frontend-testing

name	frontend-testing
description	Generate Vitest + React Testing Library tests for frontend components, hooks, and utilities. Triggers on testing, spec files, coverage, Vitest, RTL, unit tests, integration tests, or write/review test requests.

This skill enables Claude to generate high-quality, comprehensive frontend tests following established conventions and best practices.

When to Apply This Skill

Apply this skill when the user:

• Asks to write tests for a component, hook, or utility
• Asks to review existing tests for completeness
• Mentions Vitest, React Testing Library, RTL, or spec files
• Requests test coverage improvement
• Mentions testing, unit tests, or integration tests for frontend code
• Wants to understand testing patterns in the frontend codebase

Do NOT apply when:

• User is asking about E2E tests (Playwright)
• User is only asking conceptual questions without code context

Quick Reference

Tech Stack

Tool	Version	Purpose
Vitest	4+	Test runner
React Testing Library	16+	Component testing
jsdom	-	Test environment
TypeScript	5+	Type safety

Note: keep this list up to date with the project's dependencies.

Key Commands

Always prefer running specific tests over the entire suite for faster feedback:

# Run ALL tests (avoid during development)
npx vitest run

# ✅ PREFERRED: Run specific file
npx vitest run src/components/Button.spec.tsx

# ✅ PREFERRED: Run tests matching a pattern
npx vitest run --grep "Button"
npx vitest run -t "should render"

# ✅ PREFERRED: Run specific describe block
npx vitest run --grep "Button > Rendering"

# ✅ Run tests in a directory
npx vitest run src/components/

# ✅ Run single test by name
npx vitest run -t "should disable button when loading"

Watch Mode (Background Testing)

Use watch mode for efficient iterative development:

# ✅ Watch mode - reruns on file changes
npx vitest

# ✅ Watch specific file
npx vitest src/components/Button.spec.tsx

# ✅ Watch tests matching pattern
npx vitest --grep "Button"

File Naming

• Test files: ComponentName.spec.tsx (same directory as component)

Test Structure Template

import { render, screen, fireEvent, waitFor } from '@testing-library/react'
import Component from './index'

// ✅ Import real project components (DO NOT mock these)
// import Loading from '@/app/components/base/loading'
// import { ChildComponent } from './child-component'

// ✅ Mock external dependencies only
vi.mock('@/service/api')
vi.mock('next/navigation', () => ({
  useRouter: () => ({ push: vi.fn() }),
  usePathname: () => '/test',
}))

// Shared state for mocks (if needed)
let mockSharedState = false

describe('ComponentName', () => {
  beforeEach(() => {
    vi.clearAllMocks()  // ✅ Reset mocks BEFORE each test
    mockSharedState = false  // ✅ Reset shared state
  })

  // Rendering tests (REQUIRED)
  describe('Rendering', () => {
    it('should render without crashing', () => {
      // Arrange
      const props = { title: 'Test' }

      // Act
      render(<Component {...props} />)

      // Assert
      expect(screen.getByText('Test')).toBeInTheDocument()
    })
  })

  // Props tests (REQUIRED)
  describe('Props', () => {
    it('should apply custom className', () => {
      render(<Component className="custom" />)
      expect(screen.getByRole('button')).toHaveClass('custom')
    })
  })

  // User Interactions
  describe('User Interactions', () => {
    it('should handle click events', () => {
      const handleClick = vi.fn()
      render(<Component onClick={handleClick} />)

      fireEvent.click(screen.getByRole('button'))

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

  // Edge Cases (REQUIRED)
  describe('Edge Cases', () => {
    it('should handle null data', () => {
      render(<Component data={null} />)
      expect(screen.getByText(/no data/i)).toBeInTheDocument()
    })

    it('should handle empty array', () => {
      render(<Component items={[]} />)
      expect(screen.getByText(/empty/i)).toBeInTheDocument()
    })
  })
})

Testing Workflow (CRITICAL)

⚠️ Incremental Approach Required

NEVER generate all test files at once. For complex components or multi-file directories:

1. Analyze & Plan: List all files, order by complexity (simple → complex)
2. Process ONE at a time: Write test → Run test → Fix if needed → Next
3. Verify before proceeding: Do NOT continue to next file until current passes

