How to uniquely identify a socket with Node.js

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How to uniquely identify a socket with Node.js

TLDR; How to identify sockets in event based programming model.

I am just starting up with node.js , in the past i have done most of my coding
part in C++ and PHP sockets() so node.js is something extremely new to me.

In c++ to identify a socket we could have done something like writing a main socket say server to listen for new connections and changes, and then handling those connections accordingly.

Solution :

If you are looking for actual sockets and not, they do exist.

But as stated, Node.js and Javascript use an event-based programming model, so you create a (TCP) socket, listen on an IP:port (similar to bind), then accept connection events which pass a Javascript object representing the connection.

From this you can get the FD or another identifier, but this object is also a long-lived object that you can store an identifier on if you wish (this is what does).

var server = net.createServer();

server.on('connection', function(conn) { = Math.floor(Math.random() * 1000);
  conn.on('data', function(data) {
    conn.write('ID: ';

Timothy’s approach is good, the only thing to mention – Math.random() may cause id’s duplication. So the chance it will generate the same random number is really tiny, but it could happen. So I’d recommend you to use dylang’s module – shortid:

var shortid = require('shortid');
var server = net.createServer();

server.on('connection', function(conn) { = shortid.generate();
  conn.on('data', function(data) {
    conn.write('ID: ';

So in that case you can be sure that no id duplications will occur.

in typescript:

import { v4 as uuidv4 } from 'uuid';
import net from 'net';
class Socket extends net.Socket {
  id?: string;

const server = net.createServer();

server.on('connection', (conn) => { = uuidv4();
  conn.on('data', (data) => {

you need to add id first;

In c++ to identify a socket we could have done something like writing
a main socket say server to listen for new connections and then
handling those connections accordingly.but so far i havent found
anything like that in node.js . (the berkeley socket model) Does it
even exist in node.js .. if not i am going back to my C++ :$

You should go back, because JavaScript is a prototype-based, object-oriented scripting language that is dynamic, weakly typed and has first-class functions. They are both completely different languages and you will have to have a different mindset to write clean JavaScript code.

Session ID

If you made use of the sessionId property of socket in v0.6, this is now simply .id.

// v0.6.x
var sid = socket.sessionId;

// v0.7.x
var sid =;

if you found this question by looking for unique ids that you can use to differentiate between sockets on the client-side (just like i did), then here is a very simple answer:

var id = 0; //initial id value
io.sockets.on('connection', function(socket) {

    var my_id = id; //my_id = value for this exact socket connection
    id++; //increment global id for further connnections

    socket.broadcast.emit("user_connected", "user with id " + my_id + "connected");

on every new connection the id is incremented on the serverside. this guarantees a unique id.
I use this method for finding out where a broadcast came from on the clientside and saving data from concurrent sockets.

for example:


var my_coords = {x : 2, y : -5};
socket.broadcast.emit("user_position", {id: my_id, coord: my_coords});  


user = {};
socketio.on("user_position", function(data) {
    if(typeof user[] === "undefined")
        user[] = {};

    user[]["x"] = data.coord.x;
    user[]["y"] = data.coord.y;

How to identify a client based on its socket id. Useful for private messaging and other stuff.

Using v1.4.5

client side:

var socketclientid = "john"; //should be the unique login id
var iosocket = io.connect("http://localhost:5000", {query: "name=john"});

var socketmsg = JSON.stringify({
  type: "private messaging",
  to: "doe",
  message: "whats up!"

server side:

io.on('connection', function(socket){
  var sessionid =;
  var name = socket.handshake.query['name'];
  //store both data in json object and put in array or something

socket.on('message', function(msg){
  var thesessionid =;      
  var name = ???? //do lookup in the user array using the sessionid
  console.log("Message receive from: " + name);

  var msgobject = JSON.parse(msg);
  var msgtype = msgobject.type;
  var msgto =;
  var themessage = msgobject.message;

  //do something with the msg
  //john want to send private msg to doe
  var doesocketid = ???? //use socket id lookup for msgto in the array
                         //doe must be online
  //send to doe only
  if (msgtype == "private messaging")'message', 'themessage');


mmmm, i don’t really get what you’re looking for but socket-programming with node.js (and is really straight forward. take a look at some examples on the homepage:

// note, io.listen() will create a http server for you
var io = require('').listen(80);

io.sockets.on('connection', function (socket) {
  io.sockets.emit('this', { will: 'be received by everyone connected'});

  socket.on('private message', function (from, msg) {
    console.log('I received a private message by ', from, ' saying ', msg);

  socket.on('disconnect', function () {
    sockets.emit('user disconnected');

on connecting to the server, every socket get an unique id with which you can identify it later on.

hope this helps!?

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