--- 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(arr: T[]): T | undefined { return arr[0]; } // ❌ NEVER function parse(input: any): any {} ``` ## Utility Types ```typescript Pick; // Select fields Omit; // Exclude fields Partial; // All optional Required; // All required Readonly; // All readonly Record; // Object type Extract; // Extract from union Exclude; // Exclude from union NonNullable; // Remove null/undefined ReturnType; // Function return type Parameters; // 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

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