For each file:
  ┌────────────────────────────────────────────────┐
  │ 1. Write test                                  │
  │ 2. Run: npm run test-unit <file>.spec.tsx      │
  │ 3. PASS? → Mark complete, next file            │
  │    FAIL? → Fix first, then continue            │
  └────────────────────────────────────────────────┘

Complexity-Based Order

Process in this order for multi-file testing:

1. 🟢 Utility functions (simplest)
2. 🟢 Custom hooks
3. 🟡 Simple components (presentational)
4. 🟡 Medium components (state, effects)
5. 🔴 Complex components (API, routing)
6. 🔴 Integration tests (index files - last)

When to Refactor First

• Medium Complexity: Break into smaller pieces before testing
• 500+ lines: Consider splitting before testing
• Many dependencies: Extract logic into hooks first

Testing Strategy

Path-Level Testing (Directory Testing)

When assigned to test a directory/path, test ALL content within that path:

• Test all components, hooks, utilities in the directory (not just index file)
• Use incremental approach: one file at a time, verify each before proceeding
• Goal: 100% coverage of ALL files in the directory

Integration Testing First

Prefer integration testing when writing tests for a directory:

• ✅ Import real project components directly (including base components and siblings)
• ✅ Only mock: API services (@/service/*), next/navigation, complex context providers
• ❌ DO NOT mock base components (@/app/components/base/*)
• ❌ DO NOT mock sibling/child components in the same directory

Core Principles

1. AAA Pattern (Arrange-Act-Assert)

Every test should clearly separate:

• Arrange: Setup test data and render component
• Act: Perform user actions
• Assert: Verify expected outcomes

2. Black-Box Testing

• Test observable behavior, not implementation details
• Use semantic queries (getByRole, getByLabelText)
• Avoid testing internal state directly
• Prefer pattern matching over hardcoded strings in assertions:

// ❌ Avoid: hardcoded text assertions
expect(screen.getByText('Loading...')).toBeInTheDocument()

// ✅ Better: role-based queries
expect(screen.getByRole('status')).toBeInTheDocument()

// ✅ Better: pattern matching
expect(screen.getByText(/loading/i)).toBeInTheDocument()

3. Single Behavior Per Test

Each test verifies ONE user-observable behavior:

// ✅ Good: One behavior
it('should disable button when loading', () => {
  render(<Button loading />)
  expect(screen.getByRole('button')).toBeDisabled()
})

// ❌ Bad: Multiple behaviors
it('should handle loading state', () => {
  render(<Button loading />)
  expect(screen.getByRole('button')).toBeDisabled()
  expect(screen.getByText('Loading...')).toBeInTheDocument()
  expect(screen.getByRole('button')).toHaveClass('loading')
})

4. Semantic Naming

Use should <behavior> when <condition>:

it('should show error message when validation fails')
it('should call onSubmit when form is valid')
it('should disable input when isReadOnly is true')

Required Test Scenarios

Always Required (All Components)

1. Rendering: Component renders without crashing
2. Props: Required props, optional props, default values
3. Edge Cases: null, undefined, empty values, boundary conditions

Conditional (When Present)

Feature	Test Focus
useState	Initial state, transitions, cleanup
useEffect	Execution, dependencies, cleanup
Event handlers	All onClick, onChange, onSubmit, keyboard
API calls	Loading, success, error states
Routing	Navigation, params, query strings
useCallback/useMemo	Referential equality
Context	Provider values, consumer behavior
Forms	Validation, submission, error display

Coverage Goals (Per File)

For each test file generated, aim for:

• ✅ 100% function coverage
• ✅ 100% statement coverage
• ✅ >95% branch coverage
• ✅ >95% line coverage

Note: For multi-file directories, process one file at a time with full coverage each. See references/workflow.md.

Detailed Guides

For more detailed information, refer to:

• references/workflow.md - Incremental testing workflow (MUST READ for multi-file testing)
• references/mocking.md - Mock patterns and best practices
• references/async-testing.md - Async operations and API calls
• references/common-patterns.md - Frequently used testing patterns
• references/checklist.md - Test generation checklist and validation steps

Project Configuration

• vitest.config.ts - Vitest configuration
• vitest.setup.ts - Test environment setup
• Modules are not mocked automatically. Global mocks live in vitest.setup.ts (for example react-i18next, next/image); mock other modules like ky or mime locally in test files.