React Testing

For general UI testing patterns (queries, events, async, accessibility), load the front-end-testing skill. For TDD workflow, load the tdd skill.

Vitest Browser Mode with React (Preferred)

Always prefer vitest-browser-react over @testing-library/react. Tests run in a real browser, giving production-accurate rendering, events, and CSS.

Setup

npm install -D vitest @vitest/browser-playwright vitest-browser-react @vitejs/plugin-react

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

export default defineConfig({
  plugins: [react()],
  test: {
    browser: {
      enabled: true,
      provider: playwright(),
      headless: true,
      instances: [{ browser: 'chromium' }],
    },
  },
})

Component Testing

import { render } from 'vitest-browser-react'
import { expect, test } from 'vitest'

test('should display user name when provided', async () => {
  const screen = await render(<UserProfile name="Alice" email="alice@example.com" />)

  await expect.element(screen.getByText(/alice/i)).toBeVisible()
  await expect.element(screen.getByText(/alice@example.com/i)).toBeVisible()
})

Key differences from @testing-library/react:

• render() is async — use await
• Returns a screen scoped to the rendered component
• Use expect.element() for auto-retrying assertions
• No act() wrapper needed — CDP events + retry handle timing
• Auto-cleanup happens before each test (not after), so components stay visible for debugging

Testing Props and Callbacks

test('should call onSubmit when form submitted', async () => {
  const handleSubmit = vi.fn()
  const screen = await render(<LoginForm onSubmit={handleSubmit} />)

  await screen.getByLabelText(/email/i).fill('test@example.com')
  await screen.getByRole('button', { name: /submit/i }).click()

  expect(handleSubmit).toHaveBeenCalledWith({
    email: 'test@example.com',
  })
})

Testing Conditional Rendering

test('should show error message when login fails', async () => {
  server.use(
    http.post('/api/login', () => {
      return HttpResponse.json({ error: 'Invalid credentials' }, { status: 401 })
    })
  )

  const screen = await render(<LoginForm />)

  await screen.getByLabelText(/email/i).fill('wrong@example.com')
  await screen.getByRole('button', { name: /submit/i }).click()

  await expect.element(screen.getByText(/invalid credentials/i)).toBeVisible()
})

Testing Hooks with renderHook

import { renderHook } from 'vitest-browser-react'

test('should toggle value', async () => {
  const { result } = await renderHook(() => useToggle(false))

  expect(result.current.value).toBe(false)

  await act(() => {
    result.current.toggle()
  })

  expect(result.current.value).toBe(true)
})

Testing Context Providers

test('should show user menu when authenticated', async () => {
  const screen = await render(
    <AuthProvider initialUser={{ name: 'Alice', role: 'admin' }}>
      <Dashboard />
    </AuthProvider>
  )

  await expect.element(screen.getByRole('button', { name: /user menu/i })).toBeVisible()
})

For hooks that need context:

const { result } = await renderHook(() => useAuth(), {
  wrapper: ({ children }) => (
    <AuthProvider>{children}</AuthProvider>
  ),
})

---

Legacy: @testing-library/react Patterns

The patterns below apply when using @testing-library/react with jsdom. Prefer vitest-browser-react for new projects.

---

Testing React Components

React components are just functions that return JSX. Test them like functions: inputs (props) → output (rendered DOM).

Basic Component Testing

// ✅ CORRECT - Test component behavior
it('should display user name when provided', () => {
  render(<UserProfile name="Alice" email="alice@example.com" />);

  expect(screen.getByText(/alice/i)).toBeInTheDocument();
  expect(screen.getByText(/alice@example.com/i)).toBeInTheDocument();
});

// ❌ WRONG - Testing implementation
it('should set name state', () => {
  const wrapper = mount(<UserProfile name="Alice" />);
  expect(wrapper.state('name')).toBe('Alice'); // Internal state!
});

Testing Props

// ✅ CORRECT - Test how props affect rendered output
it('should call onSubmit when form submitted', async () => {
  const handleSubmit = vi.fn();
  const user = userEvent.setup();

