---
name: typescript-skill
description: TypeScript coding patterns and type safety guide. Use when writing, reviewing, or debugging TypeScript code.
---

# TypeScript Skill

## Key Rules

### Always Prefer Explicit Types
```typescript
// Bad
const process = (data: any) => data.value;

// Good
interface GameState { value: number; }
const process = (data: GameState): number => data.value;
```

### Use `unknown` Instead of `any`
```typescript
// Bad
function parse(input: any) { return input.name; }

// Good
function parse(input: unknown): string {
  if (typeof input === 'object' && input !== null && 'name' in input) {
    return String((input as { name: unknown }).name);
  }
  throw new Error('Invalid input');
}
```

### Prefer `const` Assertions for Literals
```typescript
const DIRECTIONS = ['up', 'down', 'left', 'right'] as const;
type Direction = typeof DIRECTIONS[number]; // 'up' | 'down' | 'left' | 'right'
```

## Common Patterns

| Pattern | Use Case |
|---------|----------|
| `type` | Unions, intersections, primitives |
| `interface` | Object shapes (extendable) |
| `enum` | Named constants (prefer `as const` for simple cases) |
| `generic <T>` | Reusable, type-safe utilities |