TypeScript Development Rules (Frontend)

Basic Principles

• Aggressive Refactoring - Prevent technical debt and maintain health
• Delete code when no current caller exists - YAGNI principle (Kent Beck)

Comment Writing Rules

• Function Description Focus: Describe what the code "does"
• Timeless content only: Record decisions and rationale; leave chronological history to version control
• Conciseness: Keep explanations to necessary minimum

Type Safety

Absolute Rule: Replace every any with unknown, generics, or union types. any disables type checking and causes runtime errors.

any Type Alternatives (Priority Order)

1. unknown Type + Type Guards: Use for validating external input (API responses, localStorage, URL parameters)
2. Generics: When type flexibility is needed
3. Union Types・Intersection Types: Combinations of multiple types
4. Type Assertions (Last Resort): Only when type is certain

Type Guard Implementation Pattern

function isUser(value: unknown): value is User {
  return typeof value === 'object' && value !== null && 'id' in value && 'name' in value
}

Modern Type Features

• satisfies Operator: const config = { apiUrl: '/api' } satisfies Config - Preserves inference
• const Assertion: const ROUTES = { HOME: '/' } as const satisfies Routes - Immutable and type-safe
• Branded Types: type UserId = string & { __brand: 'UserId' } - Distinguish meaning
• Template Literal Types: type EventName = \on${Capitalize<string>}`` - Express string patterns with types

Type Safety in Frontend Implementation

• React Props/State: TypeScript manages types, unknown unnecessary
• External API Responses: Always receive as unknown, validate with type guards
• localStorage/sessionStorage: Treat as unknown, validate
• URL Parameters: Treat as unknown, validate
• Form Input (Controlled Components): Type-safe with React synthetic events

Type Safety in Data Flow

• Frontend → Backend: Props/State (Type Guaranteed) → API Request (Serialization)
• Backend → Frontend: API Response (unknown) → Type Guard → State (Type Guaranteed)

Type Complexity Management

• Props Design:
  • Props count: 3-7 props ideal (consider component splitting if exceeds 10)
  • Optional Props: 50% or less (consider default values or Context if excessive)
  • Nesting: Up to 2 levels (flatten deeper structures)
• Type Assertions: Review design if used 3+ times
• External API Types: Relax constraints and define according to reality (convert appropriately internally)

Coding Conventions

Component Design Criteria

• Function components only: Official React recommendation, optimizable by modern tooling (Exception: Error Boundary requires class component)
• Custom Hooks: Standard pattern for logic reuse and dependency injection
• Component Hierarchy: Use the project's adopted component architecture. When the project uses Atomic Design: Atoms → Molecules → Organisms → Templates → Pages. When the project uses Feature-based, Container-Presenter, or another structure: follow that structure consistently and document the chosen layering in the project README or design doc
• Co-location: Place tests, styles, and related files alongside components

State Management Patterns

• Local State: useState for component-specific state
• Context API: For sharing state across component tree (theme, auth, etc.)
• Custom Hooks: Encapsulate state logic and side effects
• Server State: React Query or SWR for API data caching

Data Flow Principles

• Single Source of Truth: Each piece of state has one authoritative source
• Unidirectional Flow: Data flows top-down via props
• Immutable Updates: Use immutable patterns for state updates

// Immutable state update — always create new arrays/objects
setUsers(prev => [...prev, newUser])

Function Design

• 0-2 parameters maximum: Use object for 3+ parameters

  function createUser({ name, email, role }: CreateUserParams) {}

Props Design (Props-driven Approach)

• Props are the interface: Define all necessary information as props
• Pass all data dependencies as props; use Context only for cross-cutting concerns (theme, auth, locale)
• Type-safe: Always define Props type explicitly

Environment Variables

• Use build tool's environment variable system: process.env does not work in browsers
• Centrally manage environment variables through configuration layer
• Define a typed config object with defaults for every environment variable

// Use import.meta.env (not process.env — unavailable in browser bundles)
const config = {
  apiUrl: import.meta.env.API_URL || 'http://localhost:3000',
  appName: import.meta.env.APP_NAME || 'My App'
}

Security (Client-side Constraints)

• CRITICAL: All frontend code is public and visible in browser
• All secrets stay server-side: Store API keys, tokens, and secrets on the backend only
• Exclude .env files via .gitignore
• Limit error messages to non-sensitive context

// Backend manages secrets, frontend accesses via proxy
const response = await fetch('/api/data') // Backend handles API key authentication

Dependency Injection

• Custom Hooks for dependency injection: Ensure testability and modularity

Asynchronous Processing

• Promise Handling: Always use async/await
• Error Handling: Always handle with try-catch or Error Boundary
• Type Definition: Explicitly define return value types (e.g., Promise<Result>)

Format Rules

• Semicolon omission (follow Biome settings)
• Types in PascalCase, variables/functions in camelCase
• Imports use absolute paths (src/)

Clean Code Principles

• Delete unused code immediately
• Delete debug console.log()
• Delete commented-out code (retrieve from version control when needed)
• Comments explain "why" (not "what")

Error Handling

Absolute Rule: Every caught error must be logged with context and either re-thrown to Error Boundary, returned as a Result error variant, or displayed as user-facing error state.

Fail-Fast Principle: Fail quickly on errors to prevent continued processing in invalid states

catch (error) {
  logger.error('Processing failed', error)
  throw error // Handle with Error Boundary or higher layer
}

Result Type Pattern: Express errors with types for explicit handling

type Result<T, E> = { ok: true; value: T } | { ok: false; error: E }

// Example: Express error possibility with types
function parseUser(data: unknown): Result<User, ValidationError> {
  if (!isValid(data)) return { ok: false, error: new ValidationError() }
  return { ok: true, value: data as User }
}

Custom Error Classes

export class AppError extends Error {
  constructor(message: string, public readonly code: string, public readonly statusCode = 500) {
    super(message)
    this.name = this.constructor.name
  }
}
// Purpose-specific: ValidationError(400), ApiError(502), NotFoundError(404)

Layer-Specific Error Handling (React)

• Error Boundary: Catch React component errors, display fallback UI
• Custom Hook: Detect business rule violations, propagate AppError as-is
• API Layer: Convert fetch errors to domain errors

Structured Logging and Sensitive Information Protection Redact sensitive fields (password, token, apiKey, secret, creditCard) before logging

Asynchronous Error Handling in React

• Error Boundary setup mandatory: Catch rendering errors
• Use try-catch with all async/await in event handlers
• Always log and re-throw errors or display error state

Refactoring Techniques

Basic Policy

• Small Steps: Maintain always-working state through gradual improvements
• Safe Changes: Minimize the scope of changes at once
• Behavior Guarantee: Ensure existing behavior remains unchanged while proceeding

Implementation Procedure: Understand Current State → Gradual Changes → Behavior Verification → Final Validation

Priority: Duplicate Code Removal > Large Function Division > Complex Conditional Branch Simplification > Type Safety Improvement

Performance Optimization

• Component Memoization: Use React.memo for expensive components
• State Optimization: Minimize re-renders with proper state structure
• Lazy Loading: Use React.lazy and Suspense for code splitting
• Bundle Size: Monitor with the build script and keep under 500KB

Non-functional Requirements

• Browser Compatibility: Chrome/Firefox/Safari/Edge (latest 2 versions)
• Rendering Time: Within 5 seconds for major pages