frontend-typescript-rules

name	frontend/typescript-rules
description	Applies React/TypeScript type safety, component design, and state management rules. Use when implementing React components.

TypeScript Development Rules (Frontend)

Frontend-Specific Anti-patterns

In addition to universal anti-patterns in coding-standards skill, watch for these Frontend-specific issues:

• Prop drilling through 3+ levels - Should use Context API or state management
• Massive components (300+ lines) - Split into smaller components

Type Safety in Frontend Implementation

Type Safety in Data Flow

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

Frontend-Specific Type Scenarios:

• 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 Complexity Management (Frontend)

• 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 (Mandatory): Official React recommendation, optimizable by modern tooling
• Classes Prohibited: Class components completely deprecated (Exception: Error Boundary)
• Custom Hooks: Standard pattern for logic reuse

Function Design

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

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

Props Design (Props-driven Approach)

• Props are the interface: Define all necessary information as props
• Avoid implicit dependencies: Do not depend on global state or context without necessity
• Type-safe: Always define Props type explicitly

Environment Variables

• Use build tool's environment variable system: process.env does not work in browsers
• No secrets on client-side: All frontend code is public, manage secrets in backend

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()
• No commented-out code (manage history with version control)
• Comments explain "why" (not "what")

Error Handling

Absolute Rule: Error suppression prohibited. All errors must have log output and appropriate handling.

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

// Prohibited: Unconditional fallback
catch (error) {
  return defaultValue // Hides error
}

// Required: Explicit failure
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 Never include sensitive information (password, token, apiKey, secret, creditCard) in logs

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

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