Socket.IO - how do I get a list of connected sockets/clients?

In Socket.IO 0.7 you have a clients method on the namespaces, this returns a array of all connected sockets.

API for no namespace:

var clients = io.sockets.clients();
var clients = io.sockets.clients('room'); // all users from room `room`

For a namespace

var clients = io.of('/chat').clients();
var clients = io.of('/chat').clients('room'); // all users from room `room`

Hopes this helps someone in the future

NOTE: This Solution ONLY works with version prior to 1.0

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UPDATED 2020 Mar 06

From 1.x and above, please refer to this link: getting how many people are in a chat room in socket.io

Socket.io 1.4

Object.keys(io.sockets.sockets); gives you all the connected sockets.

Socket.io 1.0 As of socket.io 1.0, the actual accepted answer isn't valid anymore. So I made a small function that I use as a temporary fix :

function findClientsSocket(roomId, namespace) {
    var res = []
    // the default namespace is "/"
    , ns = io.of(namespace ||"/");

    if (ns) {
        for (var id in ns.connected) {
            if(roomId) {
                var index = ns.connected[id].rooms.indexOf(roomId);
                if(index !== -1) {
                    res.push(ns.connected[id]);
                }
            } else {
                res.push(ns.connected[id]);
            }
        }
    }
    return res;
}

Api for No namespace becomes

// var clients = io.sockets.clients();
// becomes : 
var clients = findClientsSocket();

// var clients = io.sockets.clients('room');
// all users from room `room`
// becomes
var clients = findClientsSocket('room');

Api for a namespace becomes :

// var clients = io.of('/chat').clients();
// becomes
var clients = findClientsSocket(null, '/chat');

// var clients = io.of('/chat').clients('room');
// all users from room `room`
// becomes
var clients = findClientsSocket('room', '/chat');

Also see this related question, in which I give a function that returns the sockets for a given room.

function findClientsSocketByRoomId(roomId) {
var res = []
, room = io.sockets.adapter.rooms[roomId];
if (room) {
    for (var id in room) {
    res.push(io.sockets.adapter.nsp.connected[id]);
    }
}
return res;
}

Socket.io 0.7

API for no namespace:

var clients = io.sockets.clients();
var clients = io.sockets.clients('room'); // all users from room `room`

For a namespace

var clients = io.of('/chat').clients();
var clients = io.of('/chat').clients('room'); // all users from room `room`

Note: Since it seems the socket.io API is prone to breaking, and some solution rely on implementation details, it could be a matter of tracking the clients yourself:

var clients = [];

io.sockets.on('connect', function(client) {
    clients.push(client); 

    client.on('disconnect', function() {
        clients.splice(clients.indexOf(client), 1);
    });
});

After socket.io 1.0 we cannot use

io.sockets.clients(); 
or
io.sockets.clients('room'); 

anymore. Instead you can use the following

var clients_in_the_room = io.sockets.adapter.rooms[roomId]; 
for (var clientId in clients_in_the_room ) {
  console.log('client: %s', clientId); //Seeing is believing 
  var client_socket = io.sockets.connected[clientId];//Do whatever you want with this
}