feature.py -v

# 8. Add next test
# Return to step 2

Pytest-specific TDD tips:

•Use descriptive test names (snake_case)
•Use pytest fixtures for setup/teardown
•Use @pytest.mark.parametrize for data-driven tests
•Use pytest.raises for exception testing
•Use pytest --cov for coverage checking

JavaScript/TypeScript TDD with Jest/Vitest

Setup (Jest):

bash

# Install Jest
npm install --save-dev jest

# Create test directory
mkdir tests

# Add test script to package.json
# "test": "jest"

Setup (Vitest):

bash

# Install Vitest
npm install --save-dev vitest

# Create test directory
mkdir tests

# Add test script to package.json
# "test": "vitest"

TDD Workflow:

bash

# 1. Create test file
touch feature.test.js

# 2. Write failing test
vim feature.test.js

# 3. Run test (should fail)
npm test feature.test.js

# 4. Write minimal production code
vim feature.js

# 5. Run test (should pass)
npm test feature.test.js

# 6. Refactor code
vim feature.js

# 7. Run tests again (should still pass)
npm test feature.test.js

# 8. Add next test
# Return to step 2

Jest/Vitest-specific TDD tips:

•Use descriptive test names (descriptive strings)
•Use describe blocks to group related tests
•Use beforeEach/afterEach for setup/teardown
•Use .toThrow() for exception testing
•Use watch mode for rapid TDD: npm test -- --watch

Next.js TDD

Setup:

bash

# Use test-generator-framework with nextjs-unit-test-creator
# Next.js standard setup includes testing infrastructure

TDD for Next.js Components:

typescript

// Button.test.tsx
import { render, screen } from '@testing-library/react';
import { Button } from './Button';

describe('Button', () => {
  it('renders button with text', () => {
    render(<Button>Click me</Button>);
    expect(screen.getByText('Click me')).toBeInTheDocument();
  });
});

Next.js-specific TDD tips:

•Test user interactions (click, form submit, etc.)
•Test component props and state
•Mock API calls with testing-library
•Use @testing-library/react for component testing
•Use @testing-library/jest-dom for custom matchers

Best Practices

Test Organization

•One test file per module/component
•Group related tests with describe blocks
•Use descriptive test names that explain the behavior
•Keep tests independent and isolated
•Arrange-Act-Assert (AAA) pattern:

python

# Arrange: Set up test data and conditions
user = create_user(email="test@example.com")

# Act: Execute the function being tested
result = user.validate_email()

# Assert: Verify the expected outcome
assert result is True

Test Naming

Good test names:

•✅ test_login_with_valid_credentials_succeeds
•✅ test_empty_email_raises_validation_error
•✅ user can login with correct password (Jest)
•✅ returns error when password is too short (Jest)

Bad test names:

•❌ test_login
•❌ test_1
•❌ testValidation

Writing Good Tests

•

Test behavior, not implementation:

python

# Good: Tests behavior
assert user.is_authenticated() is True

# Bad: Tests implementation
assert user._token is not None

•

Keep tests small and focused:

python

# Good: One behavior per test
def test_login_succeeds_with_valid_credentials():
    ...

def test_login_fails_with_invalid_password():
    ...

# Bad: Multiple behaviors in one test
def test_login():
    # Tests success and failure in one test
    ...

•

Use test data generators for edge cases:

python

@pytest.mark.parametrize("email,valid", [
    ("user@example.com", True),
    ("invalid", False),
    ("", False),
    (None, False),
])
def test_email_validation(email, valid):
    assert is_valid_email(email) is valid

Refactoring Guidelines

•Refactor only when all tests pass
•Make small, incremental changes
•Run tests after each refactoring step
•Don't add new functionality during refactoring
•Use automated refactoring tools when available
•Maintain the same test coverage

Common Issues

Tests Don't Fail Initially

Issue: Tests pass immediately without writing production code

Cause: You're testing the wrong thing or implementation already exists

Solution:

