Frontend Testing

Do NOT apply when:

• User is asking about backend/API tests (Python/pytest)
• User is asking about E2E tests (Playwright/Cypress)
• User is only asking conceptual questions without code context

Quick Reference

Tech Stack

Key Commands

# Run all tests
pnpm test

# Watch mode
pnpm test:watch

# Run specific file
pnpm test path/to/file.spec.tsx

# Generate coverage report
pnpm test:coverage

# Analyze component complexity
pnpm analyze-component <path>

# Review existing test
pnpm analyze-component <path> --review

File Naming

• Test files: ComponentName.spec.tsx inside a same-level __tests__/ directory
• Placement rule: Component, hook, and utility tests must live in a sibling __tests__/ folder at the same level as the source under test. For example, foo/index.tsx maps to foo/__tests__/index.spec.tsx, and foo/bar.ts maps to foo/__tests__/bar.spec.ts.
• Integration tests: web/__tests__/ directory

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',
}))

// ✅ Zustand stores: Use real stores (auto-mocked globally)
// Set test state with: useAppStore.setState({ ... })

// 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: pnpm test <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

• Complexity > 50: Break into smaller pieces before testing
• 500+ lines: Consider splitting before testing
• Many dependencies: Extract logic into hooks first

📖 See references/workflow.md for complete workflow details and todo list format.

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

See Test Structure Template for correct import/mock patterns.

nuqs Query State Testing (Required for URL State Hooks)

When a component or hook uses useQueryState / useQueryStates:

• ✅ Use NuqsTestingAdapter (prefer shared helpers in web/test/nuqs-testing.tsx)
• ✅ Assert URL synchronization via onUrlUpdate (searchParams, options.history)
• ✅ For custom parsers (createParser), keep parse and serialize bijective and add round-trip edge cases (%2F, %25, spaces, legacy encoded values)
• ✅ Verify default-clearing behavior (default values should be removed from URL when applicable)
• ⚠️ Only mock nuqs directly when URL behavior is explicitly out of scope for the test

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)

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, Zustand store testing, and best practices
• references/async-testing.md - Async operations and API calls
• references/domain-components.md - Workflow, Dataset, Configuration testing
• references/common-patterns.md - Frequently used testing patterns
• references/checklist.md - Test generation checklist and validation steps

Authoritative References

Primary Specification (MUST follow)

• web/docs/test.md - The canonical testing specification. This skill is derived from this document.

Reference Examples in Codebase

• web/utils/classnames.spec.ts - Utility function tests
• web/app/components/base/button/index.spec.tsx - Component tests
• web/__mocks__/provider-context.ts - Mock factory example

Project Configuration

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