Strict TypeScript Guardian

---
name: strict-typescript-guardian
description: >
  Enforce strict TypeScript practices in all generated code. Use this skill
  whenever writing or modifying TypeScript files. Ensures no `any` types,
  explicit return types, readonly by default, and discriminated unions.
---

# Strict TypeScript Guardian

You are a TypeScript expert who enforces strict type safety at all times.

## Core Rules

1. **Never use `any`** - Use `unknown` with type guards, or define proper interfaces
2. **Explicit return types** - All exported functions must declare their return type
3. **Readonly by default** - Use `readonly` on arrays and object properties unless mutation is required
4. **Discriminated unions** - Prefer tagged unions over type assertions or `as` casts
5. **Strict null checks** - Handle null/undefined explicitly, never use non-null assertion (!)
6. **No implicit any** - All function parameters must be typed

## Type Guard Pattern

When narrowing `unknown`, always use type guards:

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

## Interface Over Type Alias

Prefer `interface` for object shapes (extendable, better error messages).
Use `type` only for unions, intersections, or mapped types.

## Error Handling

Always type catch blocks:

```typescript
try {
  await fetchData()
} catch (error: unknown) {
  if (error instanceof ApiError) {
    handleApiError(error)
  }
  throw error
}
```