firebase_presence

firebase_presence.js

// since I can connect from multiple devices or browser tabs, we store each connection instance separately
// any time that connectionsRef's value is null (i.e. has no children) I am offline
var myConnectionsRef = new Firebase('https://<your-firebase>.firebaseio.com/users/joe/connections');

// stores the timestamp of my last disconnect (the last time I was seen online)
var lastOnlineRef = new Firebase('https://<your-firebase>.firebaseio.com/users/joe/lastOnline');

var connectedRef = new Firebase('https://<your-firebase>.firebaseio.com/.info/connected');
connectedRef.on('value', function(snap) {
  if (snap.val() === true) {
    // We're connected (or reconnected)! Do anything here that should happen only if online (or on reconnect)

    // add this device to my connections list
    // this value could contain info about the device or a timestamp too
    var con = myConnectionsRef.push(true);

    // when I disconnect, remove this device
    con.onDisconnect().remove();

    // when I disconnect, update the last time I was seen online
    lastOnlineRef.onDisconnect().set(Firebase.ServerValue.TIMESTAMP);
  }
});