React Testing

Overview

This skill provides comprehensive guidance for testing React applications using Vitest, React Testing Library, and Jest. Apply these patterns when writing unit tests, integration tests, and ensuring code quality.

Core Philosophy

• Test behavior, not implementation: Focus on what users see and do
• Avoid testing internal state: Test public APIs and user interactions
• Write tests that give confidence: Catch real bugs, not false positives
• Keep tests simple: Tests should be easier to understand than the code

Testing Tools

Vitest Configuration

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

export default defineConfig({
  plugins: [react()],
  test: {
    globals: true,
    environment: 'jsdom',
    setupFiles: './src/test/setup.ts',
    coverage: {
      provider: 'v8',
      reporter: ['text', 'json', 'html'],
      exclude: ['node_modules/', 'src/test/']
    }
  }
});

Test Setup

// src/test/setup.ts
import { expect, afterEach } from 'vitest';
import { cleanup } from '@testing-library/react';
import * as matchers from '@testing-library/jest-dom/matchers';

expect.extend(matchers);

afterEach(() => {
  cleanup();
});

Component Testing

Basic Component Test

import { render, screen } from '@testing-library/react';
import { expect, test } from 'vitest';
import { Button } from './Button';

test('renders button with label', () => {
  render(<Button label="Click me" onClick={vi.fn()} />);
  expect(screen.getByText('Click me')).toBeInTheDocument();
});

test('calls onClick when clicked', async () => {
  const handleClick = vi.fn();
  render(<Button label="Click me" onClick={handleClick} />);

  await userEvent.click(screen.getByRole('button'));
  expect(handleClick).toHaveBeenCalledTimes(1);
});

test('is disabled when prop is set', () => {
  render(<Button label="Click me" onClick={vi.fn()} disabled />);
  expect(screen.getByRole('button')).toBeDisabled();
});

Form Testing

import { render, screen, waitFor } from '@testing-library/react';
import userEvent from '@testing-library/user-event';
import { LoginForm } from './LoginForm';

test('submits form with email and password', async () => {
  const handleSubmit = vi.fn();
  render(<LoginForm onSubmit={handleSubmit} />);

  await userEvent.type(screen.getByLabelText('Email'), 'user@example.com');
  await userEvent.type(screen.getByLabelText('Password'), 'password123');
  await userEvent.click(screen.getByRole('button', { name: 'Login' }));

  await waitFor(() => {
    expect(handleSubmit).toHaveBeenCalledWith({
      email: 'user@example.com',
      password: 'password123'
    });
  });
});

test('shows validation error for invalid email', async () => {
  render(<LoginForm onSubmit={vi.fn()} />);

  await userEvent.type(screen.getByLabelText('Email'), 'invalid-email');
  await userEvent.click(screen.getByRole('button', { name: 'Login' }));

  expect(await screen.findByText('Invalid email address')).toBeInTheDocument();
});

Hook Testing

Custom Hook Test

import { renderHook, act } from '@testing-library/react';
import { expect, test } from 'vitest';
import { useCounter } from './useCounter';

test('increments counter', () => {
  const { result } = renderHook(() => useCounter());

  expect(result.current.count).toBe(0);

  act(() => {
    result.current.increment();
  });

  expect(result.current.count).toBe(1);
});

test('decrements counter', () => {
  const { result } = renderHook(() => useCounter(10));

  act(() => {
    result.current.decrement();
  });

  expect(result.current.count).toBe(9);
});

Hook with Dependencies

import { renderHook, waitFor } from '@testing-library/react';
import { useApi } from './useApi';

test('fetches data successfully', async () => {
  global.fetch = vi.fn(() =>
    Promise.resolve({
      ok: true,
      json: () => Promise.resolve({ name: 'John' })
    })
  ) as any;

  const { result } = renderHook(() => useApi('/api/user'));

  expect(result.current.loading).toBe(true);

  await waitFor(() => {
    expect(result.current.loading).toBe(false);
  });

  expect(result.current.data).toEqual({ name: 'John' });
  expect(result.current.error).toBeNull();
});

test('handles error', async () => {
  global.fetch = vi.fn(() =>
    Promise.reject(new Error('Network error'))
  );

  const { result } = renderHook(() => useApi('/api/user'));

  await waitFor(() => {
    expect(result.current.error).toBeTruthy();
  });

  expect(result.current.data).toBeUndefined();
});

Integration Testing

With Context

import { render, screen } from '@testing-library/react';
import { AuthProvider } from './AuthContext';
import { Dashboard } from './Dashboard';

const renderWithAuth = (ui: ReactElement, { user = null } = {}) => {
  return render(
    <AuthProvider value={{ user }}>
      {ui}
    </AuthProvider>
  );
};

test('shows dashboard when authenticated', () => {
  renderWithAuth(<Dashboard />, { user: { name: 'John' } });
  expect(screen.getByText('Welcome, John')).toBeInTheDocument();
});

test('redirects to login when not authenticated', () => {
  renderWithAuth(<Dashboard />);
  expect(screen.queryByText('Welcome')).not.toBeInTheDocument();
});

With Router

import { render, screen } from '@testing-library/react';
import { MemoryRouter } from 'react-router-dom';
import { App } from './App';

test('navigates to profile page', async () => {
  render(
    <MemoryRouter initialEntries={['/profile']}>
      <App />
    </MemoryRouter>
  );

  expect(screen.getByText('User Profile')).toBeInTheDocument();
});

Mocking

API Mocking

import { rest } from 'msw';
import { setupServer } from 'msw/node';

const server = setupServer(
  rest.get('/api/users', (req, res, ctx) => {
    return res(
      ctx.json([
        { id: '1', name: 'John' },
        { id: '2', name: 'Jane' }
      ])
    );
  })
);

beforeAll(() => server.listen());
afterEach(() => server.resetHandlers());
afterAll(() => server.close());

test('displays users from API', async () => {
  render(<UserList />);

  expect(await screen.findByText('John')).toBeInTheDocument();
  expect(await screen.findByText('Jane')).toBeInTheDocument();
});

Component Mocking

import { vi } from 'vitest';

vi.mock('./HeavyComponent', () => ({
  HeavyComponent: () => <div>Mocked Component</div>
}));

test('renders page with mocked component', () => {
  render(<Dashboard />);
  expect(screen.getByText('Mocked Component')).toBeInTheDocument();
});

Accessibility Testing

import { axe, toHaveNoViolations } from 'jest-axe';

expect.extend(toHaveNoViolations);

test('has no accessibility violations', async () => {
  const { container } = render(<LoginForm onSubmit={vi.fn()} />);
  const results = await axe(container);
  expect(results).toHaveNoViolations();
});

Best Practices

1. Use semantic queries: getByRole, getByLabelText over getByTestId
2. Test user behavior: Click, type, navigate like users do
3. Avoid implementation details: Don't test state or props directly
4. Use findBy for async: Automatically waits for elements
5. Clean up: Use afterEach(cleanup) to reset DOM
6. Mock external dependencies: APIs, timers, modules
7. Test error states: Loading, error, empty states
8. Keep tests focused: One assertion per test when possible
9. Use descriptive test names: Describe what should happen
10. Run tests in CI/CD: Ensure tests pass before deployment