WebSockets & Realtime Systems Programming · Lesson

Implementing Server-Side Logic

Write code to create a WebSocket server, listen for connections, and handle basic events.

Implementing Server-Side Logic is a free WebSockets & Realtime Systems Programming lesson on CoddyKit — lesson 2 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the WebSockets & Realtime Systems Programming learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Server Logic Unveiled

Welcome back! In the previous lesson, we set up our Node.js project. Now, let's dive into building the actual server logic for our WebSocket application.

We'll learn how to create a WebSocket server, listen for incoming client connections, and handle basic events like receiving and sending messages.

Creating the WS Server

The first step is to create an instance of our WebSocket server. We'll use the ws library we installed earlier. It provides a WebSocket.Server class.

We need to specify a port for our server to listen on. Port 8080 is a common choice for development.

Try running this basic server setup:

const WebSocket = require('ws');

const wss = new WebSocket.Server({ port: 8080 });

console.log('WebSocket server started on port 8080');

Listening for Connections

Our server is running, but it doesn't do much yet! The most fundamental event for a WebSocket server is 'connection'.

This event fires every time a new client successfully establishes a WebSocket connection with our server. The event handler receives a ws object, representing the individual client connection.

Let's add a handler to log when a client connects:

const WebSocket = require('ws');

const wss = new WebSocket.Server({ port: 8080 });

wss.on('connection', function connection(ws) {
  console.log('Client connected!');
});

console.log('WebSocket server started on port 8080');

The Client WebSocket Object

When a client connects, the wss.on('connection') callback receives a ws object. This isn't the server itself, but an instance of WebSocket specific to that client.

It represents the communication channel with that single client.
We'll use this ws object to send messages to, and receive messages from, that particular client.

Think of it as a direct line to one person in a crowded room.

Handling Client Messages

Once a client is connected, it can send messages to our server. We need to listen for these messages using the 'message' event on the individual client's ws object.

The message handler receives the data sent by the client. For simplicity, we'll assume text messages for now.

Update your server to log any incoming messages:

const WebSocket = require('ws');

const wss = new WebSocket.Server({ port: 8080 });

wss.on('connection', function connection(ws) {
  console.log('Client connected!');

  ws.on('message', function incoming(message) {
    console.log('Received: %s', message);
  });
});

console.log('WebSocket server started on port 8080');

Sending Data Back to Client

A key aspect of WebSockets is bidirectional communication. Not only can clients send messages to the server, but the server can also send messages back to specific clients.

We use the ws.send() method on the client's WebSocket object to send data. Let's create a simple 'echo' server that sends back whatever it receives!

const WebSocket = require('ws');

const wss = new WebSocket.Server({ port: 8080 });

wss.on('connection', function connection(ws) {
  console.log('Client connected!');

  ws.on('message', function incoming(message) {
    console.log('Received: %s', message);
    ws.send(`Server received: ${message}`); // Echo back!
  });
});

console.log('WebSocket server started on port 8080');

Handling Client Disconnections

Clients won't stay connected forever. They might close their browser tab, lose internet, or deliberately disconnect. It's crucial to handle these disconnections gracefully.

The 'close' event on the client's ws object tells us when a client has disconnected. This is useful for cleanup or logging.

Add a 'close' event handler to your server:

const WebSocket = require('ws');

const wss = new WebSocket.Server({ port: 8080 });

wss.on('connection', function connection(ws) {
  console.log('Client connected!');

  ws.on('message', function incoming(message) {
    console.log('Received: %s', message);
    ws.send(`Server received: ${message}`);
  });

  ws.on('close', function close() {
    console.log('Client disconnected!');
  });
});

console.log('WebSocket server started on port 8080');

Dealing with Errors

Errors can occur at various stages �� network issues, malformed messages, or server problems. It's good practice to include error handling.

Client-specific errors: Use ws.on('error', ...) for issues related to a single client connection.
Server-wide errors: Use wss.on('error', ...) for problems affecting the entire WebSocket server.

Let's add basic error logging to our server:

const WebSocket = require('ws');

const wss = new WebSocket.Server({ port: 8080 });

wss.on('connection', function connection(ws) {
  console.log('Client connected!');

  ws.on('message', function incoming(message) {
    console.log('Received: %s', message);
    ws.send(`Server received: ${message}`);
  });

  ws.on('close', function close() {
    console.log('Client disconnected!');
  });

  ws.on('error', function error(err) {
    console.error('Client WS Error:', err.message);
  });
});

wss.on('error', function serverError(err) {
  console.error('Server WS Error:', err.message);
});

console.log('WebSocket server started on port 8080');

Full Basic Server Example

You've now learned how to set up a WebSocket server, handle new connections, listen for incoming messages, send responses, and manage disconnections and errors.

This complete example brings all those concepts together, forming a robust foundation for your WebSocket applications.

const WebSocket = require('ws');

const wss = new WebSocket.Server({ port: 8080 });

wss.on('connection', function connection(ws) {
  console.log('Client connected!');

  ws.on('message', function incoming(message) {
    console.log(`Received from client: ${message}`);
    ws.send(`Echo: ${message}`); // Send message back to the same client
  });

  ws.on('close', function close() {
    console.log('Client disconnected!');
  });

  ws.on('error', function error(err) {
    console.error('Client connection error:', err);
  });
});

wss.on('listening', () => {
  console.log('WebSocket server is listening on port 8080');
});

wss.on('error', function serverError(err) {
  console.error('WebSocket server error:', err);
});

Server Logic Quiz

Which event handler is used on the server instance (wss) to detect when a new client connects?

Recap & Next Steps

Great work! You've successfully implemented the core server-side logic for a WebSocket application.

You learned to create a WebSocket.Server instance.
You handled 'connection', 'message', 'close', and 'error' events.
You can now send messages back to individual connected clients using ws.send().

Next, we'll explore how to send messages to ALL connected clients, a crucial feature for chat applications and other broadcast scenarios!

Frequently asked questions

Is the “Implementing Server-Side Logic” lesson free?

Yes — the full text of “Implementing Server-Side Logic” is free to read here on the web, and the WebSockets & Realtime Systems Programming course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the WebSockets & Realtime Systems Programming course, upgrade to CoddyKit PRO.

What will I learn in “Implementing Server-Side Logic”?

Write code to create a WebSocket server, listen for connections, and handle basic events. You practise WebSockets & Realtime Systems Programming with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start WebSockets & Realtime Systems Programming?

No prior experience is required. WebSockets & Realtime Systems Programming on CoddyKit is structured for beginners through advanced learners; this is — lesson 2 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Implementing Server-Side Logic” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this WebSockets & Realtime Systems Programming lesson?

Yes. Every WebSockets & Realtime Systems Programming lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

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