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Brian Neville-O'Neill
Brian Neville-O'Neill

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Writing a working chat server in Node

Written by Fernando Doglio✏️

This is probably a topic that has been beaten to death since Node.js and (especially) were released. The problem I see is most of the articles out there tend to stay above the surface of what a chat server should do and even though they end up solving the initial predicament, it is such a basic use case that taking that code and turning it into a production-ready chat server is the equivalent of the following image:

So instead of doing that, in this article, I want to share with you an actual chat server, one that is a bit basic due to the restrictions of the medium, mind you, but one that you’ll be able to use from day one. One that in fact, I’m using already in one of my personal projects.

What does a chat server do?

But first, let’s quickly review what is needed for a chat server to indeed, be useful. Leaving aside your particular requirements, a chat server should be capable of doing the following:

  • Receive messages from client applications
  • Distribute received messages to other clients who’re interested in getting them
  • Broadcast general notifications, such as user logins and logouts
  • Be able to send private messages between two users (i.e private messages)

That is the extent of what our little chat server will be capable of doing.

For the purposes of this article, I will create this server as a back-end service, able to work without a defined front end and I will also create a basic HTML application using jQuery and vanilla JavaScript.

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Defining the chat server

Now that we know what the chat server is going to be doing, let’s define the basic interface for it. Needless to say, the entire thing will be based on, so this tutorial assumes you’re already familiar with the library. If you aren’t though, I strongly recommend you check it out before moving forward.

With that out of the way, let’s go into more details about our server’s tasks:

  • This server needs to be able to receive and distribute messages. Which will turn into two of the major methods we’ll be using
  • Other than that I’ll also add some code to handle joining actions in order to notify the rest of the clients connected in the same room
  • Messages will be sent normally and private messages will be those that start with a @ followed by another user’s name (i.e *“@fernando Hey, how are you?”* )

The way I’ll structure the code, is by creating a single class called ChatServer , inside it, we can abstract the inner workings of the socket, like this:

// Setup basic express server
const config = require("config");
const ChatServer  = require("./lib/chat-server")

const port = process.env.PORT || config.get('app.port');
// Chatroom

let numUsers = 0;

const chatServer = new ChatServer({

chatServer.start( socket => {
  console.log('Server listening at port %d', port);
    chatServer.onMessage( socket, (newmsg) => {
        if(newmsg.type = config.get("chat.message_types.generic")) {
            console.log("New message received: ", newmsg)
           chatServer.distributeMsg(socket, newmsg, _ => {
               console.log("Distribution sent")

        if(newmsg.type == config.get('chat.message_types.private')) {
            chatServer.sendMessage(socket, newmsg, _ => {
                console.log("PM sent")

    chatServer.onJoin( socket, newUser => {
        console.log("New user joined: ", newUser.username)
        chatServer.distributeMsg(socket, newUser.username + ' has joined !', () => {
            console.log("Message sent")
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Notice how I’m just starting the server, and once it’s up and running, I just set up two different callback functions:

  • One for incoming messages which simply receives the messages, then formats it into a convenient JSON and then returns it as the attribute of the callback function
  • One for joining events, when users join the room a message is distributed among all others letting them know who just joined
  • Standard messages are tagged with the type “generic” and they end up on a broadcast to the entire room (with the exception of the sending client of course) with the content of the received message
  • And private messages (those starting with an @ character) are tagged as “private” and are directly sent to the intended user through its unique socket connection (I’ll show you how in a minute)

Let me show you now how the methods from the chat server were implemented.

How do sockets work?

To make a long story short, a socket is a persistent bi-directional connection between two computers, usually, one acting as a client and other acting as a server ( in other words: a service provider and a consumer).

There are two main differences (if we keep to the high level definition I just gave you) between sockets and the other, very well known method of communication between client and server (i.e REST APIs):

  1. The connection is persistent, which means that once client and server connect, every new message sent by the client will be received by the exact same server. This is not the case for REST APIs, which need to be stateless. A load-balanced set of REST servers does not require (in fact, it’s not even recommended) the same server to reply to requests from the same client.
  2. Communication can be started by the server, which is also one of the benefits of using sockets over REST (or HTTP to be honest). This simplifies a lot of the logistics when a piece of data needs to move from server to client, since with an open socket, there are no other pre-requisites and the data just flows from one end to the other. This is also one of the features that make socket-based chat servers such an easy and direct use case, if you wanted to use REST or a similar protocol, you would need a lot of extra network traffic to trigger data transfer between parties (like having client apps doing active polling to request pending messages from the server).

That being said, the following code tries to simplify the logic needed by to handle and manage socket connections:

let express = require('express');
let config = require("config")
let app = express();
let socketIO = require("")
let http = require('http')

module.exports = class ChatServer {

    constructor(opts) {
        this.server = http.createServer(app); = socketIO(this.server);
        this.opts = opts 
        this.userMaps = new Map()

    start(cb) {
        this.server.listen(this.opts.port, () => {
            console.log("Up and running...")
  'connection', socket => {

    sendMessage(socket, msgObj, done) {
        // we tell the client to execute 'new message'
        let target =
        this.userMaps[target].emit(config.get(""), msgObj)

    onJoin(socket, cb) {
        socket.on(config.get(''), (data) => {
            console.log("Requesting to join a room: ", data)

            socket.roomname = data.roomname
            socket.username = data.username

            this.userMaps.set(data.username, socket)

            socket.join(data.roomname, _ => {
                    username: data.username, 
                    roomname: data.roomname

