Testing Strategy

This skill provides comprehensive guidance for implementing effective testing strategies across your entire application stack.

Test Pyramid

The Testing Hierarchy

        /\
       /  \
      /E2E \       10% - End-to-End Tests (slowest, most expensive)
     /______\
    /        \
   /Integration\  20% - Integration Tests (medium speed/cost)
  /____________\
 /              \
/   Unit Tests   \ 70% - Unit Tests (fast, cheap, focused)
/__________________\

Rationale:

• 70% Unit Tests: Fast, isolated, catch bugs early
• 20% Integration Tests: Test component interactions
• 10% E2E Tests: Test critical user journeys

Why This Distribution?

Unit tests are cheap:

• Run in milliseconds
• No external dependencies
• Easy to debug
• High code coverage per test

Integration tests are moderate:

• Test real interactions
• Catch integration bugs
• Slower than unit tests
• More complex setup

E2E tests are expensive:

• Test entire system
• Catch UX issues
• Very slow (seconds/minutes)
• Brittle and hard to maintain

TDD (Test-Driven Development)

Red-Green-Refactor Cycle

1. Red - Write a failing test:

describe('Calculator', () => {
  test('adds two numbers', () => {
    const calculator = new Calculator();
    expect(calculator.add(2, 3)).toBe(5); // FAILS - method doesn't exist
  });
});

2. Green - Write minimal code to pass:

class Calculator {
  add(a: number, b: number): number {
    return a + b; // Simplest implementation
  }
}
// Test now PASSES

3. Refactor - Improve the code:

class Calculator {
  add(a: number, b: number): number {
    // Add validation
    if (!Number.isFinite(a) || !Number.isFinite(b)) {
      throw new Error('Arguments must be finite numbers');
    }
    return a + b;
  }
}

TDD Benefits

Design benefits:

• Forces you to think about API before implementation
• Leads to more testable, modular code
• Encourages SOLID principles

Quality benefits:

• 100% test coverage by design
• Catches bugs immediately
• Provides living documentation

Workflow benefits:

• Clear next step (make test pass)
• Confidence when refactoring
• Prevents over-engineering

Arrange-Act-Assert Pattern

The AAA Pattern

Every test should follow this structure:

test('user registration creates account and sends welcome email', async () => {
  // ARRANGE - Set up test conditions
  const userData = {
    email: 'test@example.com',
    password: 'SecurePass123',
    name: 'Test User',
  };
  const mockEmailService = jest.fn();
  const userService = new UserService(mockEmailService);

  // ACT - Execute the behavior being tested
  const result = await userService.register(userData);

  // ASSERT - Verify the outcome
  expect(result.id).toBeDefined();
  expect(result.email).toBe(userData.email);
  expect(mockEmailService).toHaveBeenCalledWith({
    to: userData.email,
    subject: 'Welcome!',
    template: 'welcome',
  });
});

Why AAA?

• Clear structure: Easy to understand what's being tested
• Consistent: All tests follow same pattern
• Maintainable: Easy to modify and debug

Mocking Strategies

When to Mock

✅ DO mock:

• External APIs
• Databases
• File system operations
• Time/dates
• Random number generators
• Network requests
• Third-party services

// Mock external API
jest.mock('axios');

test('fetches user data from API', async () => {
  const mockData = { id: 1, name: 'John' };
  (axios.get as jest.Mock).mockResolvedValue({ data: mockData });

  const user = await fetchUser(1);

  expect(user).toEqual(mockData);
});

When NOT to Mock

❌ DON'T mock:

• Pure functions (test them directly)
• Simple utility functions
• Domain logic
• Value objects
• Internal implementation details

// ❌ BAD - Over-mocking
test('validates email', () => {
  const validator = new EmailValidator();
  jest.spyOn(validator, 'isValid').mockReturnValue(true);
  expect(validator.isValid('test@example.com')).toBe(true);
  // This test is useless - you're testing the mock, not the code
});

