typescript-testing

name	typescript-testing
description	Frontend testing rules with Vitest, React Testing Library, and MSW. Includes coverage requirements, test design principles, and quality criteria. Use when writing frontend tests or reviewing test quality.

TypeScript Testing Rules (Frontend)

Test Framework

• Vitest: This project uses Vitest
• React Testing Library: For component testing
• MSW (Mock Service Worker): For API mocking
• Test imports: import { describe, it, expect, beforeEach, vi } from 'vitest'
• Component test imports: import { render, screen, fireEvent } from '@testing-library/react'
• Mock creation: Use vi.mock()

Basic Testing Policy

Quality Requirements

• Coverage: Unit test coverage must be 60% or higher (Frontend standard 2025)
• Independence: Each test can run independently without depending on other tests
• Reproducibility: Tests are environment-independent and always return the same results
• Readability: Test code maintains the same quality as production code

Coverage Requirements (ADR-0002 Compliant)

Component-specific targets:

• Atoms (Button, Text, etc.): 70% or higher
• Molecules (FormField, etc.): 65% or higher
• Organisms (Header, Footer, etc.): 60% or higher
• Custom Hooks: 65% or higher
• Utils: 70% or higher

Metrics: Statements, Branches, Functions, Lines

Test Types and Scope

1. Unit Tests (React Testing Library)

   • Verify behavior of individual components or functions
   • Mock all external dependencies
   • Most numerous, implemented with fine granularity
   • Focus on user-observable behavior

2. Integration Tests (React Testing Library + MSW)

   • Verify coordination between multiple components
   • Mock APIs with MSW (Mock Service Worker)
   • No actual DB connections (backend manages DB)
   • Verify major functional flows

3. Cross-functional Verification in E2E Tests

   • Mandatory verification of impact on existing features when adding new features
   • Cover integration points with "High" and "Medium" impact levels from Design Doc's "Integration Point Map"
   • Verification pattern: Existing feature operation → Enable new feature → Verify continuity of existing features
   • Success criteria: No change in displayed content, rendering time within 5 seconds
   • Designed for automatic execution in CI/CD pipelines

Red-Green-Refactor Process (Test-First Development)

Recommended Principle: Always start code changes with tests

Background:

• Ensure behavior before changes, prevent regression
• Clarify expected behavior before implementation
• Ensure safety during refactoring

Development Steps:

1. Red: Write test for expected behavior (it fails)
2. Green: Pass test with minimal implementation
3. Refactor: Improve code while maintaining passing tests

NG Cases (Test-first not required):

• Pure configuration file changes (vite.config.ts, tailwind.config.js, etc.)
• Documentation-only updates (README, comments, etc.)
• Emergency production incident response (post-incident tests mandatory)

Test Design Principles

Test Case Structure

• Tests consist of three stages: "Arrange," "Act," "Assert"
• Clear naming that shows purpose of each test
• One test case verifies only one behavior

Test Data Management

• Manage test data in dedicated directories or co-located with tests
• Define test-specific environment variable values
• Always mock sensitive information
• Keep test data minimal, using only data directly related to test case verification purposes

Mock and Stub Usage Policy

✅ Recommended: Mock external dependencies in unit tests

• Merit: Ensures test independence and reproducibility
• Practice: Mock API calls with MSW, mock external libraries

❌ Avoid: Actual API connections in unit tests

• Reason: Slows test speed and causes environment-dependent problems

Test Failure Response Decision Criteria

Fix tests: Wrong expected values, references to non-existent features, dependence on implementation details, implementation only for tests Fix implementation: Valid specifications, business logic, important edge cases When in doubt: Confirm with user

Test Helper Utilization Rules

Basic Principles

Use test helpers to reduce duplication and improve maintainability.

