react-testing

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React component testing patterns including components, hooks, context, and forms. Covers Vitest Browser Mode with vitest-browser-react (preferred) and @testing-library/react. Use when testing React applications. For general UI testing patterns, see the front-end-testing skill.

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Sourcecitypaul/.dotfiles

Added on2026-05-24

NPX Install

npx skill4agent add citypaul/.dotfiles react-testing

SKILL.md Content

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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

bash

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

typescript

// 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

tsx

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

tsx

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

tsx

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

tsx

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

tsx

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:

tsx

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

tsx

// ✅ 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();
});

tsx

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

Testing Props

tsx

// ✅ 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

tsx

// ✅ 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):

tsx

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

tsx

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:

tsx

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

tsx

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

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

Testing Components with Context

tsx

// ✅ 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

tsx

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

tsx

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

tsx

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

tsx

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

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

✅ CORRECT - RTL handles it

tsx

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

tsx

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

✅ CORRECT - No cleanup needed

tsx

// Cleanup happens automatically after each test

3. beforeEach render pattern

❌ WRONG - Shared render in beforeEach

tsx

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

tsx

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

tsx

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

✅ CORRECT - Test rendered output

tsx

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

5. Shallow rendering

❌ WRONG - Shallow rendering

tsx

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

✅ CORRECT - Full rendering

tsx

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

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

Testing Loading States

tsx

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

tsx

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

tsx

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

tsx

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)

react-testing

NPX Install

Tags

SKILL.md Content

React Testing

Vitest Browser Mode with React (Preferred)

Setup

Component Testing

Testing Props and Callbacks

Testing Conditional Rendering

Testing Hooks with renderHook

Testing Context Providers

Legacy: @testing-library/react Patterns

Testing React Components

Basic Component Testing

Testing Props

Testing Conditional Rendering

Testing React Hooks

Custom Hooks with renderHook

Hooks with Props

Testing Context

wrapper Option

Multiple Providers

Testing Components with Context

Testing Forms

Controlled Inputs

Form Submissions

Form Validation

React-Specific Anti-Patterns

1. Unnecessary act() wrapping

2. Manual cleanup() calls

3. beforeEach render pattern

4. Testing component internals

5. Shallow rendering

Testing Loading States

Testing Error Boundaries

Testing Portals

Testing Suspense

Summary Checklist