// ✅ GOOD - Test real implementation
test('validates email', () => {
  const validator = new EmailValidator();
  expect(validator.isValid('test@example.com')).toBe(true);
  expect(validator.isValid('invalid')).toBe(false);
});

Mocking Patterns

Stub (return predetermined values):

const mockDatabase = {
  findUser: jest.fn().mockResolvedValue({ id: 1, name: 'John' }),
  saveUser: jest.fn().mockResolvedValue(true),
};

Spy (track calls, use real implementation):

const emailService = new EmailService();
const sendSpy = jest.spyOn(emailService, 'send');

await emailService.send('test@example.com', 'Hello');

expect(sendSpy).toHaveBeenCalledTimes(1);
expect(sendSpy).toHaveBeenCalledWith('test@example.com', 'Hello');

Fake (lightweight implementation):

class FakeDatabase {
  private data = new Map();

  async save(key: string, value: any) {
    this.data.set(key, value);
  }

  async get(key: string) {
    return this.data.get(key);
  }
}

Test Coverage Goals

Coverage Metrics

Line Coverage: Percentage of code lines executed

• Target: 80-90% for critical paths

Branch Coverage: Percentage of if/else branches tested

• Target: 80%+ (more important than line coverage)

Function Coverage: Percentage of functions called

• Target: 90%+

Statement Coverage: Percentage of statements executed

• Target: 80%+

Coverage Configuration

// package.json
{
  "jest": {
    "collectCoverage": true,
    "coverageThreshold": {
      "global": {
        "branches": 80,
        "functions": 90,
        "lines": 80,
        "statements": 80
      },
      "./src/critical/": {
        "branches": 95,
        "functions": 95,
        "lines": 95,
        "statements": 95
      }
    },
    "coveragePathIgnorePatterns": [
      "/node_modules/",
      "/tests/",
      "/migrations/",
      "/.config.ts$/"
    ]
  }
}

What to Prioritize

High priority (aim for 95%+ coverage):

• Business logic
• Security-critical code
• Payment/billing code
• Data validation
• Authentication/authorization

Medium priority (aim for 80%+ coverage):

• API endpoints
• Database queries
• Utility functions
• Error handling

Low priority (optional coverage):

• UI components (use integration tests instead)
• Configuration files
• Type definitions
• Third-party library wrappers

Integration Testing

Database Integration Tests

import { PrismaClient } from '@prisma/client';

describe('UserRepository', () => {
  let prisma: PrismaClient;
  let repository: UserRepository;

  beforeAll(async () => {
    // Use test database
    prisma = new PrismaClient({
      datasources: { db: { url: process.env.TEST_DATABASE_URL } },
    });
    repository = new UserRepository(prisma);
  });

  beforeEach(async () => {
    // Clean database before each test
    await prisma.user.deleteMany();
  });

  afterAll(async () => {
    await prisma.$disconnect();
  });

  test('creates user and retrieves by email', async () => {
    // ARRANGE
    const userData = {
      email: 'test@example.com',
      name: 'Test User',
      password: 'hashed_password',
    };

    // ACT
    const created = await repository.create(userData);
    const retrieved = await repository.findByEmail(userData.email);

    // ASSERT
    expect(retrieved).toBeDefined();
    expect(retrieved?.id).toBe(created.id);
    expect(retrieved?.email).toBe(userData.email);
  });
});

API Integration Tests

import request from 'supertest';
import { app } from '../src/app';

describe('User API', () => {
  test('POST /api/users creates user and returns 201', async () => {
    const response = await request(app)
      .post('/api/users')
      .send({
        email: 'test@example.com',
        password: 'SecurePass123',
        name: 'Test User',
      })
      .expect(201);

    expect(response.body).toMatchObject({
      email: 'test@example.com',
      name: 'Test User',
    });
    expect(response.body.password).toBeUndefined(); // Never return password
  });

  test('POST /api/users returns 400 for invalid email', async () => {
    const response = await request(app)
      .post('/api/users')
      .send({
        email: 'invalid-email',
        password: 'SecurePass123',
        name: 'Test User',
      })
      .expect(400);

    expect(response.body.error.code).toBe('VALIDATION_ERROR');
  });
});

