Testing Typescript

TypeScript Testing

Target: $ARGUMENTS

Writes focused, behavior-driven tests following project testing strategy.

Quick Reference

TDD methodology (language-agnostic): See tdd-core plugin (testing-tdd skill)

TypeScript-specific documentation: references/

• references/testing-strategy-ts.md — TypeScript tools (vitest, @testing-library/react, MSW)
• references/tdd-best-practices-ts.md — TypeScript TDD examples (extends tdd-core)

Quick Decision

vitest (default): Use it/test for known cases, vi.mock for module mocking. Works at unit/integration levels.

@testing-library/react (components): Use for React component behavior testing with render, screen, userEvent.

See references/testing-strategy-ts.md for full methodology comparison.

TDD Essentials (Quick Reference)

Cycle: RED (failing test / type error) -> GREEN (minimal pass) -> REFACTOR (clean up)

Structure: Arrange-Act-Assert (AAA)

it('calculates order total from item prices', () => {
  // ARRANGE
  const items = [{ price: 10, qty: 2 }, { price: 5, qty: 1 }];
  const calculator = new OrderCalculator();

  // ACT
  const total = calculator.total(items);

  // ASSERT
  expect(total).toBe(25);
});

Component Testing Priorities

What to Test (KISS/DRY/YAGNI)

High-Value: Business logic, error paths, component behavior, async flows, custom hooks

Avoid: Type system behavior, library internals, CSS styling, implementation details

See references/testing-strategy-ts.md -> "Patterns to Remove" for full list.

Naming Convention

Format: {module} > {component} > {behavior}

describe('UserService', () => {
  it('creates a new user with valid data', () => { ... });
  it('rejects duplicate email addresses', () => { ... });
});

Execution

npx vitest                          # Watch mode (default)
npx vitest run                      # Single run
npx vitest run src/utils            # Filter by path
npx vitest run --reporter=verbose   # Verbose output
npx vitest --ui                     # Browser UI

Quality Gates

• All tests pass (npx vitest run)
• TDD Red-Green-Refactor followed
• Arrange-Act-Assert structure used
• Naming convention followed
• Behavior-focused (not implementation)
• No library/framework behavior tested
• Mocks use vi.mock() with proper restore (vi.restoreAllMocks())