Agent skill

designing-tests

Designs and implements testing strategies for any codebase. Use when adding tests, improving coverage, setting up testing infrastructure, debugging test failures, or when asked about unit tests, integration tests, or E2E testing.

View SKILL.md on GitHub Repository
Stars 232
Forks 15

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SKILL.md

Designing Tests

Test Implementation Workflow

Copy this checklist and track progress:

Test Implementation Progress:
- [ ] Step 1: Identify what to test
- [ ] Step 2: Select appropriate test type
- [ ] Step 3: Write tests following templates
- [ ] Step 4: Run tests and verify passing
- [ ] Step 5: Check coverage meets targets
- [ ] Step 6: Fix any failing tests

Testing Pyramid

Apply the testing pyramid for balanced coverage:

        /\
       /  \     E2E Tests (10%)
      /----\    - Critical user journeys
     /      \   - Slow but comprehensive
    /--------\  Integration Tests (20%)
   /          \ - Component interactions
  /------------\ - API contracts
 /              \ Unit Tests (70%)
/________________\ - Fast, isolated
                   - Business logic focus

Framework Selection

JavaScript/TypeScript

Type Recommended Alternative
Unit Vitest Jest
Integration Vitest + MSW Jest + SuperTest
E2E Playwright Cypress
Component Testing Library Enzyme

Python

Type Recommended Alternative
Unit pytest unittest
Integration pytest + httpx pytest + requests
E2E Playwright Selenium
API pytest + FastAPI TestClient -

Go

Type Recommended
Unit testing + testify
Integration testing + httptest
E2E testing + chromedp

Test Structure Templates

Unit Test

javascript
describe('[Unit] ComponentName', () => {
  describe('methodName', () => {
    it('should [expected behavior] when [condition]', () => {
      // Arrange
      const input = createTestInput();

      // Act
      const result = methodName(input);

      // Assert
      expect(result).toEqual(expectedOutput);
    });

    it('should throw error when [invalid condition]', () => {
      expect(() => methodName(invalidInput)).toThrow(ExpectedError);
    });
  });
});

Integration Test

javascript
describe('[Integration] API /users', () => {
  beforeAll(async () => {
    await setupTestDatabase();
  });

  afterAll(async () => {
    await teardownTestDatabase();
  });

  it('should create user and return 201', async () => {
    const response = await request(app)
      .post('/users')
      .send({ name: 'Test', email: 'test@example.com' });

    expect(response.status).toBe(201);
    expect(response.body.id).toBeDefined();
  });
});

E2E Test

javascript
describe('[E2E] User Registration Flow', () => {
  it('should complete registration successfully', async ({ page }) => {
    await page.goto('/register');

    await page.fill('[data-testid="email"]', 'new@example.com');
    await page.fill('[data-testid="password"]', 'SecurePass123!');
    await page.click('[data-testid="submit"]');

    await expect(page.locator('.welcome-message')).toBeVisible();
    await expect(page).toHaveURL('/dashboard');
  });
});

Coverage Strategy

What to Cover

  • ✅ Business logic (100%)
  • ✅ Edge cases and error handling (90%+)
  • ✅ API contracts (100%)
  • ✅ Critical user paths (E2E)
  • ⚠️ UI components (snapshot + interaction)
  • ❌ Third-party library internals
  • ❌ Simple getters/setters

Coverage Thresholds

json
{
  "coverageThreshold": {
    "global": {
      "branches": 80,
      "functions": 80,
      "lines": 80,
      "statements": 80
    },
    "src/core/": {
      "branches": 95,
      "functions": 95
    }
  }
}

Test Data Management

Factories/Builders

javascript
// factories/user.js
export const userFactory = (overrides = {}) => ({
  id: faker.string.uuid(),
  name: faker.person.fullName(),
  email: faker.internet.email(),
  createdAt: new Date(),
  ...overrides,
});

// Usage
const admin = userFactory({ role: 'admin' });

Fixtures

javascript
// fixtures/users.json
{
  "validUser": { "name": "Test", "email": "test@example.com" },
  "invalidUser": { "name": "", "email": "invalid" }
}

Mocking Strategy

When to Mock

  • ✅ External APIs and services
  • ✅ Database in unit tests
  • ✅ Time/Date for determinism
  • ✅ Random values
  • ❌ Internal modules (usually)
  • ❌ The code under test

Mock Examples

javascript
// API mocking with MSW
import { http, HttpResponse } from 'msw';

export const handlers = [
  http.get('/api/users', () => {
    return HttpResponse.json([
      { id: 1, name: 'John' },
    ]);
  }),
];

// Time mocking
vi.useFakeTimers();
vi.setSystemTime(new Date('2024-01-01'));

Test Validation Loop

After writing tests, run this validation:

Test Validation:
- [ ] All tests pass: `npm test`
- [ ] Coverage meets thresholds: `npm test -- --coverage`
- [ ] No flaky tests (run multiple times)
- [ ] Tests are independent (order doesn't matter)
- [ ] Test names clearly describe behavior

If any tests fail, fix them before proceeding. If coverage is below target, add more tests for uncovered code paths.

bash
# Run tests
npm test

# Run with coverage
npm test -- --coverage

# Run specific test file
npm test -- path/to/test.spec.ts

# Run in watch mode during development
npm test -- --watch

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