HTMLDialogElement: returnValue プロパティ

<dialog id="termsDialog">
  <p>利用規約に同意しますか？（リンク）</p>
  <button id="declineButton" value="declined">拒否</button>
  <button id="acceptButton" value="accepted">承諾</button>
</dialog>
<p>
  <button id="openDialogButton">利用規約の確認</button>
</p>
<p id="statusText"></p>

const dialog = document.getElementById("termsDialog");
const statusText = document.getElementById("statusText");
const openDialogButton = document.getElementById("openDialogButton");
const declineButton = document.getElementById("declineButton");
const acceptButton = document.getElementById("acceptButton");
openDialogButton.addEventListener("click", () => {
  dialog.showModal();
});
declineButton.addEventListener("click", closeDialog);
acceptButton.addEventListener("click", closeDialog);
function closeDialog(event) {
  const button = event.target;
  dialog.close(button.value);
}
dialog.addEventListener("close", () => {
  statusText.innerText = dialog.returnValue
    ? `返値: ${dialog.returnValue}`
    : "返値はありません";
});