Test-Driven Development (TDD)

Red → Green → Refactor cycle for all code changes.

The TDD Cycle

• RED: Write failing test

• GREEN: Write minimal code to pass

• 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