typescript-strict-patterns

name	typescript-strict-patterns
description	Enforces maximum TypeScript type safety for Black Trigram — strict mode, no 'any', readonly by default, exhaustive pattern matching, explicit return types for compile-time error detection
license	MIT

📐 TypeScript Strict Patterns Skill

Strategic Principle: TypeScript's type system is your first line of defense. Use it to prevent bugs at compile time.

🎯 Purpose

Enforce strict TypeScript patterns for Black Trigram, ensuring maximum type safety, compile-time error detection, and maintainable code through TypeScript best practices.

Reference: Hack23 ISMS Secure Development Policy

Enforcement Rules

Rule 1: Strict Mode Compliance

IF (TypeScript code)
THEN (must compile with strict: true, no type errors, no suppressions)
ELSE (reject - strict mode catches real bugs)

Rule 2: No 'any' Type

IF (variable, parameter, or return type is 'any')
THEN (replace with proper type, generic, or 'unknown' with type guard)
ELSE (reject - 'any' defeats the purpose of TypeScript)

Rule 3: Readonly by Default

IF (interface property OR function parameter)
THEN (use 'readonly' unless mutation is required and justified)
ELSE (accidental mutation causes hard-to-find bugs)

Rule 4: Exhaustive Pattern Matching

IF (switch/if-else over union type or enum)
THEN (handle all cases with exhaustive check using 'never')
ELSE (new enum values silently fall through)

Rule 5: Explicit Return Types

IF (exported function)
THEN (declare explicit return type)
ELSE (inferred return types can change unexpectedly)

Core Patterns

Strict Configuration

// tsconfig.json - must include
{
  "compilerOptions": {
    "strict": true,
    "noUncheckedIndexedAccess": true,
    "exactOptionalPropertyTypes": true,
    "noImplicitReturns": true,
    "noFallthroughCasesInSwitch": true,
    "noImplicitOverride": true
  }
}

Type Guards

// ✅ GOOD: Proper type narrowing
function isTrigramStance(value: unknown): value is TrigramStance {
  return typeof value === 'string' &&
    (['geon', 'tae', 'li', 'jin', 'son', 'gam', 'gan', 'gon'] as const)
      .includes(value as TrigramStance);
}

// Usage
const stance: unknown = getStanceFromInput();
if (isTrigramStance(stance)) {
  // TypeScript knows stance is TrigramStance here
  applyStance(stance);
}

Exhaustive Matching

// ✅ Compile-time safety for union types
function getStanceColor(stance: TrigramStance): number {
  switch (stance) {
    case 'geon': return KOREAN_COLORS.ACCENT_GOLD;
    case 'tae': return KOREAN_COLORS.PRIMARY_CYAN;
    case 'li': return KOREAN_COLORS.CARDINAL_SOUTH;
    case 'jin': return KOREAN_COLORS.ACCENT_BLUE;
    case 'son': return KOREAN_COLORS.CARDINAL_EAST;
    case 'gam': return KOREAN_COLORS.PRIMARY_CYAN;
    case 'gan': return KOREAN_COLORS.CARDINAL_WEST;
    case 'gon': return KOREAN_COLORS.CARDINAL_CENTER;
    default: {
      const _exhaustive: never = stance;
        ();
    }
  }
}

Readonly Interfaces

// ✅ Immutable by default
export interface VitalPoint {
  readonly id: string;
  readonly names: Readonly<BilingualText>;
  readonly position: Readonly<Position>;
  readonly category: VitalPointCategory;
  readonly baseDamage: number;
  readonly effects: readonly VitalPointEffect[];
}

// ✅ Readonly arrays
function processEffects(effects: readonly StatusEffect[]): void {
  // effects.push(...) ← compile error!
}

Discriminated Unions

// ✅ Type-safe state management
type CombatAction =
  | { readonly type: 'attack'; readonly targetId: string; readonly damage: number }
  | { readonly type: 'defend'; readonly blockStrength: number }
  | { readonly type: 'special'; readonly techniqueId: string; readonly kiCost: number }
  | { readonly type: 'move'; readonly direction: Vector2 };

function executeCombatAction(action: CombatAction): void {
  switch (action.type) {
    case 'attack':
      applyDamage(action.targetId, action.damage); // TypeScript knows the shape
      break;
    case 'defend':
      activateBlock(action.blockStrength);
      break;
    
  }
}

Anti-Patterns

❌ Anti-Pattern	✅ Correct Pattern
value as Type	Type guard or generic
value! (non-null assertion)	Null check or optional chaining
// @ts-ignore	Fix the type error
any	unknown + type guard
Function	Specific function signature
Object	Record or specific interface
Mutable interface props	readonly properties
Implicit any in callbacks	Explicit parameter types

Testing Requirements

• ✅ npm run check passes with zero errors
• ✅ No @ts-ignore or @ts-expect-error without justification
• ✅ No any types in production code
• ✅ All exported functions have explicit return types
• ✅ Discriminated unions use exhaustive matching

Compliance

• ISO 27001:2022: A.8.25 (Secure development lifecycle)
• CWE-843: Type Confusion prevention
• Hack23 ISMS: Secure Development Policy

---

흑괘의 타입 안전성 - Type Safety of the Black Trigram