Component Tester

Expert skill for testing UI component libraries with Vitest and React Testing Library. Specializes in writing comprehensive tests, analyzing coverage, validating accessibility, and ensuring component quality.

Core Capabilities

1. Test Writing

• Unit tests for individual components
• Integration tests for component interactions
• Snapshot tests for visual regression
• Accessibility tests (a11y)
• User interaction tests (clicks, typing, keyboard nav)
• Async behavior testing (loading states, data fetching)
• Edge case and error state testing

2. Testing Tools Mastery

• Vitest: Fast test runner with native ESM support
• React Testing Library: User-centric testing
• @testing-library/user-event: Realistic user interactions
• @testing-library/jest-dom: Custom matchers
• @axe-core/react: Accessibility testing
• msw: API mocking

3. Test Patterns

• Arrange-Act-Assert (AAA) pattern
• Test fixtures and factories
• Custom render functions
• Reusable test utilities
• Mock management
• Test data builders

4. Coverage Analysis

• Line coverage metrics
• Branch coverage analysis
• Function coverage tracking
• Statement coverage
• Identify untested code paths
• Generate coverage reports (HTML, LCOV, JSON)

5. Accessibility Validation

• Screen reader compatibility
• Keyboard navigation testing
• ARIA attribute validation
• Color contrast checks
• Focus management tests
• Semantic HTML validation

6. Component Quality Checks

• PropTypes validation
• TypeScript type checking
• Performance testing
• Memory leak detection
• Render efficiency
• Bundle size impact

Workflow

Phase 1: Test Planning

1. Analyze Component

   • Understand component behavior
   • Identify user interactions
   • List edge cases and error states
   • Determine accessibility requirements
   • Note performance considerations

2. Define Test Strategy

   • What to test (user behavior, not implementation)
   • Which interactions to cover
   • Which edge cases matter
   • What NOT to test (implementation details)

3. Set Up Test Environment

   • Create test file
   • Import necessary utilities
   • Set up custom render function
   • Prepare mocks and fixtures

Phase 2: Writing Tests

1. Basic Rendering Tests

   • Component renders without errors
   • Default props render correctly
   • Required props are handled

2. User Interaction Tests

   • Click events work
   • Keyboard navigation functions
   • Form inputs update correctly
   • Hover/focus states trigger

3. State Management Tests

   • Internal state updates correctly
   • Controlled component behavior
   • Uncontrolled component behavior
   • State persistence

4. Accessibility Tests

   • ARIA roles and labels present
   • Keyboard navigation works
   • Screen reader announcements
   • Focus management correct

5. Edge Case Tests

   • Empty states
   • Loading states
   • Error states
   • Boundary values
   • Invalid inputs

Phase 3: Running & Analysis

1. Execute Tests

   • Run test suite
   • Watch mode for development
   • Generate coverage report
   • Check for failures

2. Analyze Coverage

   • Review coverage percentages
   • Identify untested code
   • Prioritize missing tests
   • Set coverage thresholds

3. Review Results

   • Fix failing tests
   • Improve test quality
   • Refactor when needed
   • Document findings

Testing Patterns & Examples

Custom Render Function

// test-utils.tsx
import { render, RenderOptions } from '@testing-library/react'
import { ReactElement } from 'react'
import { ThemeProvider } from './theme-provider'

interface CustomRenderOptions extends Omit<RenderOptions, 'wrapper'> {
  theme?: 'light' | 'dark'
}

function customRender(
  ui: ReactElement,
  { theme = 'light', ...options }: CustomRenderOptions = {}
) {
  function Wrapper({ children }: { children: React.ReactNode }) {
    return <ThemeProvider theme={theme}>{children}</ThemeProvider>
  }

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

export * from '@testing-library/react'
export { customRender as render }

Basic Component Test

// Button.test.tsx
import { render, screen } from './test-utils'
import { userEvent } from '@testing-library/user-event'
import { Button } from './Button'

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

  it('calls onClick when clicked', async () => {
    const handleClick = vi.fn()
    const user = userEvent.setup()

    render(<Button onClick={handleClick}>Click me</Button>)

    await user.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()
  })

  it('applies variant styles', () => {
    const { rerender } = render(<Button variant="primary">Primary</Button>)
    expect(screen.getByRole('button')).toHaveClass('btn-primary')

    rerender(<Button variant="secondary">Secondary</Button>)
    expect(screen.getByRole('button')).toHaveClass('btn-secondary')
  })
})

