Utility Types

interface User {
  id: number;
  name: string;
  email: string;
  role: "admin" | "user";
  createdAt: Date;
}

// Partial<T> — all properties become optional
// Perfect for UPDATE operations where you only send changed fields
type UserUpdate = Partial<User>;
// { id?: number; name?: string; email?: string; ... }

async function updateUser(id: number, changes: Partial<User>) {
  // changes might only have { name: "New Name" }
}

// Required<T> — all optional become required
type RequiredUser = Required<User>; // removes all ?

// Pick<T, Keys> — select specific properties
type UserPreview = Pick<User, "id" | "name">;
// { id: number; name: string }

// Omit<T, Keys> — exclude specific properties
type UserWithoutDates = Omit<User, "createdAt">;
// { id, name, email, role } — no createdAt

type NewUser = Omit<User, "id" | "createdAt">;
// { name, email, role } — for creating new user (no id yet)

// Record<Keys, Type> — object with specific key/value types
type RolePermissions = Record<"admin" | "user", string[]>;
const permissions: RolePermissions = {
  admin: ["read", "write", "delete"],
  user: ["read"],
};

// ReturnType<T> — extract return type of function
async function getUser() { return { id: 1, name: "Ali" }; }
type FetchedUser = Awaited<ReturnType<typeof getUser>>;
// { id: number; name: string }

// Readonly<T> — prevent mutation
type ReadonlyUser = Readonly<User>;
const frozen: ReadonlyUser = { id: 1, name: "Ali", /* ... */ };
// frozen.name = "X"; // ❌ Error: cannot assign to readonly property