app.js

function addNotifyToAllAccounts(input, sender) {
  console.log("Trying to find user");
  User.find({ username: input[1] }, function(err, userdb) {
    if(!err && userdb){
      console.log("Found user: " + userdb[0].username);
      let newNotification = '';
      // rebuilding message
      for(var i = 2; i < input.length; i++){
        newNotification += input[i] + " ";
      }

      userdb.forEach(function(user){
        user.notification.hasNotification = true;
        let notification = {
          text: newNotification,
          date: new Date(),
          fromUser: await User.findOne({ username: sender }, function(err, senderFound) {
            if(err) console.log(err);
            else console.log(`Found sender: ${senderFound.name} (id: ${senderFound._id})`);
          }) 
        }
        user.notification.notifications.push(notification);
        user.save(function(err, newUser) {
          if(err) console.log(err)
          else console.log(`Notification updates for ${newUser.username} (id: ${newUser._id})`);
        });
      });
    }
  });
}