Async Testing

// DataFetcher.test.tsx
import { render, screen, waitFor } from './test-utils'
import { DataFetcher } from './DataFetcher'
import { server } from './mocks/server'
import { rest } from 'msw'

describe('DataFetcher', () => {
  it('displays loading state initially', () => {
    render(<DataFetcher url="/api/data" />)
    expect(screen.getByText(/loading/i)).toBeInTheDocument()
  })

  it('displays data after successful fetch', async () => {
    render(<DataFetcher url="/api/data" />)

    await waitFor(() => {
      expect(screen.getByText(/data loaded/i)).toBeInTheDocument()
    })
  })

  it('displays error on failed fetch', async () => {
    server.use(
      rest.get('/api/data', (req, res, ctx) => {
        return res(ctx.status(500), ctx.json({ error: 'Server error' }))
      })
    )

    render(<DataFetcher url="/api/data" />)

    await waitFor(() => {
      expect(screen.getByText(/error/i)).toBeInTheDocument()
    })
  })
})

User Interaction Testing

// Form.test.tsx
import { render, screen } from './test-utils'
import { userEvent } from '@testing-library/user-event'
import { Form } from './Form'

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

    render(<Form onSubmit={handleSubmit} />)

    // Type in input
    await user.type(screen.getByLabelText(/username/i), 'john_doe')

    // Select option
    await user.selectOptions(screen.getByLabelText(/role/i), 'admin')

    // Check checkbox
    await user.click(screen.getByLabelText(/agree to terms/i))

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

    expect(handleSubmit).toHaveBeenCalledWith({
      username: 'john_doe',
      role: 'admin',
      agreedToTerms: true,
    })
  })

  it('validates required fields', async () => {
    const user = userEvent.setup()

    render(<Form />)

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

    expect(screen.getByText(/username is required/i)).toBeInTheDocument()
  })
})

Keyboard Navigation Testing

// Menu.test.tsx
import { render, screen } from './test-utils'
import { userEvent } from '@testing-library/user-event'
import { Menu } from './Menu'

describe('Menu keyboard navigation', () => {
  it('opens menu with Enter key', async () => {
    const user = userEvent.setup()
    render(<Menu />)

    const trigger = screen.getByRole('button', { name: /open menu/i })
    await user.tab() // Focus trigger
    await user.keyboard('{Enter}')

    expect(screen.getByRole('menu')).toBeInTheDocument()
  })

  it('navigates items with arrow keys', async () => {
    const user = userEvent.setup()
    render(<Menu defaultOpen />)

    const items = screen.getAllByRole('menuitem')

    await user.tab() // Focus first item
    expect(items[0]).toHaveFocus()

    await user.keyboard('{ArrowDown}')
    expect(items[1]).toHaveFocus()

    await user.keyboard('{ArrowUp}')
    expect(items[0]).toHaveFocus()
  })

  it('closes menu with Escape key', async () => {
    const user = userEvent.setup()
    render(<Menu defaultOpen />)

    expect(screen.getByRole('menu')).toBeInTheDocument()

    await user.keyboard('{Escape}')

    expect(screen.queryByRole('menu')).not.toBeInTheDocument()
  })
})

Accessibility Testing

// Button.a11y.test.tsx
import { render } from './test-utils'
import { axe, toHaveNoViolations } from 'jest-axe'
import { Button } from './Button'

expect.extend(toHaveNoViolations)

describe('Button accessibility', () => {
  it('has no accessibility violations', async () => {
    const { container } = render(<Button>Click me</Button>)
    const results = await axe(container)
    expect(results).toHaveNoViolations()
  })

  it('has correct ARIA label when icon-only', async () => {
    const { container } = render(
      <Button aria-label="Close dialog">
        <XIcon />
      </Button>
    )
    const results = await axe(container)
    expect(results).toHaveNoViolations()
  })
})

