Mongoose find latest entry

I have an entry which looks like the below:

{
  "__v": 10,
  "_id": "50f8050ed714645500000002",
  "sid": "DI7gZ0sprJ",
  "lines": [
    {
      "text": "Well that's what some people say anyway",
      "_id": "50f80526d714645500000003",
      "user": "Player456"
    },
    {
      "text": "They're people just like the rest of us",
      "_id": "50f805925543712caf000005",
      "user": "Player123"
    },
  ],
  "title": "Test",
  "maxlines": 0
}

and have a socket.io script like below:

socket.on('sendchat', function (data) {
    Story.findOne({ sid: socket.room }, function(err, story){
        if (err) {
            console.log(err);
        }
        else if(story == null) {
            console.log(err);
        }
        else {
            io.sockets.in(socket.room).emit('updatechat', socket.username, data);
            story.maxlines = story.maxlines - 1;

            if (story.maxlines == 0) {
                // End Game Script - Redirect to view page
                io.sockets.in(socket.room).emit('gameover', slugs(story.title), socket.room);
            }
            //Save Story to DB
            story.lines.push({
                user: socket.username,
                text: data
            });
            story.save(function(err, story){
                if (err) {
                    console.log(err);
                }
                else {
                    console.log("Story Updated");
                }
            });
        };
    });
});

Is there a way I can check to see if socket.username equals the user on the newest line that has been entered?

For example above Player123 is the last one to enter a line, so if Player123 tries again it just emits an error, but if Player456 enters a line, then Player123 is able to again

I'm trying to create a simple turn based game, which checks to make sure a person cannot enter a line when it's not their turn

You can directly check the last entry in the lines array in the story object. As in:

if (story.lines.length &&
    story.lines[story.lines.length-1].user === socket.username) {
    // error, same user is trying to enter another line
}