TypeScript

TypeScript

Overview

A brief summary of what the skill does and its core philosophy.

When to Use

Context for when this skill is applicable.

Negative Constraints (What NOT to Do)

1. NEVER use any: Use unknown with type guards, discriminated unions, or Zod schemas.
2. NEVER use type assertions (as Type or as unknown as Type) to bypass safety: Fix the underlying type signature or use runtime narrowing (instanceof, typeof, in).
3. NEVER use non-null assertions (foo!.bar): Handle null and undefined with optional chaining (?.) or explicit error guards.
4. NEVER export mutable global arrays or object constants: Always mark constant objects and arrays with as const and readonly.
5. NEVER omit explicit return types on exported functions: Exported public APIs must declare explicit return types to protect consumers.

Rules & Patterns

Strict Mode

Always use strict TypeScript configuration:

{
  "compilerOptions": {
    "strict": true,
    "noUncheckedIndexedAccess": true,
    "noImplicitReturns": true,
    "noFallthroughCasesInSwitch": true
  }
}

Types

• Prefer interface for object shapes, type for unions/intersections
• No any — use unknown if type is truly unknown, then narrow
• Explicit return types for exported functions
• Const assertions — as const for literal types

// Good
interface User {
  id: string;
  name: string;
  role: 'admin' | 'user';
}

// Unions
type Result<T> = { ok: true; data: T } | { ok: false; error: string };

Utility Types

• Partial<T> — all properties optional
• Required<T> — all properties required
• Pick<T, K> — select specific properties
• Omit<T, K> — remove specific properties
• Record<K, V> — key-value map

Type Guards

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

Generic Patterns

// Repository pattern
interface Repository<T extends { id: string }> {
  findById(id: string): Promise<T | null>;
  create(data: Omit<T, 'id'>): Promise<T>;
  update(id: string, data: Partial<T>): Promise<T>;
  delete(id: string): Promise<void>;
}