Service Integration Tests

describe('OrderService Integration', () => {
  test('complete order flow', async () => {
    // Create order
    const order = await orderService.create({
      userId: 'user_123',
      items: [{ productId: 'prod_1', quantity: 2 }],
    });

    // Process payment
    const payment = await paymentService.process({
      orderId: order.id,
      amount: order.total,
    });

    // Verify inventory updated
    const product = await inventoryService.getProduct('prod_1');
    expect(product.stock).toBe(originalStock - 2);

    // Verify order status updated
    const updatedOrder = await orderService.getById(order.id);
    expect(updatedOrder.status).toBe('paid');
  });
});

E2E Testing

Playwright Setup

// playwright.config.ts
import { defineConfig } from '@playwright/test';

export default defineConfig({
  testDir: './e2e',
  fullyParallel: true,
  retries: process.env.CI ? 2 : 0,
  workers: process.env.CI ? 1 : undefined,
  use: {
    baseURL: 'http://localhost:3000',
    trace: 'on-first-retry',
    screenshot: 'only-on-failure',
  },
  projects: [
    { name: 'chromium', use: { browserName: 'chromium' } },
    { name: 'firefox', use: { browserName: 'firefox' } },
    { name: 'webkit', use: { browserName: 'webkit' } },
  ],
});

E2E Test Example

import { test, expect } from '@playwright/test';

test.describe('User Registration Flow', () => {
  test('user can register and login', async ({ page }) => {
    // Navigate to registration page
    await page.goto('/register');

    // Fill registration form
    await page.fill('[name="email"]', 'test@example.com');
    await page.fill('[name="password"]', 'SecurePass123');
    await page.fill('[name="confirmPassword"]', 'SecurePass123');
    await page.fill('[name="name"]', 'Test User');

    // Submit form
    await page.click('button[type="submit"]');

    // Wait for redirect to dashboard
    await page.waitForURL('/dashboard');

    // Verify welcome message
    await expect(page.locator('h1')).toContainText('Welcome, Test User');
  });

  test('shows validation errors for invalid input', async ({ page }) => {
    await page.goto('/register');

    await page.fill('[name="email"]', 'invalid-email');
    await page.fill('[name="password"]', '123'); // Too short

    await page.click('button[type="submit"]');

    // Verify error messages displayed
    await expect(page.locator('[data-testid="email-error"]'))
      .toContainText('Invalid email');
    await expect(page.locator('[data-testid="password-error"]'))
      .toContainText('at least 8 characters');
  });
});

Critical E2E Scenarios

Test these critical user journeys:

• User registration and login
• Checkout and payment flow
• Password reset
• Profile updates
• Critical business workflows

Performance Testing

Load Testing with k6

import http from 'k6/http';
import { check, sleep } from 'k6';

export const options = {
  stages: [
    { duration: '30s', target: 20 },   // Ramp up to 20 users
    { duration: '1m', target: 20 },    // Stay at 20 users
    { duration: '30s', target: 100 },  // Ramp up to 100 users
    { duration: '1m', target: 100 },   // Stay at 100 users
    { duration: '30s', target: 0 },    // Ramp down to 0 users
  ],
  thresholds: {
    http_req_duration: ['p(95)<500'], // 95% of requests under 500ms
    http_req_failed: ['rate<0.01'],   // Less than 1% error rate
  },
};

export default function() {
  const response = http.get('https://api.example.com/users');

  check(response, {
    'status is 200': (r) => r.status === 200,
    'response time < 500ms': (r) => r.timings.duration < 500,
  });

  sleep(1);
}

Benchmark Testing

import { performance } from 'perf_hooks';

describe('Performance Benchmarks', () => {
  test('database query completes in under 100ms', async () => {
    const start = performance.now();

    await database.query('SELECT * FROM users WHERE email = ?', ['test@example.com']);

    const duration = performance.now() - start;
    expect(duration).toBeLessThan(100);
  });

  test('API endpoint responds in under 200ms', async () => {
    const start = performance.now();

    await request(app).get('/api/users/123');

    const duration = performance.now() - start;
    expect(duration).toBeLessThan(200);
  });
});

