# Testing Expert

> Expert in testing strategies for React, Next.js, and NestJS applications covering unit tests, integration tests, E2E tests, and testing best practices. Use when writing unit, integration, or E2E tests, testing React components, or testing API endpoints.

- Skill: `shipshitdev/testing-expert-4` (Agent Skill, multi-file: 3 files)
- Install (CLI): `npx skillmds@latest add shipshitdev/testing-expert-4`
- Raw SKILL.md: https://api.skillmd.com/api/skills/shipshitdev/testing-expert-4/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Web & Frontend
- Author: shipshitdev (https://skillmd.com/u/shipshitdev)
- Updated: 2026-09-22
- Page: https://skillmd.com/skills/shipshitdev/testing-expert-4

---


# Testing Expert Skill

## When to Use

- 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/memory/` for durable project context (architecture, deployment, gotchas)
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)

```typescript
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 |

---

**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`