Snapshot Testing

// Card.test.tsx
import { render } from './test-utils'
import { Card } from './Card'

describe('Card snapshots', () => {
  it('matches snapshot with default props', () => {
    const { container } = render(
      <Card>
        <Card.Header>Title</Card.Header>
        <Card.Content>Content</Card.Content>
      </Card>
    )
    expect(container.firstChild).toMatchSnapshot()
  })

  it('matches snapshot with all variants', () => {
    const variants = ['default', 'bordered', 'elevated'] as const

    variants.forEach((variant) => {
      const { container } = render(
        <Card variant={variant}>
          <Card.Content>Content</Card.Content>
        </Card>
      )
      expect(container.firstChild).toMatchSnapshot()
    })
  })
})

Mock Management

// setupTests.ts
import { beforeAll, afterEach, afterAll } from 'vitest'
import { server } from './mocks/server'

beforeAll(() => server.listen({ onUnhandledRequest: 'error' }))
afterEach(() => server.resetHandlers())
afterAll(() => server.close())

// mocks/server.ts
import { setupServer } from 'msw/node'
import { handlers } from './handlers'

export const server = setupServer(...handlers)

// mocks/handlers.ts
import { rest } from 'msw'

export const handlers = [
  rest.get('/api/data', (req, res, ctx) => {
    return res(
      ctx.status(200),
      ctx.json({ data: 'Mock data' })
    )
  }),
]

Test Configuration

Vitest Config

// 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',
    css: true,
    coverage: {
      provider: 'v8',
      reporter: ['text', 'html', 'lcov'],
      exclude: [
        'node_modules/',
        'src/test/',
        '**/*.d.ts',
        '**/*.config.*',
        '**/mockData',
        'dist/',
      ],
      thresholds: {
        lines: 80,
        functions: 80,
        branches: 80,
        statements: 80,
      },
    },
  },
})

Setup File

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

// Cleanup after each test
afterEach(() => {
  cleanup()
})

// Mock window.matchMedia
Object.defineProperty(window, 'matchMedia', {
  writable: true,
  value: vi.fn().mockImplementation(query => ({
    matches: false,
    media: query,
    onchange: null,
    addListener: vi.fn(),
    removeListener: vi.fn(),
    addEventListener: vi.fn(),
    removeEventListener: vi.fn(),
    dispatchEvent: vi.fn(),
  })),
})

// Mock IntersectionObserver
global.IntersectionObserver = class IntersectionObserver {
  constructor() {}
  disconnect() {}
  observe() {}
  takeRecords() {
    return []
  }
  unobserve() {}
} as any

Testing Best Practices

What to Test

✅ DO Test:

• User-visible behavior
• Accessibility features
• User interactions (clicks, typing, keyboard nav)
• Different prop combinations
• Edge cases and error states
• Loading and async states
• Integration between components

❌ DON'T Test:

• Implementation details
• Internal state directly
• Styling (use visual regression tools instead)
• Third-party libraries
• Framework internals

Writing Effective Tests

1. Use Queries in Priority Order

   // 1. Accessible to all (best)
   getByRole('button', { name: /submit/i })
   getByLabelText(/username/i)
   getByPlaceholderText(/enter email/i)
   getByText(/welcome/i)
   
   // 2. Semantic (good)
   getByAltText(/profile picture/i)
   getByTitle(/tooltip/i)
   
   // 3. Test IDs (last resort)
   getByTestId('submit-button')
   

2. Wait for Async Changes

   // Use waitFor for assertions
   await waitFor(() => {
     expect(screen.getByText(/data loaded/i)).toBeInTheDocument()
   })
   
   // Use findBy for queries (combines getBy + waitFor)
   expect(await screen.findByText(/data loaded/i)).toBeInTheDocument()
   

3. User-Event Over FireEvent

   // Prefer userEvent (more realistic)
   await user.click(button)
   await user.type(input, 'hello')
   
   // Avoid fireEvent when possible
   fireEvent.click(button) // Less realistic
   

4. Descriptive Test Names

   // Good
   it('displays error message when email is invalid', () => {})
   it('disables submit button while form is submitting', () => {})
   
   // Bad
   it('works', () => {})
   it('test button', () => {})
   

5. Arrange-Act-Assert Pattern

   it('increments counter on click', async () => {
     // Arrange
     const user = userEvent.setup()
     render(<Counter />)
   
     // Act
     await user.click(screen.getByRole('button', { name: /increment/i }))
   
