サンプルコードで身につける: 交差型 & — 型を合成する

type Priced = { price: number };
type Stocked = { stock: number };

// 両方のプロパティをすべて持つ型になる
type StoreItem = Priced & Stocked;

const coffeeBean: StoreItem = { price: 1200, stock: 8 };

console.log(coffeeBean.price);
console.log(coffeeBean.stock);
console.log("在庫総額: " + coffeeBean.price * coffeeBean.stock + "円");

// const wrong: StoreItem = { price: 500 }; // stock がないのでエラー

// 小さな「部品」の型を用意しておく
type Identified = { id: number };
type Timestamped = { createdAt: string };
type PostContent = { postTitle: string; content: string };

// 部品を重ねてブログ記事の型を合成する
type BlogPost = Identified & Timestamped & PostContent;

const firstPost: BlogPost = {
  id: 1,
  createdAt: "2026-06-01",
  postTitle: "交差型入門",
  content: "& で型を合成します",
};

console.log(firstPost.postTitle + "(" + firstPost.createdAt + ")");
console.log("記事ID: " + firstPost.id);

type Order = { orderId: number; total: number };

// 引数の型の位置で、その場で & を使って合成できる
function printReceipt(order: Order & { note?: string }): void {
  let line = "注文" + order.orderId + ": " + order.total + "円";
  if (order.note !== undefined) {
    line = line + "(" + order.note + ")";
  }
  console.log(line);
}

printReceipt({ orderId: 1, total: 1500 });
printReceipt({ orderId: 2, total: 980, note: "ギフト包装" });

interface HasId {
  id: number;
}
interface HasEmail {
  email: string;
}

// interface 同士を & で束ねて type にできる
type Contact = HasId & HasEmail;

const supportDesk: Contact = { id: 10, email: "support@example.com" };

function sendMail(target: Contact): string {
  return target.email + " 宛に送信(会員ID: " + target.id + ")";
}

console.log(sendMail(supportDesk));

type Viewer = { viewableMenus: string[] };
type Editor = Viewer & { editableMenus: string[] };
type Admin = Editor & { canDeleteUser: boolean };

const adminAccount: Admin = {
  viewableMenus: ["home", "report", "admin"],
  editableMenus: ["report"],
  canDeleteUser: true,
};

// Viewer のプロパティを全部持っているので、Viewer として渡せる
function listMenus(account: Viewer): string {
  return "閲覧可能: " + account.viewableMenus.join(" / ");
}

console.log(listMenus(adminAccount));
console.log("ユーザー削除権限: " + adminAccount.canDeleteUser);