サンプルコードで身につける: readonly プロパティ

const memberCard: { readonly memberId: number; rank: string } = {
  memberId: 1001,
  rank: "ブロンズ",
};

console.log(memberCard.memberId);
console.log(memberCard.rank);

memberCard.rank = "シルバー"; // readonly でないプロパティは変更できる
console.log(memberCard.rank);
// memberCard.memberId = 9999; // readonly なので再代入はエラー

function printReceipt(receipt: { readonly receiptNo: string; readonly total: number }): void {
  // receipt.total = 0; // 受け取った側でも書き換えはエラー
  console.log("領収書番号: " + receipt.receiptNo);
  console.log("合計: " + receipt.total + "円");
}

printReceipt({ receiptNo: "R-2026-001", total: 1480 });
printReceipt({ receiptNo: "R-2026-002", total: 980 });

const appInfo: {
  readonly appName: string;
  readonly version: string;
  readonly maxUploadMb: number;
} = {
  appName: "PhotoShare",
  version: "2.1.0",
  maxUploadMb: 20,
};

console.log(appInfo.appName + " v" + appInfo.version);
console.log("アップロード上限: " + appInfo.maxUploadMb + "MB");
// appInfo.version = "3.0.0"; // どのプロパティも書き換え不可

const player: { readonly playerId: number; score: number } = {
  playerId: 7,
  score: 0,
};

function addScore(target: { readonly playerId: number; score: number }, point: number): void {
  target.score = target.score + point; // score は変更できる
  // target.playerId = 0; // playerId の変更はエラー
}

addScore(player, 100);
addScore(player, 250);
console.log("プレイヤー" + player.playerId + "の得点: " + player.score);

const originalOrder: { readonly orderId: string; quantity: number } = {
  orderId: "A-100",
  quantity: 1,
};

function changeQuantity(
  order: { readonly orderId: string; quantity: number },
  newQuantity: number
): { readonly orderId: string; quantity: number } {
  return { ...order, quantity: newQuantity }; // コピーして新しく作る
}

const updatedOrder = changeQuantity(originalOrder, 3);
console.log(originalOrder.quantity); // 元は変わらない
console.log(updatedOrder.quantity);
console.log(updatedOrder.orderId);