name: tdd
description: Test-driven development workflow. Write tests first, then implement. Use when building new features, fixing bugs, or refactoring code.
license: MIT
metadata:
author: thesimpleapp
version: "1.0"

Test-Driven Development

Write tests first, then implement the minimum code to pass them.

The TDD Cycle

1. RED    → Write a failing test
2. GREEN  → Write minimum code to pass
3. REFACTOR → Clean up while keeping tests green

Workflow

Step 1: Understand the Requirement

• What behavior are we implementing?
• What are the inputs and expected outputs?
• What are the edge cases?

Step 2: Write the Test First

// Example: Testing a function that validates email
describe('validateEmail', () => {
  it('returns true for valid email', () => {
    expect(validateEmail('[email protected]')).toBe(true);
  });

  it('returns false for missing @', () => {
    expect(validateEmail('userexample.com')).toBe(false);
  });

  it('returns false for empty string', () => {
    expect(validateEmail('')).toBe(false);
  });
});

Step 3: Run Test (Confirm it Fails)

The test MUST fail before you write implementation. If it passes, either:
- The feature already exists
- The test is wrong

Step 4: Write Minimum Implementation

Only write enough code to make the test pass. Resist the urge to add "extra" logic.

Step 5: Refactor

• Remove duplication
• Improve naming
• Extract functions if needed
• Run tests after each change

Test Types

Best Practices

Good Tests Are:

• Fast - Run in milliseconds
• Isolated - Don't depend on other tests
• Repeatable - Same result every time
• Self-validating - Pass or fail, no manual checking
• Timely - Written before or with the code

Test Naming

Use descriptive names that explain the behavior:

✓ "returns empty array when no items match filter"
✗ "test1" or "filterTest"

Arrange-Act-Assert Pattern

it('calculates total with discount', () => {
  // Arrange
  const cart = new Cart();
  cart.addItem({ price: 100 });
  cart.applyDiscount(0.1);

  // Act
  const total = cart.getTotal();

  // Assert
  expect(total).toBe(90);
});

When to Skip TDD

TDD works best for logic-heavy code. Consider skipping for:
- Exploratory prototypes (but add tests before merging)
- Pure UI layout (use visual testing instead)
- One-off scripts

Commands

Run tests continuously:

npm test -- --watch
# or
pytest --watch