Flaky Test Prevention

Common Causes of Flaky Tests

1. Race Conditions:

// ❌ BAD - Race condition
test('displays data', async () => {
  fetchData();
  expect(screen.getByText('Data loaded')).toBeInTheDocument();
  // Fails intermittently if fetchData takes longer than expected
});

// ✅ GOOD - Wait for async operation
test('displays data', async () => {
  fetchData();
  await screen.findByText('Data loaded'); // Waits up to 1 second
});

2. Time Dependencies:

// ❌ BAD - Depends on current time
test('shows message for new users', () => {
  const user = { createdAt: new Date() };
  expect(isNewUser(user)).toBe(true);
  // Fails if test runs slowly
});

// ✅ GOOD - Mock time
test('shows message for new users', () => {
  jest.useFakeTimers();
  jest.setSystemTime(new Date('2025-10-16'));

  const user = { createdAt: new Date('2025-10-15') };
  expect(isNewUser(user)).toBe(true);

  jest.useRealTimers();
});

3. Shared State:

// ❌ BAD - Tests share state
let counter = 0;

test('increments counter', () => {
  counter++;
  expect(counter).toBe(1);
});

test('increments counter again', () => {
  counter++;
  expect(counter).toBe(1); // Fails if first test ran
});

// ✅ GOOD - Isolated state
test('increments counter', () => {
  const counter = new Counter();
  counter.increment();
  expect(counter.value).toBe(1);
});

Flaky Test Best Practices

1. Always clean up after tests:

afterEach(async () => {
  await database.truncate();
  jest.clearAllMocks();
  jest.useRealTimers();
});

2. Use explicit waits, not delays:

// ❌ BAD
await sleep(1000);

// ✅ GOOD
await waitFor(() => expect(element).toBeInTheDocument());

3. Isolate test data:

test('creates user', async () => {
  const uniqueEmail = `test-${Date.now()}@example.com`;
  const user = await createUser({ email: uniqueEmail });
  expect(user.email).toBe(uniqueEmail);
});

Test Data Management

Test Fixtures

// fixtures/users.ts
export const testUsers = {
  admin: {
    email: 'admin@example.com',
    password: 'AdminPass123',
    role: 'admin',
  },
  regular: {
    email: 'user@example.com',
    password: 'UserPass123',
    role: 'user',
  },
};

// Usage in tests
import { testUsers } from './fixtures/users';

test('admin can delete users', async () => {
  const admin = await createUser(testUsers.admin);
  // Test admin functionality
});

Factory Pattern

class UserFactory {
  static create(overrides = {}) {
    return {
      id: faker.datatype.uuid(),
      email: faker.internet.email(),
      name: faker.name.fullName(),
      createdAt: new Date(),
      ...overrides,
    };
  }

  static createMany(count: number, overrides = {}) {
    return Array.from({ length: count }, () => this.create(overrides));
  }
}

// Usage
test('displays user list', () => {
  const users = UserFactory.createMany(5);
  render(<UserList users={users} />);
  expect(screen.getAllByRole('listitem')).toHaveLength(5);
});

Database Seeding

// seeds/test-seed.ts
export async function seedTestDatabase() {
  // Create admin user
  const admin = await prisma.user.create({
    data: { email: 'admin@test.com', role: 'admin' },
  });

  // Create test products
  const products = await Promise.all([
    prisma.product.create({ data: { name: 'Product 1', price: 10 } }),
    prisma.product.create({ data: { name: 'Product 2', price: 20 } }),
  ]);

  return { admin, products };
}

// Usage
beforeEach(async () => {
  await prisma.$executeRaw`TRUNCATE TABLE users CASCADE`;
  const { admin, products } = await seedTestDatabase();
});

