TypeScript Types and Interfaces

interface User {
  id: string;
  email: string;
  name?: string;         // optional
  readonly createdAt: Date;
}

interface AdminUser extends User {
  role: "admin";
}

const u: AdminUser = {
  id: "u_1",
  email: "ada@example.com",
  createdAt: new Date(),
  role: "admin",
};

// u.createdAt = new Date(); // error: readonly

type ID = string & { readonly __brand: "ID" };  // branded nominal type
type Result<T> =
  | { ok: true; value: T }
  | { ok: false; error: string };

function parse(input: string): Result<number> {
  const n = Number(input);
  return Number.isNaN(n)
    ? { ok: false, error: "not a number" }
    : { ok: true, value: n };
}

const r = parse("42");
if (r.ok) console.log(r.value * 2);
else      console.error(r.error);