Limiting entries in JSON Object

Alrighty! I'm working on small chat add-on for my website, and when a user logs on they'll see the chat history, I'm using a JSON Object to store all messages in my NodeJS server, now I'd like it so whenever more than fifty entries are in the Object it adds the latest message and removes the oldest, I'd like this to limit my server from handling a lot of messages every time a user logs on. How would I be doing this?

Here's how I store my messages,

            var messages = {
                "session":[

                ]
            };

            messages.session.push(
                {
                    "name":user.name,
                    "message":safe_tags_replace(m.msg),
                    "image":user.avatar,
                    "user":user.steamid,
                    "rank":user.rank,
                }               
            );

I could also just do loading the last fifty messages in the JSON Object but whenever I run my server for a long time without restarting it this Object will become extremly big, would this be a problem?

via Martijn Ebbens