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

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

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

Testing Conditional Rendering

// ✅ CORRECT - Test what user sees in different states
it('should show error message when login fails', async () => {
  server.use(
    http.post('/api/login', () => {
      return HttpResponse.json({ error: 'Invalid credentials' }, { status: 401 });
    })
  );

  const user = userEvent.setup();
  render(<LoginForm />);

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

  await screen.findByText(/invalid credentials/i);
});

---

Testing React Hooks

Custom Hooks with renderHook

Built into @testing-library/react (import directly, no separate package needed):

import { renderHook } from '@testing-library/react';

it('should toggle value', () => {
  const { result } = renderHook(() => useToggle(false));

  expect(result.current.value).toBe(false);

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

  expect(result.current.value).toBe(true);
});

Pattern:

• result.current - Current return value of hook
• act() - Wrap state updates
• rerender() - Re-run hook with new props

Hooks with Props

it('should accept initial value', () => {
  const { result, rerender } = renderHook(
    ({ initialValue }) => useCounter(initialValue),
    { initialProps: { initialValue: 10 } }
  );

  expect(result.current.count).toBe(10);

  // Test with different initial value
  rerender({ initialValue: 20 });
  expect(result.current.count).toBe(20);
});

---

Testing Context

wrapper Option

For hooks that need context providers:

const { result } = renderHook(() => useAuth(), {
  wrapper: ({ children }) => (
    <AuthProvider>
      {children}
    </AuthProvider>
  ),
});

expect(result.current.user).toBeNull();

act(() => {
  result.current.login({ email: 'test@example.com' });
});

expect(result.current.user).toEqual({ email: 'test@example.com' });

Multiple Providers

const AllProviders = ({ children }) => (
  <AuthProvider>
    <ThemeProvider>
      <RouterProvider>
        {children}
      </RouterProvider>
    </ThemeProvider>
  </AuthProvider>
);

const { result } = renderHook(() => useMyHook(), {
  wrapper: AllProviders,
});

Testing Components with Context

// ✅ CORRECT - Wrap component in provider
const renderWithAuth = (ui, { user = null, ...options } = {}) => {
  return render(
    <AuthProvider initialUser={user}>
      {ui}
    </AuthProvider>,
    options
  );
};

it('should show user menu when authenticated', () => {
  renderWithAuth(<Dashboard />, {
    user: { name: 'Alice', role: 'admin' },
  });

  expect(screen.getByRole('button', { name: /user menu/i })).toBeInTheDocument();
});

---

Testing Forms

Controlled Inputs

it('should update input value as user types', async () => {
  const user = userEvent.setup();

  render(<SearchInput />);

  const input = screen.getByLabelText(/search/i);

  await user.type(input, 'react');

  expect(input).toHaveValue('react');
});

Form Submissions

it('should submit form with user input', async () => {
  const handleSubmit = vi.fn();
  const user = userEvent.setup();

  render(<RegistrationForm onSubmit={handleSubmit} />);

  await user.type(screen.getByLabelText(/name/i), 'Alice');
  await user.type(screen.getByLabelText(/email/i), 'alice@example.com');
  await user.type(screen.getByLabelText(/password/i), 'password123');
  await user.click(screen.getByRole('button', { name: /sign up/i }));

  expect(handleSubmit).toHaveBeenCalledWith({
    name: 'Alice',
    email: 'alice@example.com',
    password: 'password123',
  });
});

Form Validation

it('should show validation errors for invalid input', async () => {
  const user = userEvent.setup();

  render(<RegistrationForm />);

  // Submit empty form
  await user.click(screen.getByRole('button', { name: /sign up/i }));

  // Validation errors appear
  expect(screen.getByText(/name is required/i)).toBeInTheDocument();
  expect(screen.getByText(/email is required/i)).toBeInTheDocument();
  expect(screen.getByText(/password is required/i)).toBeInTheDocument();
});

