Testing Expert Skill

Expert in testing strategies for React, Next.js, and NestJS applications.

When to Use This Skill

• Writing unit tests
• Creating integration tests
• Setting up E2E tests
• Testing React components
• Testing API endpoints
• Testing database operations
• Setting up test infrastructure
• Reviewing test coverage

Project Context Discovery

1. Scan Documentation: Check .agents/SYSTEM/ARCHITECTURE.md for testing architecture
2. Identify Tools: Jest/Vitest, React Testing Library, Supertest, Playwright/Cypress
3. Discover Patterns: Review existing test files, utilities, mocking patterns
4. Use Project-Specific Skills: Check for [project]-testing-expert skill

Core Testing Principles

Testing Pyramid

• Unit Tests (70%): Fast, isolated, test individual functions/components
• Integration Tests (20%): Test component interactions
• E2E Tests (10%): Test full user flows

Coverage Targets

• Line coverage: > 80%
• Branch coverage: > 75%
• Function coverage: > 85%
• Critical paths: 100%

Test Organization

src/
  users/
    users.controller.ts
    users.controller.spec.ts  # Unit tests
    users.service.ts
    users.service.spec.ts
  __tests__/
    integration/
    e2e/

Test Quality (AAA Pattern)

it('should return users filtered by organization', async () => {
  // Arrange: Set up test data
  const organizationId = 'org1';
  const expectedUsers = [{ organization: organizationId }];

  // Act: Execute the code being tested
  const result = await service.findAll(organizationId);

  // Assert: Verify the result
  expect(result).toEqual(expectedUsers);
});

Good Tests Are

• Independent (no test dependencies)
• Fast (< 100ms each)
• Repeatable (same result every time)
• Meaningful (test real behavior)
• Maintainable (easy to update)

Testing Best Practices Summary

1. Test Isolation: Each test independent, clean up after
2. Meaningful Tests: Test behavior, not implementation
3. Mocking Strategy: Mock external dependencies, not what you're testing
4. Test Data: Use factories, keep data minimal, clean up
5. Coverage: High coverage, focus on critical paths

Integration

Test Type	Tools	Use Case
Unit	Jest/Vitest	Functions, components, services
Integration	Supertest + Jest	Controller + Service + DB
E2E	Playwright/Cypress	Full user flows
Component	React Testing Library	React component behavior

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For complete React Testing Library examples, hook testing, Next.js page/API testing, NestJS service/controller testing, integration test setup, E2E test patterns, MongoDB testing, authentication helpers, test fixtures, and mocking patterns, see: references/full-guide.md