---

name: test-driven-development description: Use when writing new functions, adding features, fixing bugs, or refactoring by applying TDD principles - write failing tests before implementation code, make them pass, then refactor. allowed-tools:

• Write
• Read
• Edit
• Bash
• Grep

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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

# 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

# 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

# Backend
mix test path/to/test.exs

# Frontend
yarn test path/to/test.tsx