Enum Button

// Display a dialog box with a message and "Yes" and "No" buttons.
const ui = DocumentApp.getUi();
const response = ui.alert(
    'Are you sure you want to continue?',
    ui.ButtonSet.YES_NO,
);

// Process the user's response.
if (response === ui.Button.YES) {
  Logger.log('The user clicked "Yes."');
} else {
  Logger.log('The user clicked "No" or the dialog\'s close button.');
}