React Testing

React Testing

Full Reference: See advanced.md for MSW setup, testing hooks, testing context, forms, accessibility testing, snapshots, and test patterns.

Deep Knowledge: Use mcp__documentation__fetch_docs with technology: react topic: testing for comprehensive documentation.

Setup with Vitest + Testing Library

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

export default defineConfig({
  plugins: [react()],
  test: {
    environment: 'jsdom',
    globals: true,
    setupFiles: './src/test/setup.ts',
    css: true,
  },
});

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

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

Basic Component Testing

import { render, screen } from '@testing-library/react';
import userEvent from '@testing-library/user-event';
import { describe, it, expect, vi } from 'vitest';

describe('Button', () => {
  it('renders children', () => {
    render(<Button => {}}>Click me</Button>);
    expect(screen.getByRole('button', { name: /click me/i })).toBeInTheDocument();
  });

  it('calls onClick when clicked', async () => {
    const handleClick = vi.fn();
    render(<Button me</Button>);

    await userEvent.click(screen.getByRole('button'));

    expect(handleClick).toHaveBeenCalledTimes(1);
  });

  it('is disabled when disabled prop is true', () => {
    render(<Button => {}} disabled>Click me</Button>);
    expect(screen.getByRole('button')).toBeDisabled();
  });
});

Query Methods

// Priority order (prefer accessible queries)
// 1. getByRole - accessible to everyone
screen.getByRole('button', { name: /submit/i });
screen.getByRole('textbox', { name: /email/i });
screen.getByRole('heading', { level: 1 });

// 2. getByLabelText - for form fields
screen.getByLabelText(/email address/i);

// 3. getByPlaceholderText
screen.getByPlaceholderText(/enter your email/i);

// 4. getByText - for non-interactive elements
screen.getByText(/welcome to our app/i);

// 5. getByDisplayValue - for input values
screen.getByDisplayValue(/john@example.com/i);

// 6. getByAltText - for images
screen.getByAltText(/user avatar/i);

// 7. getByTitle
screen.getByTitle(/close/i);

// 8. getByTestId - last resort
screen.getByTestId('custom-element');

// Query variants
screen.getByRole('button');     // Throws if not found
screen.queryByRole('button');   // Returns null if not found
screen.findByRole('button');    // Returns Promise, waits for element
screen.getAllByRole('button');  // Returns array, throws if none
screen.queryAllByRole('button'); // Returns array (possibly empty)
screen.findAllByRole('button'); // Returns Promise of array

User Events

import userEvent from '@testing-library/user-event';

describe('Form', () => {
  it('submits with user input', async () => {
    const user = userEvent.setup();
    const handleSubmit = vi.fn();

    render(<LoginForm />);

    // Type in inputs
    await user.type(screen.getByLabelText(/email/i), 'john@example.com');
    await user.type(screen.getByLabelText(/password/i), 'secret123');

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

    expect(handleSubmit).toHaveBeenCalledWith({
      email: 'john@example.com',
      password: 'secret123',
    });
  });

  it('handles keyboard navigation', async () => {
    const user = userEvent.setup();
    render(<Form />);

    // Tab through inputs
    await user.tab();
    expect(screen.getByLabelText(/email/i)).toHaveFocus();

    await user.tab();
    expect(screen.getByLabelText(/password/i)).toHaveFocus();

    // Type and submit with Enter
    await user.type(screen.getByLabelText(/password/i), 'secret{Enter}');
  });

  it('handles select and checkbox', async () => {
    const user = userEvent.setup();
    render(<SettingsForm />);

    // Select option
    await user.selectOptions(screen.getByRole('combobox'), ['dark']);

    // Toggle checkbox
    await user.click(screen.getByRole('checkbox', { name: /notifications/i }));

    expect(screen.getByRole('checkbox')).toBeChecked();
  });
});

Async Testing

describe('UserProfile', () => {
  it('shows loading then user data', async () => {
    render(<UserProfile userId="123" />);

    // Initially shows loading
    expect(screen.getByText(/loading/i)).toBeInTheDocument();

    // Wait for data to load
    await waitFor(() => {
      expect(screen.getByText('John Doe')).toBeInTheDocument();
    });

    // Loading is gone
    expect(screen.queryByText(/loading/i)).not.toBeInTheDocument();
  });
});

// Using findBy (combines getBy + waitFor)
it('loads and displays items', async () => {
  render(<ItemList />);

  // findBy waits for element
  const items = await screen.findAllByRole('listitem');
  expect(items).toHaveLength(3);
});

// waitForElementToBeRemoved
it('removes loading indicator', async () => {
  render(<DataLoader />);

  await waitForElementToBeRemoved(() => screen.queryByText(/loading/i));

  expect(screen.getByText('Data loaded')).toBeInTheDocument();
});

Mocking

Mock Functions

import { vi } from 'vitest';

const mockFn = vi.fn();
mockFn.mockReturnValue('default');
mockFn.mockReturnValueOnce('first call');
mockFn.mockImplementation((x) => x * 2);
mockFn.mockResolvedValue({ data: [] });
mockFn.mockRejectedValue(new Error('Failed'));

// Assertions
expect(mockFn).toHaveBeenCalled();
expect(mockFn).toHaveBeenCalledTimes(2);
expect(mockFn).toHaveBeenCalledWith('arg1', 'arg2');
expect(mockFn).toHaveBeenLastCalledWith('last arg');

Mock Modules

// Mock a module
vi.mock('@/lib/api', () => ({
  fetchUsers: vi.fn(() => Promise.resolve([{ id: 1, name: 'John' }])),
}));

// Import after mocking
import { fetchUsers } from '@/lib/api';

it('fetches users', async () => {
  render(<UserList />);

  expect(fetchUsers).toHaveBeenCalled();
  await screen.findByText('John');
});

Common Pitfalls

Best Practices

• Test behavior, not implementation
• Use accessible queries (getByRole)
• Use userEvent over fireEvent
• Test error states
• Use MSW for API mocking
• Don't test implementation details
• Don't test third-party libraries
• Don't overuse snapshots

When NOT to Use This Skill

• End-to-end testing - Use Playwright skill for full E2E flows
• Backend testing - Use framework-specific testing skills
• Performance testing - Use specialized performance testing tools
• Visual regression testing - Use tools like Percy or Chromatic

Anti-Patterns

Quick Troubleshooting

Reference Documentation

• Testing Library
• Vitest
• MSW
• MCP: mcp__documentation__fetch_docs → technology: react, topic: testing