react-testing

React Testing Skill

Patterns for testing React components with React Testing Library.

Testing Philosophy

React Testing Library encourages testing components the way users interact with them, not implementation details.

The Guiding Principles

1. Test behavior, not implementation
2. Find elements like a user would
3. Interact like a user would
4. Assert on what the user sees

Rendering Components

Basic Rendering

import { render, screen } from '@testing-library/react'

test('renders greeting', () => {
  render(<Greeting name="World" />)

  expect(screen.getByText('Hello, World!')).toBeInTheDocument()
})

Rendering with Providers

function renderWithProviders(
  ui: ReactElement,
  {
    preloadedState = {},
    ...renderOptions
  } = {}
) {
  function Wrapper({ children }: { children: ReactNode }) {
    return (
      <Provider store={setupStore(preloadedState)}>
        <ThemeProvider>
          <Router>
            {children}
          </Router>
        </ThemeProvider>
      </Provider>
    )
  }

  return render(ui, { wrapper: Wrapper, ...renderOptions })
}

// Usage
test('renders with providers', () => {
  renderWithProviders(<UserDashboard />)
  expect(screen.getByText('Dashboard')).toBeInTheDocument()
})

Queries

Query Priority

Use queries in this order (most to least preferred):

// 1. Accessible queries (preferred)
screen.getByRole('button', { name: /submit/i })
screen.getByLabelText('Email')
screen.getByPlaceholderText('Enter email')
screen.getByText('Hello World')
screen.getByDisplayValue('current value')

// 2. Semantic queries
screen.getByAltText('Profile picture')
screen.getByTitle('Close')

// 3. Test IDs (last resort)
screen.getByTestId('custom-element')

Query Variants

// getBy* - Throws if not found (use for elements that should exist)
screen.getByRole('button')

// queryBy* - Returns null if not found (use for asserting absence)
expect(screen.queryByText('Loading')).not.toBeInTheDocument()

// findBy* - Returns promise, waits for element (use for async)
const button = await screen.findByRole('button')

// *AllBy* - Returns array of all matches
const items = screen.getAllByRole('listitem')

Role-Based Queries

// Buttons
screen.getByRole('button', { name: /submit/i })

// Links
screen.getByRole('link', { name: /home/i })

// Form elements
screen.getByRole('textbox', { name: /email/i })
screen.getByRole('checkbox', { name: /remember me/i })
screen.getByRole('combobox', { name: /country/i })

// Headings
screen.getByRole('heading', { name: /welcome/i, level: 1 })

// Navigation
screen.getByRole('navigation')

// Lists
screen.getByRole('list')
screen.getAllByRole('listitem')

User Interactions

Click Events

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

test('button click increments counter', async () => {
  const user = userEvent.setup()
  render(<Counter />)

  const button = screen.getByRole('button', { name: /increment/i })
  await user.click(button)

  expect(screen.getByText('Count: 1')).toBeInTheDocument()
})

Typing

test('form submission', async () => {
  const user = userEvent.setup()
  const onSubmit = jest.fn()
  render(<LoginForm onSubmit={onSubmit} />)

  await user.type(screen.getByLabelText(/email/i), '[email protected]')
  await user.type(screen.getByLabelText(/password/i), 'password123')
  await user.click(screen.getByRole('button', { name: /sign in/i }))

  expect(onSubmit).toHaveBeenCalledWith({
    email: '[email protected]',
    password: 'password123',
  })
})

Select and Keyboard

test('select option', async () => {
  const user = userEvent.setup()
  render(<CountrySelect />)

  await user.selectOptions(
    screen.getByRole('combobox', { name: /country/i }),
    'US'
  )

  expect(screen.getByRole('combobox')).toHaveValue('US')
})

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

  await user.tab()  // Focus first input
  await user.type(screen.getByLabelText(/name/i), 'John')
  await user.tab()  // Focus next input
  await user.keyboard('{Enter}')  // Submit form
})

Async Testing

Waiting for Elements

test('loads user data', async () => {
  render(<UserProfile userId="123" />)

  // Wait for loading to finish
  expect(screen.getByText(/loading/i)).toBeInTheDocument()

  // Wait for data to appear
  expect(await screen.findByText('John Doe')).toBeInTheDocument()
})

waitFor

import { waitFor } from '@testing-library/react'

test('form validation', async () => {
  const user = userEvent.setup()
  render(<Form />)

