Testing Best Practices

// Pattern 1: "should" pattern
test('should return sum of two numbers', () => {
  expect(add(2, 3)).toBe(5);
});

// Pattern 2: Behavioral description
test('returns user data when ID exists', () => {
  const user = findUser(1);
  expect(user).toBeDefined();
});

// Pattern 3: Given-When-Then format
test('given valid email, when user signs up, then account is created', () => {
  const result = signup('test@example.com', 'password');
  expect(result.success).toBe(true);
});

// Pattern 4: Error scenarios
test('throws error when dividing by zero', () => {
  expect(() => divide(10, 0)).toThrow('Division by zero');
});

// Pattern 5: Edge cases
test('returns empty array when input is null', () => {
  expect(filterItems(null)).toEqual([]);
});

// ❌ BAD: Tests depend on execution order
let user;

test('creates user', () => {
  user = createUser('Alice'); // Sets global variable
  expect(user).toBeDefined();
});

test('updates user', () => {
  updateUser(user, { age: 25 }); // Depends on previous test!
  expect(user.age).toBe(25);
});

// ✅ GOOD: Each test is independent
test('creates user with correct data', () => {
  const user = createUser('Alice');
  expect(user.name).toBe('Alice');
  expect(user.id).toBeDefined();
});

test('updates user age successfully', () => {
  const user = createUser('Bob'); // Create fresh user
  updateUser(user, { age: 30 });
  expect(user.age).toBe(30);
});

// ✅ BETTER: Use beforeEach for common setup
describe('User operations', () => {
  let user;
  
  beforeEach(() => {
    user = createUser('Alice'); // Fresh user for each test
  });
  
  test('has correct name', () => {
    expect(user.name).toBe('Alice');
  });
  
  test('can update age', () => {
    updateUser(user, { age: 25 });
    expect(user.age).toBe(25);
  });
});

// test-helpers.js - Shared test utilities
export const createTestUser = (overrides = {}) => ({
  id: 1,
  name: 'Test User',
  email: 'test@example.com',
  role: 'user',
  ...overrides
});

export const createTestProduct = (overrides = {}) => ({
  id: 1,
  name: 'Test Product',
  price: 99.99,
  stock: 10,
  ...overrides
});

// Using in tests
import { createTestUser, createTestProduct } from './test-helpers';

test('applies employee discount', () => {
  const employee = createTestUser({ role: 'employee' });
  const product = createTestProduct({ price: 100 });
  
  const discountedPrice = calculatePrice(product, employee);
  
  expect(discountedPrice).toBe(80); // 20% employee discount
});

test('regular users pay full price', () => {
  const regularUser = createTestUser(); // Default role: 'user'
  const product = createTestProduct({ price: 100 });
  
  const price = calculatePrice(product, regularUser);
  
  expect(price).toBe(100); // No discount
});

describe('Shopping Cart', () => {
  let cart;
  
  beforeEach(() => {
    cart = new ShoppingCart(); // Fresh cart for each test
  });
  
  describe('adding items', () => {
    test('adds item to cart', () => {
      cart.add({ id: 1, name: 'Book', price: 20 });
      expect(cart.items).toHaveLength(1);
    });
    
    test('increases quantity for duplicate items', () => {
      const item = { id: 1, name: 'Book', price: 20 };
      cart.add(item);
      cart.add(item);
      
      expect(cart.items).toHaveLength(1);
      expect(cart.items[0].quantity).toBe(2);
    });
  });
  
  describe('calculating total', () => {
    beforeEach(() => {
      cart.add({ id: 1, name: 'Book', price: 20 });
      cart.add({ id: 2, name: 'Pen', price: 5 });
    });
    
    test('calculates total without discount', () => {
      expect(cart.getTotal()).toBe(25);
    });
    
    test('applies discount code', () => {
      cart.applyDiscount('SAVE10'); // 10% off
      expect(cart.getTotal()).toBe(22.5);
    });
  });
  
  afterEach(() => {
    cart.clear(); // Cleanup
  });
});