test-driven-development

# Test-Driven Development (TDD)

Red → Green → Refactor cycle for all code changes.

## The TDD Cycle

1. **RED**: Write failing test
2. **GREEN**: Write minimal code to pass
3. **REFACTOR**: Improve code quality

### Repeat for each requirement

## When to Apply TDD

✅ **Always use TDD for:**

- New functions/methods
- New features
- Bug fixes (reproduce first)
- Refactoring existing code
- API changes

❌ **Skip TDD for:**

- UI styling tweaks
- Configuration changes
- Documentation updates

## Process

### 1. Write Failing Test First

```elixir
# Start with test
test "calculates total with tax" do
  result = Calculator.calculate_total([100, 200])
  assert Money.equal?(result, Money.new(:USD, 324))
end

# Run test - should FAIL
mix test
```

### 2. Implement Minimal Code

```elixir
# Just enough to pass
def calculate_total(prices) do
  prices |> Enum.sum() |> Kernel.*(1.08) |> Money.new(:USD)
end
```

### 3. Refactor

Extract constants, improve naming, etc.

## Test Patterns by Stack

### Backend (Elixir)

- File: `test/path/module_test.exs`
- Pattern: `apps/api/test/your_app/task/task_test.exs`

### Frontend (TypeScript)

- File: `ComponentName.test.tsx`
- Pattern: `mobile/libraries/atorasu/atoms/Button/Button.test.tsx`

## Critical Rules

- Tests MUST fail first (verify test works)
- One test per requirement
- Test behavior, not implementation
- Run FULL test suite before commit
- NEVER skip failing tests

## Common Pitfalls

- Writing implementation before test
- Tests that pass without implementation (false positive)
- Testing implementation details instead of behavior
- Not running test to verify it fails first

## Verification

```bash
# Backend
mix test path/to/test.exs

# Frontend
yarn test path/to/test.tsx
```