     // Assert
     expect(screen.getByText('Count: 1')).toBeInTheDocument()
   })
   

Coverage Best Practices

1. Set Realistic Thresholds

   • Aim for 80%+ coverage
   • 100% coverage doesn't mean bug-free
   • Focus on critical paths

2. Ignore Appropriate Files

   • Config files
   • Type definitions
   • Test utilities
   • Generated code
   • Mock data

3. Review Coverage Reports

   • Look for untested branches
   • Identify missed edge cases
   • Check critical code paths

Running Tests

Common Commands

# Run all tests
npm test

# Watch mode
npm test -- --watch

# Run specific file
npm test Button.test.tsx

# Run with coverage
npm test -- --coverage

# Run in UI mode
npm test -- --ui

# Run only changed files
npm test -- --changed

# Update snapshots
npm test -- -u

CI/CD Integration

# .github/workflows/test.yml
name: Test

on: [push, pull_request]

jobs:
  test:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v3
      - uses: actions/setup-node@v3
        with:
          node-version: '18'
      - run: npm ci
      - run: npm test -- --coverage
      - uses: codecov/codecov-action@v3
        with:
          file: ./coverage/lcov.info

Common Testing Scenarios

Testing Controlled Components

it('calls onChange when value changes', async () => {
  const handleChange = vi.fn()
  const user = userEvent.setup()

  const { rerender } = render(
    <Input value="" onChange={handleChange} />
  )

  await user.type(screen.getByRole('textbox'), 'hello')

  expect(handleChange).toHaveBeenCalledTimes(5)
  expect(handleChange).toHaveBeenLastCalledWith('hello')
})

Testing Compound Components

it('activates tab when clicked', async () => {
  const user = userEvent.setup()

  render(
    <Tabs defaultValue="tab1">
      <TabsList>
        <TabsTrigger value="tab1">Tab 1</TabsTrigger>
        <TabsTrigger value="tab2">Tab 2</TabsTrigger>
      </TabsList>
      <TabsContent value="tab1">Content 1</TabsContent>
      <TabsContent value="tab2">Content 2</TabsContent>
    </Tabs>
  )

  expect(screen.getByText('Content 1')).toBeInTheDocument()
  expect(screen.queryByText('Content 2')).not.toBeInTheDocument()

  await user.click(screen.getByRole('tab', { name: /tab 2/i }))

  expect(screen.queryByText('Content 1')).not.toBeInTheDocument()
  expect(screen.getByText('Content 2')).toBeInTheDocument()
})

Testing Context Providers

it('provides theme context to children', () => {
  render(
    <ThemeProvider theme="dark">
      <ComponentUsingTheme />
    </ThemeProvider>
  )

  expect(screen.getByTestId('theme-display')).toHaveTextContent('dark')
})

Testing Custom Hooks

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

describe('useCounter', () => {
  it('increments counter', () => {
    const { result } = renderHook(() => useCounter())

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

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

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

Troubleshooting

Common Issues

Act Warnings

// Wrap state updates in act()
await act(async () => {
  await someAsyncFunction()
})

Query Not Found

// Use queryBy for assertions about absence
expect(screen.queryByText(/not here/i)).not.toBeInTheDocument()

// Use waitFor for async elements
await waitFor(() => {
  expect(screen.getByText(/async content/i)).toBeInTheDocument()
})

Timer Issues

// Use fake timers when needed
vi.useFakeTimers()

act(() => {
  vi.advanceTimersByTime(1000)
})

vi.useRealTimers()

When to Use This Skill

Activate this skill when you need to:

• Write unit tests for components
• Create integration tests
• Set up test infrastructure
• Analyze test coverage
• Fix failing tests
• Improve test quality
• Add accessibility tests
• Test user interactions
• Mock API calls
• Set up MSW handlers
• Configure Vitest
• Debug test issues

Output Format

When writing tests, provide:

1. Complete Test Suite: All test cases for the component
2. Coverage Report: What's tested and what's not
3. Setup Instructions: Any configuration needed
4. Mock Data: Test fixtures if required
5. Accessibility Notes: A11y test results
6. Next Steps: Recommendations for additional tests

Always write tests that are maintainable, readable, and focused on user behavior.