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

  await waitFor(() => {
    expect(screen.getByText(/email is required/i)).toBeInTheDocument()
  })
})

test('disappearing element', async () => {
  render(<Notification />)

  expect(screen.getByText(/success/i)).toBeInTheDocument()

  await waitFor(() => {
    expect(screen.queryByText(/success/i)).not.toBeInTheDocument()
  })
})

waitForElementToBeRemoved

test('loading state disappears', async () => {
  render(<DataLoader />)

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

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

Mocking

Mocking API Calls

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

const server = setupServer(
  rest.get('/api/user/:id', (req, res, ctx) => {
    return res(ctx.json({ id: req.params.id, name: 'John Doe' }))
  })
)

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

test('fetches user data', async () => {
  render(<UserProfile userId="123" />)

  expect(await screen.findByText('John Doe')).toBeInTheDocument()
})

test('handles error', async () => {
  server.use(
    rest.get('/api/user/:id', (req, res, ctx) => {
      return res(ctx.status(500))
    })
  )

  render(<UserProfile userId="123" />)

  expect(await screen.findByText(/error loading user/i)).toBeInTheDocument()
})

Mocking Hooks

jest.mock('../hooks/useUser', () => ({
  useUser: jest.fn(),
}))

import { useUser } from '../hooks/useUser'

test('renders user when loaded', () => {
  (useUser as jest.Mock).mockReturnValue({
    user: { name: 'John' },
    loading: false,
    error: null,
  })

  render(<UserProfile />)
  expect(screen.getByText('John')).toBeInTheDocument()
})

Mocking Router

import { MemoryRouter, Route, Routes } from 'react-router-dom'

test('navigates on click', async () => {
  const user = userEvent.setup()

  render(
    <MemoryRouter initialEntries={['/']}>
      <Routes>
        <Route path="/" element={<Home />} />
        <Route path="/about" element={<About />} />
      </Routes>
    </MemoryRouter>
  )

  await user.click(screen.getByRole('link', { name: /about/i }))

  expect(screen.getByText(/about page/i)).toBeInTheDocument()
})

Component Testing Patterns

Testing Forms

describe('RegistrationForm', () => {
  test('shows validation errors for empty required fields', async () => {
    const user = userEvent.setup()
    render(<RegistrationForm />)

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

    expect(await screen.findByText(/name is required/i)).toBeInTheDocument()
    expect(screen.getByText(/email is required/i)).toBeInTheDocument()
  })

  test('submits with valid data', async () => {
    const user = userEvent.setup()
    const onSubmit = jest.fn()
    render(<RegistrationForm onSubmit={onSubmit} />)

    await user.type(screen.getByLabelText(/name/i), 'John Doe')
    await user.type(screen.getByLabelText(/email/i), '[email protected]')
    await user.click(screen.getByRole('button', { name: /register/i }))

    expect(onSubmit).toHaveBeenCalledWith({
      name: 'John Doe',
      email: '[email protected]',
    })
  })
})

Testing Modals

test('opens and closes modal', async () => {
  const user = userEvent.setup()
  render(<App />)

  // Modal not visible initially
  expect(screen.queryByRole('dialog')).not.toBeInTheDocument()

  // Open modal
  await user.click(screen.getByRole('button', { name: /open modal/i }))
  expect(screen.getByRole('dialog')).toBeInTheDocument()

  // Close modal
  await user.click(screen.getByRole('button', { name: /close/i }))
  await waitForElementToBeRemoved(() => screen.queryByRole('dialog'))
})

Testing Lists

test('renders and filters list items', async () => {
  const user = userEvent.setup()
  render(<TodoList />)

  // Verify initial items
  const items = screen.getAllByRole('listitem')
  expect(items).toHaveLength(3)

  // Filter items
  await user.type(screen.getByRole('searchbox'), 'buy')

  const filteredItems = screen.getAllByRole('listitem')
  expect(filteredItems).toHaveLength(1)
  expect(filteredItems[0]).toHaveTextContent('Buy groceries')
})

Accessibility Testing

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

expect.extend(toHaveNoViolations)

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

Debug Utilities

test('debugging', () => {
  render(<Component />)

  // Print current DOM
  screen.debug()

  // Print specific element
  screen.debug(screen.getByRole('button'))

  // Log accessible roles
  screen.logTestingPlaygroundURL()  // Opens Testing Playground with current DOM
})

Integration

Used by:

• frontend-developer agent
• e2e-test-specialist agent
• unit-test-specialist agent