CI/CD Integration

GitHub Actions Configuration

# .github/workflows/test.yml
name: Tests

on:
  push:
    branches: [main, develop]
  pull_request:
    branches: [main]

jobs:
  test:
    runs-on: ubuntu-latest

    services:
      postgres:
        image: postgres:15
        env:
          POSTGRES_PASSWORD: postgres
        options: >-
          --health-cmd pg_isready
          --health-interval 10s
          --health-timeout 5s
          --health-retries 5

    steps:
      - uses: actions/checkout@v3

      - name: Setup Node.js
        uses: actions/setup-node@v3
        with:
          node-version: '18'
          cache: 'npm'

      - name: Install dependencies
        run: npm ci

      - name: Run linter
        run: npm run lint

      - name: Run type check
        run: npm run type-check

      - name: Run unit tests
        run: npm run test:unit

      - name: Run integration tests
        run: npm run test:integration
        env:
          DATABASE_URL: postgresql://postgres:postgres@localhost:5432/test

      - name: Run E2E tests
        run: npm run test:e2e

      - name: Upload coverage
        uses: codecov/codecov-action@v3
        with:
          files: ./coverage/coverage-final.json
          fail_ci_if_error: true

Test Scripts Organization

// package.json
{
  "scripts": {
    "test": "npm run test:unit && npm run test:integration && npm run test:e2e",
    "test:unit": "jest --testPathPattern=\\.test\\.ts$",
    "test:integration": "jest --testPathPattern=\\.integration\\.ts$",
    "test:e2e": "playwright test",
    "test:watch": "jest --watch",
    "test:coverage": "jest --coverage",
    "test:ci": "jest --ci --coverage --maxWorkers=2"
  }
}

Test Performance in CI

Parallel execution:

jobs:
  test:
    strategy:
      matrix:
        shard: [1, 2, 3, 4]
    steps:
      - run: npm test -- --shard=${{ matrix.shard }}/4

Cache dependencies:

- uses: actions/cache@v3
  with:
    path: ~/.npm
    key: ${{ runner.os }}-node-${{ hashFiles('**/package-lock.json') }}

Test Organization

File Structure

tests/
├── unit/                   # Fast, isolated tests
│   ├── services/
│   │   ├── user-service.test.ts
│   │   └── order-service.test.ts
│   └── utils/
│       ├── validator.test.ts
│       └── formatter.test.ts
├── integration/            # Database, API tests
│   ├── api/
│   │   ├── users.integration.ts
│   │   └── orders.integration.ts
│   └── database/
│       └── repository.integration.ts
├── e2e/                   # End-to-end tests
│   ├── auth.spec.ts
│   ├── checkout.spec.ts
│   └── profile.spec.ts
├── fixtures/              # Test data
│   ├── users.ts
│   └── products.ts
└── helpers/               # Test utilities
    ├── setup.ts
    └── factories.ts

Test Naming Conventions

// Pattern: describe('Component/Function', () => test('should...when...'))

describe('UserService', () => {
  describe('register', () => {
    test('should create user when valid data provided', async () => {
      // Test implementation
    });

    test('should throw error when email already exists', async () => {
      // Test implementation
    });

    test('should hash password before saving', async () => {
      // Test implementation
    });
  });

  describe('login', () => {
    test('should return token when credentials are valid', async () => {
      // Test implementation
    });

    test('should throw error when password is incorrect', async () => {
      // Test implementation
    });
  });
});

When to Use This Skill

Use this skill when:

• Setting up testing infrastructure
• Writing unit, integration, or E2E tests
• Implementing TDD methodology
• Reviewing test coverage
• Debugging flaky tests
• Optimizing test performance
• Configuring CI/CD pipelines
• Establishing testing standards
• Training team on testing practices
• Improving code quality through testing

---

Remember: Good tests give you confidence to refactor, catch bugs early, and serve as living documentation. Invest in your test suite and it will pay dividends throughout the project lifecycle.