---

React-Specific Anti-Patterns

1. Unnecessary act() wrapping

❌ WRONG - Manual act() everywhere

act(() => {
  render(<MyComponent />);
});

await act(async () => {
  await user.click(button);
});

✅ CORRECT - RTL handles it

render(<MyComponent />);
await user.click(button);

Modern RTL auto-wraps:

• render()
• userEvent methods
• fireEvent
• waitFor, findBy

When you DO need manual act():

• Custom hook state updates (renderHook)
• Direct state mutations (rare, usually bad practice)

---

2. Manual cleanup() calls

❌ WRONG - Manual cleanup

afterEach(() => {
  cleanup(); // Automatic since RTL 9!
});

✅ CORRECT - No cleanup needed

// Cleanup happens automatically after each test

---

3. beforeEach render pattern

❌ WRONG - Shared render in beforeEach

let button;
beforeEach(() => {
  render(<MyComponent />);
  button = screen.getByRole('button'); // Shared state across tests
});

it('test 1', () => {
  // Uses shared button from beforeEach
});

✅ CORRECT - Factory function per test

const renderComponent = () => {
  render(<MyComponent />);
  return {
    button: screen.getByRole('button'),
  };
};

it('test 1', () => {
  const { button } = renderComponent(); // Fresh state
});

For factory patterns, see testing skill.

---

4. Testing component internals

❌ WRONG - Accessing component internals

const wrapper = shallow(<MyComponent />);
expect(wrapper.state('isOpen')).toBe(true); // Internal state
expect(wrapper.instance().handleClick).toBeDefined(); // Internal method

✅ CORRECT - Test rendered output

render(<MyComponent />);
expect(screen.getByRole('dialog')).toBeInTheDocument(); // What user sees

---

5. Shallow rendering

❌ WRONG - Shallow rendering

const wrapper = shallow(<MyComponent />);
// Child components not rendered - incomplete test

✅ CORRECT - Full rendering

render(<MyComponent />);
// Full component tree rendered - realistic test

Why: Shallow rendering hides integration bugs between parent/child components.

---

Testing Loading States

it('should show loading then data', async () => {
  render(<UserList />);

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

  // Wait for data
  await screen.findByText(/alice/i);

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

---

Testing Error Boundaries

it('should catch errors with error boundary', () => {
  // Suppress console.error for this test
  const spy = vi.spyOn(console, 'error').mockImplementation(() => {});

  render(
    <ErrorBoundary fallback={<div>Something went wrong</div>}>
      <ThrowsError />
    </ErrorBoundary>
  );

  expect(screen.getByText(/something went wrong/i)).toBeInTheDocument();

  spy.mockRestore();
});

---

Testing Portals

it('should render modal in portal', () => {
  render(<Modal isOpen={true}>Modal content</Modal>);

  // Portal renders outside root, but Testing Library finds it
  expect(screen.getByText(/modal content/i)).toBeInTheDocument();
});

Testing Library queries the entire document, so portals work automatically.

---

Testing Suspense

it('should show fallback then content', async () => {
  render(
    <Suspense fallback={<div>Loading...</div>}>
      <LazyComponent />
    </Suspense>
  );

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

  // Wait for component
  await screen.findByText(/lazy content/i);
});

---

Summary Checklist

React-specific checks:

• Preferred: Using vitest-browser-react with Vitest Browser Mode (real browser)
• Fallback: Using @testing-library/react if Browser Mode not yet configured
• All Playwright/Browser Mode tests are idempotent (no shared state between tests)
• Using renderHook() for custom hooks
• Using wrapper option for context providers
• No manual act() calls (handled automatically)
• No manual cleanup() calls (automatic)
• Testing component output, not internal state
• Using factory functions, not beforeEach render
• Using expect.element() for auto-retrying assertions (Browser Mode)
• Following TDD workflow (see tdd skill)
• Using general UI testing patterns (see front-end-testing skill)
• Using test factories for data (see testing skill)