Delete old newsletters in Gmail as soon as the the latest issue arrives, using Google Script

DeleteOlderNewslettersInGmail.js

function processInbox() {
  Logger.log("Starting");
   var mailingLists = [
     "list:'faa8eb4ef3a111cef92c4f3d4mc list <faa8eb4ef3a111cef92c4f3d4.583821.list-id.mcsv.net>'", 
     "list:'e2e180baf855ac797ef407fc7mc list <e2e180baf855ac797ef407fc7.654029.list-id.mcsv.net>'",
     "list:'9f4b80a35728f7271fe3ea6ffmc list <9f4b80a35728f7271fe3ea6ff.511493.list-id.mcsv.net>'"
   ];
  
  for(var i = 0; i < mailingLists.length; i++){
    // process all threads in the Inbox
    var threads = searchMessages(mailingLists[i]);
    
    if(threads.length > 1){
      Logger.log("Found: " + threads.length + " messages to process, for " + mailingLists[i]);
      var msgToSort = getMessagesFromThreads(threads);
      
      msgToSort.sort(function(a,b) { 
        return new Date(b.date).getTime() - new Date(a.date).getTime();
      });
      
      msgToSort.splice(0, 1); // Remove latest mail
      
      for (var i = 0; i < msgToSort.length; i++) {
        Logger.log("Moving message: " + msgToSort[i].subject + " to trash");
        msgToSort[i].message.moveToTrash();
      }
      
      
      Logger.log(msgToSort);
    }
  }
}

function searchMessages(filter){
  return GmailApp.search(filter);
}

function getMessagesFromThreads(threads){
 var msgsToSort=[];
    for (var i = 0; i < threads.length; i++) {
      // get all messages in a given thread
      var messages = threads[i].getMessages();
      for (var j = 0; j < messages.length; j++) {
        msgsToSort.push(processMessage(messages[j]));
      }
    }
  return msgsToSort;
}

function processMessage(message) {
  var subject = message.getSubject();
  var date = message.getDate();
  Logger.log(date + " " + subject);
  return {"subject":subject, "message": message, "date": date }
}