Purge unread Gmail messages older than x days

purgeInbox.js

/**
 * Removes all unread threads matching the query
 * older_than:<interval> label:purge-after-<interval>
 */
var intervals = ["1d", "7d", "1m"];
function purgeAll() {
  intervals.forEach(function(interval) {
    var query = constructQuery(interval);
    var threads = GmailApp.search(query);
    Logger.log("Found " + threads.length + " matching query: " + query);
    inBatchesOf(threads, 100, function(slice) {
      GmailApp.moveThreadsToTrash(slice);
    })
  })
  
  function constructQuery(interval) {
    var labelName = "purge-after-" + interval;
    return "is:unread label:" + labelName + " older_than:" + interval;
  }
  
  function inBatchesOf(collection, batchSize, f) {
    for(var i = 0; i < collection.length; i += batchSize) {
      f(collection.slice(i, i + batchSize));
    }
  }
}