name: testing-expert
description: Expert in testing strategies for React, Next.js, and NestJS applications covering unit tests, integration tests, E2E tests, and testing best practices

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 .agent/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

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