Testing Strategies

When to use this skill

• New project: define a testing strategy
• Quality issues: bugs happen frequently
• Before refactoring: build a safety net
• CI/CD setup: automated tests

Instructions

Step 1: Understand the Test Pyramid

       /\
      /E2E\          ← few (slow, expensive)
     /______\
    /        \
   /Integration\    ← medium
  /____________\
 /              \
/   Unit Tests   \  ← many (fast, inexpensive)
/________________\

Ratio guide:

• Unit: 70%
• Integration: 20%
• E2E: 10%

Step 2: Unit testing strategy

Given-When-Then pattern:

describe('calculateDiscount', () => {
  it('should apply 10% discount for orders over $100', () => {
    // Given: setup
    const order = { total: 150, customerId: '123' };

    // When: perform action
    const discount = calculateDiscount(order);

    // Then: verify result
    expect(discount).toBe(15);
  });

  it('should not apply discount for orders under $100', () => {
    const order = { total: 50, customerId: '123' };
    const discount = calculateDiscount(order);
    expect(discount).toBe(0);
  });

  it('should throw error for invalid order', () => {
    const order = { total: -10, customerId: '123' };
    expect(() => calculateDiscount(order)).toThrow('Invalid order');
  });
});

Mocking strategy:

// Mock external dependencies
jest.mock('../services/emailService');
import { sendEmail } from '../services/emailService';

describe('UserService', () => {
  it('should send welcome email on registration', async () => {
    // Arrange
    const mockSendEmail = sendEmail as jest.MockedFunction<typeof sendEmail>;
    mockSendEmail.mockResolvedValueOnce(true);

    // Act
    await userService.register({ email: '[email protected]', password: 'pass' });

    // Assert
    expect(mockSendEmail).toHaveBeenCalledWith({
      to: '[email protected]',
      subject: 'Welcome!',
      body: expect.any(String)
    });
  });
});

Step 3: Integration Testing

API endpoint tests:

describe('POST /api/users', () => {
  beforeEach(async () => {
    await db.user.deleteMany();  // Clean DB
  });

  it('should create user with valid data', async () => {
    const response = await request(app)
      .post('/api/users')
      .send({
        email: '[email protected]',
        username: 'testuser',
        password: 'Password123!'
      });

    expect(response.status).toBe(201);
    expect(response.body.user).toMatchObject({
      email: '[email protected]',
      username: 'testuser'
    });

    // Verify it was actually saved to the DB
    const user = await db.user.findUnique({ where: { email: '[email protected]' } });
    expect(user).toBeTruthy();
  });

  it('should reject duplicate email', async () => {
    // Create first user
    await request(app)
      .post('/api/users')
      .send({ email: '[email protected]', username: 'user1', password: 'Pass123!' });

    // Attempt duplicate
    const response = await request(app)
      .post('/api/users')
      .send({ email: '[email protected]', username: 'user2', password: 'Pass123!' });

    expect(response.status).toBe(409);
  });
});

Step 4: E2E Testing (Playwright)

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

test.describe('User Registration Flow', () => {
  test('should complete full registration process', async ({ page }) => {
    // 1. Visit homepage
    await page.goto('http://localhost:3000');

    // 2. Click Sign Up button
    await page.click('text=Sign Up')