Decision Criteria

Mock Characteristics	Response Policy
Simple and stable	Consolidate in common helpers
Complex or frequently changing	Individual implementation
Duplicated in 3+ places	Consider consolidation
Test-specific logic	Individual implementation

Test Helper Usage Examples

// ✅ Builder pattern for test data
const testUser = createTestUser({ name: 'Test User', email: '[e-mail]' })

// ✅ Custom render function with providers
function renderWithProviders(ui: React.ReactElement) {
  return render(<TestProvider>{ui}</TestProvider>)
}

// ❌ Individual implementation of duplicate complex mocks

Test Implementation Conventions

Directory Structure (Co-location Principle)

src/
└── components/
    └── Button/
        ├── Button.tsx
        ├── Button.test.tsx  # Co-located with component
        └── index.ts

Rationale:

• React Testing Library best practice
• ADR-0002 Co-location principle
• Easy to find and maintain tests alongside implementation

Naming Conventions

• Test files: {ComponentName}.test.tsx
• Integration test files: {FeatureName}.integration.test.tsx
• Test suites: Names describing target components or features
• Test cases: Names describing expected behavior from user perspective

Test Code Quality Rules

✅ Recommended: Keep all tests always active

• Merit: Guarantees test suite completeness
• Practice: Fix problematic tests and activate them

❌ Avoid: test.skip() or commenting out

• Reason: Creates test gaps and incomplete quality checks
• Solution: Completely delete unnecessary tests

Test Granularity Principles

Core Principle: User-Observable Behavior Only

MUST Test: Rendered output, user interactions, accessibility, error states MUST NOT Test: Component internal state, implementation details, CSS class names

// ✅ Test user-observable behavior
expect(screen.getByRole('button', { name: 'Submit' })).toBeInTheDocument()

// ❌ Test implementation details
expect(component.state.count).toBe(0)

Test Quality Criteria

These criteria ensure reliable, maintainable tests.

Literal Expected Values

Use hardcoded literal values for assertions. This ensures independent verification of implementation correctness.

expect(formatPrice(1000)).toBe('¥1,000')
expect(calculateTax(100)).toBe(10)
expect(user.role).toBe('admin')

Result-Based Verification

Verify final results and outcomes. Use toHaveBeenCalledWith for argument verification.

expect(mockOnSubmit).toHaveBeenCalledWith({ name: 'test' })
expect(result).toEqual({ id: '1', status: 'success' })
expect(screen.getByText('Submitted')).toBeInTheDocument()

Meaningful Assertions

Every test must include at least one expect() that validates observable behavior.

it('displays error message on invalid input', () => {
  render(<Form />)
  fireEvent.click(screen.getByRole('button', { name: 'Submit' }))
  expect(screen.getByText('Required field')).toBeInTheDocument()
})

Appropriate Mock Scope

Mock only direct external I/O dependencies (API clients, database connections). Internal utilities should use real implementations.

vi.mock('./api/userApi')  // External API - mock
vi.mock('./lib/database') // External I/O - mock
// Internal utils like validators/formatters - use real implementations

Mock Type Safety Enforcement

MSW (Mock Service Worker) Setup

// ✅ Type-safe MSW handler
import { rest } from 'msw'

const handlers = [
  rest.get('/api/users/:id', (req, res, ctx) => {
    return res(ctx.json({ id: '1', name: 'John' } satisfies User))
  })
]

Component Mock Type Safety

// ✅ Only required parts
type TestProps = Pick<ButtonProps, 'label' | 'onClick'>
const mockProps: TestProps = { label: 'Click', onClick: vi.fn() }

// Only when absolutely necessary, with clear justification
const mockRouter = {
  push: vi.fn()
} as unknown as Router // Complex router type structure

Continuity Test Scope

Limited to verifying existing feature impact when adding new features. Long-term operations and performance testing are infrastructure responsibilities, not test scope.

Basic React Testing Library Example

import { describe, it, expect, vi } from 'vitest'
import { render, screen, fireEvent } from '@testing-library/react'
import { Button } from './Button'

describe('Button', () => {
  it('should call onClick when clicked', () => {
    const onClick = vi.fn()
    render(<Button label="Click me" onClick={onClick} />)
    fireEvent.click(screen.getByRole('button', { name: 'Click me' }))
    expect(onClick).toHaveBeenCalledOnce()
  })
})