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.
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name: designing-tests description: 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.
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
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
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
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
{
"coverageThreshold": {
"global": {
"branches": 80,
"functions": 80,
"lines": 80,
"statements": 80
},
"src/core/": {
"branches": 95,
"functions": 95
}
}
}
Test Data Management
Factories/Builders
// 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
// 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
// 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.
# 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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