    distributeMsg(socket, msg, done) {''), msg);

    onMessage(socket, cb) {
        socket.on(config.get(''), (data) => {
            let room = socket.roomname
            if(!socket.roomname) {
                socket.emit(config.get(''), )
                return cb({
                    error: true, 
                    msg: "You're not part of a room yet"

            let newMsg = {
                room: room,
                type: config.get("chat.message_types.generic"),
                username: socket.username,
                message: data

            return cb(newMsg)

        socket.on(config.get(''), (data) => {
            let room = socket.roomname

            let captureTarget = /(@[a-zA-Z0-9]+)(.+)/
            let matches = data.match(captureTarget)
            let targetUser = matches[1]
            console.log("New pm received, target: ", matches)

            let newMsg = {
                room: room,
                type: config.get("chat.message_types.private"),
                username: socket.username,
                message: matches[2].trim(),
                target: targetUser
            return cb(newMsg)
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The start method takes care of starting the socket server, using the Express HTTP server as a basis (this is a requirement from the library). There is not a lot more you can do here, the result of this initialization will be a call to whatever callback you set up on your code. The point here is to ensure you can’t start doing anything until the server is actually up and running (which is when your callback gets called).

Inside this callback, we set up a handler for the connection event, which is the one that gets triggered every time a new client connects. This callback will receive the actual socket instance, so we need to make sure we keep it safe because that’ll be the object we’ll use to communicate with the client application.

As you noticed in the first code sample, the socket actually gets passed as the first parameter for all methods that require it. That is how I’m making sure I don’t overwrite existing instances of the socket created by other clients.

Joining the room

After the socket connection is established, client apps need to manually join the chat and a particular room inside it. This implies the client is sending a username and a room name as part of the request, and the server is, among other things, keeping record of the username-socket pairs in a Map object. I’ll show you in a second the need for this map, but right now, that is all we take care of doing.

The join method of the socket instance makes sure that particular socket is assigned to the correct room. By doing this, we can limit the scope of broadcast messages (those that need to be sent to every relevant user). Lucky for us, this method and the entire room management logistics are provided by out of the box, so we don’t really need to do anything other than using the methods.

Receiving messages

This is probably the most complex method of the module, and as you’ve probably seen, it’s not that complicated. This method takes care of setting up a handler for every new message received. This could be interpreted as the equivalent of a route handler for your REST API using Express.

Now, if we go down the abstraction rabbit hole you’ll notice that sockets don’t really understand “messages”, instead, they just care about events. And for this module, we’re only allowing two different event names, “new message” and “new pm”, to be a message received or sent event, so both server and client need to make sure they use the same event names. This is part of a contract that has to happen, just like how clients need to know the API endpoints in order to use them, this should be specified in the documentation of your server.

Now, upon reception of a message event we do similar things:

  • For generic messages, we make sure the room name targeted is actually one where the user has previously joined. This is just a small check preventing issues while sending messages.
  • For private messages, we capture the two parts of it: the targeted user and the actual message using a quick and simple regular expression.

Once that is done, we create a JSON payload and pass it along to the provided callback. So basically, this method is meant to receive the message, check it, parse it and return it. There is no extra logic associated to it.

Whatever logic is needed after this step, will be inside your custom callback, which as you can see in the first example takes care distributing the message to the correct destination based on the type (either doing a broadcast to everyone on the same chat room) or delivering a private message to the targeted user.

Delivering private messages

Although quite straightforward, the sendMessage method is using the map I originally mentioned, so I wanted to cover it as well.

The way we can deliver a message to a particular client app (thus delivering it to the actual user) is by using the socket connection that lives between the server and that user, which is where our userMaps property comes into play. With it, the server can quickly find the correct connection based on the targeted username and use that to send the message with the emit method.

Broadcasting to the entire room

This is also something that we don’t really need to worry about, takes care of doing all the heavy lifting for us. In order to send a message to the entire room skipping the source client (basically, the client that sent the original message to the room) is by calling the emit method for the room, using as a connection source the socket for that particular client.

The logic to repeat the message for everyone on the room except the source client is completely outside our control (just the way I like it! ).

And you’re done!

That’s right, there is nothing else relevant to cover for the code, between both examples, you have all the information you need to replicate the server and start using it in your code.

I’ll leave you with a very simple client that you can use to test your progress in case you haven’t done one before:

const io = require('')

// Use https or wss in production.
let url = 'ws://localhost:8000/'

let usrname = process.argv[2] //grab the username from the command line
console.log("Username: ", usrname)

// Connect to a server.
let socket = io(url)

// Rooms messages handler (own messages are here too).
socket.on("new message", function (msg) {
  console.log("New message received")

socket.on('connect', _ => {
socket.emit("new message", "Hey World!")

socket.emit("join room", {
  roomname: "testroom",
  username: usrname

socket.emit("new message", 'Hello there!')
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This is a very simple client, but it covers the message sending and the room joining events. You can quickly edit it to send private messages to different users or add input gathering code to actually create a working chat client.

In either case, this example should be enough to get your chat server jump-started! There are tons of ways to keep improving this, as is expected, since once of the main problems with it, is that there is no persistence, should the service die, upon being restarted, all connection information would be lost. Same for user information and room history, you can quickly add storage support in order to save that information permanently and then restore it during startup.

Let me know in the comments below if you’ve implemented this type of socket-based chat services in the past and what else have you done with it, I’d love to know!

Otherwise, see you on the next one!

Editor's note: Seeing something wrong with this post? You can find the correct version here.

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Top comments (1)

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