Typescript

Const Types Pattern (REQUIRED)

// ✅ 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)

// ✅ 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

// ✅ 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

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

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

Import Types

import type { User } from "./types";
import { createUser, type Config } from "./utils";

Keywords

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

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: Gentleman-Programming
• Source: Gentleman-Programming/Gentleman-Skills
• License: MIT

Install and usage instructions live in the source repository linked above.