I have a node.js server which keeps track of all "clients" connected to it, using an associative array of sockets, because I need to send some data to specific clients sometimes. The keys in that array are strings composed of

socket.remoteAddress + ':' + socket.remotePort

and values are socket instances passed to the callback provided to net.createServer(...) Sometimes the clients disconnect for one reason or another and I receive the 'end' and then 'close' events. I'd like to remove the disconnected clients from my client registry but in the close/end event callbacks, the remoteAddress and remotePort variables are undefined. Can they be retrieved somehow?

To illustrate:

var registry = {}
var server = net.createServer(function (socket) {
    socket.on('connect', function(){
        registry [socket.remoteAddress + ':' + socket.remotePort] = socket;
    });

    socket.on('close', function(had_error){
            // ******************************************************
            // socket.remoteAddress and remotePort are undefined here
            // ******************************************************
            var id = socket.remoteAddress + ':' + socket.remotePort;
            // *************************************************************
        if(registry.hasOwnProperty(id)
                delete socket.id;
    });
});

I think you should use socket.id instead of creating your own object property that might not be unique

socket.on('connect', function(){
        registry [socket.id] = socket;
});

UPDATE

Node provides basic http API. Http is stateless. HTTP sessions allow associating information with individual visitors but node doesn't have a native sessions support.

Take a look at express, it is a nice web framework with sessions support

Also, if you need to send messages in a realtime fashion, take a look at socket.io

UPDATE V2

You can add your own ID property to the socket object:

function randomID(len){
  //Generate a random String
  return randID;
}
var registry = {}
var server = net.createServer(function (socket) {
    socket.on('connect', function(){
        var uid = randomID(32);
        socket.id = uid;
        registry [uid] = socket;
    });

    socket.on('close', function(had_error){
            var uid = socket.id;
            // *************************************************************
            console.log("Socket ID " + uid + " disconnected")
    });
});