Typescript

---
name: typescript
description: >
  TypeScript strict patterns and best practices.
  Trigger: When writing TypeScript code - types, interfaces, generics.
license: Apache-2.0
metadata:
  version: '1.0'
---

## Const Types Pattern (REQUIRED)

```typescript
// ✅ ALWAYS: Create const object first, then extract type
const STATUS = {
  ACTIVE: 'active',
  INACTIVE: 'inactive',
  PENDING: 'pending'
} as const;

type Status = (typeof STATUS)[keyof typeof STATUS];

// ❌ NEVER: Direct union types
type Status = 'active' | 'inactive' | 'pending';
```

**Why?** Single source of truth, runtime values, autocomplete, easier refactoring.

## Flat Interfaces (REQUIRED)

```typescript
// ✅ ALWAYS: One level depth, nested objects → dedicated interface
interface UserAddress {
  street: string;
  city: string;
}

interface User {
  id: string;
  name: string;
  address: UserAddress; // Reference, not inline
}

interface Admin extends User {
  permissions: string[];
}

// ❌ NEVER: Inline nested objects
interface User {
  address: { street: string; city: string }; // NO!
}
```

## Never Use `any`

```typescript
// ✅ Use unknown for truly unknown types
function parse(input: unknown): User {
  if (isUser(input)) return input;
  throw new Error('Invalid input');
}

// ✅ Use generics for flexible types
function first<T>(arr: T[]): T | undefined {
  return arr[0];
}

// ❌ NEVER
function parse(input: any): any {}
```

## Utility Types

```typescript
Pick<User, 'id' | 'name'>; // Select fields
Omit<User, 'id'>; // Exclude fields
Partial<User>; // All optional
Required<User>; // All required
Readonly<User>; // All readonly
Record<string, User>; // Object type
Extract<Union, 'a' | 'b'>; // Extract from union
Exclude<Union, 'a'>; // Exclude from union
NonNullable<T | null>; // Remove null/undefined
ReturnType<typeof fn>; // Function return type
Parameters<typeof fn>; // Function params tuple
```

## Type Guards

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

## Import Types

```typescript
import type { User } from './types';
import { createUser, type Config } from './utils';
```

## Keywords

typescript, ts, types, interfaces, generics, strict mode, utility types