Testing Strategies: Unit, Integration & E2E

Why Testing Matters

Quality software requires a solid testing strategy. Tests serve multiple purposes: they catch bugs early, document how code should behave, enable refactoring with confidence, and improve overall code quality. Without tests, you're essentially flying blind when making changes to your codebase.

Unit Testing

Unit tests verify individual functions or components in isolation. They're fast, focused, and essential for catching logic errors.

// Using Jest
describe('calculateTotal', () => {
  it('should sum array of numbers', () => {
    const items = [10, 20, 30];
    const result = calculateTotal(items);
    expect(result).toBe(60);
  });

  it('should return 0 for empty array', () => {
    expect(calculateTotal([])).toBe(0);
  });

  it('should handle negative numbers', () => {
    expect(calculateTotal([10, -5, 3])).toBe(8);
  });
});

Integration Testing

Integration tests verify that different parts of your application work together correctly. They test interactions between components, database calls, and API integrations.

// Testing API endpoint
describe('POST /api/posts', () => {
  it('should create a new post', async () => {
    const response = await request(app)
      .post('/api/posts')
      .send({
        title: 'Test Post',
        content: 'Test content'
      });

    expect(response.status).toBe(201);
    expect(response.body).toHaveProperty('id');
    expect(response.body.title).toBe('Test Post');
  });
});

End-to-End Testing

E2E tests simulate real user behavior, testing the complete flow from UI interaction through backend processing. Cypress and Playwright are popular frameworks:

// Using Cypress
describe('Blog Post Creation', () => {
  it('should create and display a new blog post', () => {
    cy.visit('/blog');
    cy.contains('New Post').click();
    
    cy.get('input[name="title"]').type('My New Post');
    cy.get('textarea[name="content"]').type('Great content');
    cy.get('button[type="submit"]').click();
    
    cy.contains('My New Post').should('be.visible');
  });
});

Testing Best Practices

• Test behavior, not implementation: Focus on what the code does, not how it does it
• Keep tests simple: One test, one concept
• Use descriptive test names: Clearly describe what's being tested
• Test error cases: Don't just test the happy path
• Mock external dependencies: Use mocks for APIs and databases in unit tests
• Aim for high coverage: Target 80%+ code coverage as a goal

Test Pyramid Strategy

The test pyramid suggests having many unit tests, fewer integration tests, and even fewer E2E tests. This approach provides fast feedback while maintaining comprehensive coverage:

• Bottom (Many): Unit tests - fast, isolated, specific
• Middle: Integration tests - test component interactions
• Top (Few): E2E tests - test critical user flows

Conclusion

A well-balanced testing strategy is an investment in code quality and developer confidence. By combining unit, integration, and E2E tests, you create a safety net that allows you to refactor and improve your code without fear of breaking functionality.