•Ensure you're testing a new feature or behavior
•Check if the function/class already exists
•Write a test for a specific edge case or error condition
•Make the test more specific about expected behavior

Too Much Production Code Written

Issue: Writing more code than needed to make test pass

Cause: Getting ahead of TDD cycle, thinking about implementation

Solution:

•Focus only on making the failing test pass
•Return hard-coded values if necessary (they'll be replaced by next tests)
•Write the absolute minimum code
•Trust that future tests will drive more implementation

Tests Become Brittle

Issue: Tests break when refactoring code

Cause: Testing implementation details instead of behavior

Solution:

•Test public interfaces and behavior
•Avoid testing private methods
•Use mocking appropriately for external dependencies
•Focus on what the code should do, not how

Slow TDD Cycle

Issue: Writing and running tests takes too long

Solution:

•Use test frameworks with watch mode (Jest/Vitest --watch, pytest-xdist)
•Keep test files small and focused
•Use fixtures to reduce setup duplication
•Run only the test you're working on
•Consider unit tests over integration tests for TDD

Test Coverage Too Low

Issue: Missing edge cases and error handling

Solution:

•Write tests for all code paths
•Use parametrized tests for edge cases
•Test error conditions explicitly
•Check coverage with pytest --cov or Jest coverage
•Review coverage reports for untested code

Verification Commands

After implementing TDD workflow, verify with these commands:

bash

# Python TDD Verification
pytest tests/ -v --cov
# Expected: All tests pass, coverage 80%+

# JavaScript TDD Verification
npm test -- --coverage
# Expected: All tests pass, coverage 80%+

# Check test file exists and is named correctly
test -f tests/test_feature.py && echo "✓ Test file exists"

# Verify tests follow TDD principles
# Check for descriptive names
grep "def test_" tests/test_feature.py | wc -l
# Expected: Multiple test functions with descriptive names

# Verify production code exists
test -f feature.py && echo "✓ Production code exists"

TDD Verification Checklist:

Example TDD Session

Feature: Validate Email Address

Step 1: Write failing test

python

# tests/test_email.py
def test_valid_email_format():
    assert is_valid_email("user@example.com") is True

Step 2: Run test (fails)

bash

$ pytest tests/test_email.py
# FAILED - is_valid_email not defined

Step 3: Write minimal code

python

# email.py
def is_valid_email(email):
    return True  # Pass the test with minimal code

Step 4: Run test (passes)

bash

$ pytest tests/test_email.py
# PASSED

Step 5: Refactor

python

# email.py
import re

def is_valid_email(email):
    """Validate email format using regex."""
    if not email:
        return False
    pattern = r'^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$'
    return re.match(pattern, email) is not None

Step 6: Run test (still passes)

bash

$ pytest tests/test_email.py
# PASSED

Step 7: Add next test (invalid email)

python

def test_invalid_email_format():
    assert is_valid_email("invalid") is False

Step 8: Run test (fails)

bash

$ pytest tests/test_email.py
# FAILED - "invalid" passes validation incorrectly

Step 9: Write minimal code (already done!)

python

# The regex in is_valid_email handles this case

Step 10: Run test (passes)

bash

$ pytest tests/test_email.py
# PASSED

Step 11: Add more edge cases

python

@pytest.mark.parametrize("email,expected", [
    ("", False),
    (None, False),
    ("user@", False),
    ("@example.com", False),
    ("user@example", False),
    ("user@example.", False),
])
def test_email_edge_cases(email, expected):
    assert is_valid_email(email) is expected

Continue until all behaviors are tested!

Related Skills

•test-generator-framework: Generic test generation patterns that complement TDD workflow
•python-pytest-creator: Python-specific test generation for TDD
•nextjs-unit-test-creator: Next.js/React test generation for TDD
•linting-workflow: Code quality checks after TDD implementation
•typescript-dry-principle: Refactoring guidance for TDD refactor step