auto-archive.js

function searchThreads(offset, max, threads) {
  const searchedThreads = GmailApp.search("in: inbox older_than:365d", offset, max);
  var maxDate = new Date();
  maxDate.setDate(maxDate.getDate()-365);
  return searchedThreads.filter(function(thread) {
    return thread.getLastMessageDate() < maxDate;
  });
}

function moveThreads(threads) {
  var threadList = threads;
  if (threadList.length > 100) {
    GmailApp.moveThreadsToArchive(threadList.splice(0, 100));
    moveThreads(threadList);
  } else {
    return GmailApp.moveThreadsToArchive(threadList);
  }
}

function archive() {
  var searchedThreads = searchThreads(0, 500, []);
  moveThreads(searchedThreads);
  const totalThreadsString = 'Archived a total of ' + searchedThreads.length + ' threads this week.';
  GmailApp.sendEmail('email@example.com', 'Archived threads summary', totalThreadsString);
}