beforeAll  → runs once before all tests in a describe block
beforeEach → runs before each individual test
afterEach  → runs after each individual test (cleanup)
afterAll   → runs once after all tests in a describe block

// src/cart.test.ts
import { describe, it, expect, beforeEach } from 'vitest';
import { Cart } from './cart';

describe('Cart', () => {
  let cart: Cart;

  beforeEach(() => {
    // Arrange - fresh cart for each test, no shared state
    cart = new Cart();
  });

  it('starts empty', () => {
    // Assert only - trivial arrange already done
    expect(cart.itemCount()).toBe(0);
    expect(cart.total()).toBe(0);
  });

  it('adds items and updates total', () => {
    // Act
    cart.add({ id: '1', name: 'Widget', price: 9.99, quantity: 2 });

    // Assert
    expect(cart.itemCount()).toBe(2);
    expect(cart.total()).toBeCloseTo(19.98);
  });

  it('throws when adding an item with zero quantity', () => {
    expect(() =>
      cart.add({ id: '1', name: 'Widget', price: 9.99, quantity: 0 })
    ).toThrow('Quantity must be positive');
  });
});

// src/order-service.test.ts
import { describe, it, expect, vi, beforeEach } from 'vitest';

// Module mock hoisted to top of file by Vitest/Jest
vi.mock('./payment-gateway', () => ({
  charge: vi.fn(),
}));
vi.mock('./mailer', () => ({
  sendConfirmation: vi.fn(),
}));

import { placeOrder } from './order-service';
import { charge } from './payment-gateway';
import { sendConfirmation } from './mailer';

describe('placeOrder', () => {
  beforeEach(() => {
    vi.resetAllMocks();
  });

  it('charges the customer and sends a confirmation on success', async () => {
    // Arrange
    vi.mocked(charge).mockResolvedValue({ success: true, transactionId: 'txn_123' });
    const order = { id: 'ord_1', total: 49.99, customer: { email: 'a@b.com' } };

    // Act
    await placeOrder(order);

    // Assert
    expect(charge).toHaveBeenCalledWith({ amount: 49.99, orderId: 'ord_1' });
    expect(sendConfirmation).toHaveBeenCalledWith('a@b.com', 'ord_1');
  });

  it('throws OrderFailedError when payment is declined', async () => {
    vi.mocked(charge).mockResolvedValue({ success: false, error: 'Insufficient funds' });
    const order = { id: 'ord_2', total: 200, customer: { email: 'a@b.com' } };

    await expect(placeOrder(order)).rejects.toThrow('OrderFailedError');
    expect(sendConfirmation).not.toHaveBeenCalled();
  });
});

import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
import { fetchUser } from './user-api';

// --- Promises ---
it('resolves with user data', async () => {
  const user = await fetchUser('user-1');
  expect(user).toMatchObject({ id: 'user-1', name: expect.any(String) });
});

it('rejects when user is not found', async () => {
  await expect(fetchUser('nonexistent')).rejects.toThrow('User not found');
});

// --- Fake timers (debounce, throttle, setTimeout) ---
describe('debounced search', () => {
  beforeEach(() => { vi.useFakeTimers(); });
  afterEach(() => { vi.useRealTimers(); });

  it('fires callback once after debounce delay', () => {
    const callback = vi.fn();
    const search = createDebouncedSearch(callback, 300);

    search('re');
    search('rea');
    search('react');

    expect(callback).not.toHaveBeenCalled();
    vi.advanceTimersByTime(300);
    expect(callback).toHaveBeenCalledOnce();
    expect(callback).toHaveBeenCalledWith('react');
  });
});

// --- Event emitters ---
it('emits "ready" after initialization', () =>
  new Promise<void>((resolve) => {
    const service = new DataService();
    service.on('ready', () => {
      expect(service.isReady()).toBe(true);
      resolve();
    });
    service.init();
  })
);

import { describe, it, expect } from 'vitest';
import { serializeCartSummary } from './cart-serializer';

describe('serializeCartSummary', () => {
  it('produces stable JSON for a standard cart', () => {
    const cart = buildCart([
      { sku: 'A1', qty: 2, price: 10 },
      { sku: 'B3', qty: 1, price: 25.5 },
    ]);

    // Snapshot is useful here: the serialization format is complex and
    // must remain stable for API consumers.
    expect(serializeCartSummary(cart)).toMatchSnapshot();
  });
});

// When output changes intentionally, review the diff then run:
// npx vitest --updateSnapshot
// Do NOT blindly run -u without reading the diff first.

// vitest.config.ts
import { defineConfig } from 'vitest/config';
import react from '@vitejs/plugin-react';

export default defineConfig({
  plugins: [react()],
  test: {
    environment: 'jsdom',       // use 'node' for server-side code
    globals: true,              // avoids importing describe/it/expect in every file
    setupFiles: ['./src/test-setup.ts'],
    coverage: {
      provider: 'v8',
      reporter: ['text', 'lcov', 'html'],
      thresholds: {
        branches: 80,
        functions: 80,
        lines: 80,
        statements: 80,
      },
      exclude: [
        'src/**/*.d.ts',
        'src/**/index.ts',      // barrel files
        'src/**/*.stories.tsx', // Storybook
      ],
    },
  },
});

// src/test-setup.ts
import '@testing-library/jest-dom'; // extends expect with .toBeInTheDocument() etc.
import { afterEach } from 'vitest';
import { cleanup } from '@testing-library/react';

afterEach(() => {
  cleanup(); // unmount React trees after each test
});

// src/components/LoginForm.test.tsx
import { describe, it, expect, vi } from 'vitest';
import { render, screen, fireEvent, waitFor } from '@testing-library/react';
import userEvent from '@testing-library/user-event';
import { LoginForm } from './LoginForm';

describe('LoginForm', () => {
  it('submits email and password when the form is valid', async () => {
    const user = userEvent.setup();
    const onSubmit = vi.fn().mockResolvedValue(undefined);

    render(<LoginForm onSubmit={onSubmit} />);

    await user.type(screen.getByLabelText(/email/i), 'user@example.com');
    await user.type(screen.getByLabelText(/password/i), 'secret123');
    await user.click(screen.getByRole('button', { name: /log in/i }));

    await waitFor(() => {
      expect(onSubmit).toHaveBeenCalledWith({
        email: 'user@example.com',
        password: 'secret123',
      });
    });
  });

  it('shows a validation error when email is empty', async () => {
    const user = userEvent.setup();
    render(<LoginForm onSubmit={vi.fn()} />);

    await user.click(screen.getByRole('button', { name: /log in/i }));

    expect(screen.getByText(/email is required/i)).toBeInTheDocument();
  });
});

undefined

To find untested branches, look for `E` (else not taken) and `I` (if not taken)
markers in the Istanbul HTML report. Focus on:
1. Error paths - what happens when a fetch fails, input is invalid, or a service throws
2. Guard clauses - early returns and null checks
3. Complex conditionals - expressions with multiple `&&` / `||` operators

---