How to make TCP communications with Node.js and the NET module

3 min
Intermediate

In Node.js, we can use the net module to create TCP servers and clients easily.

The TCP protocol provides a stable and reliable way to transmit data over the web, ensuring the ordered and error-free delivery of packets.

Some of its features are:

Stability: TCP establishes persistent connections between the client and the server, ensuring the ordered and error-free delivery of transmitted data.
Reliability: The acknowledgment mechanism for packet receipt ensures that TCP guarantees that data reaches its destination intact and in the correct order.
Congestion Control: TCP includes congestion control algorithms that regulate the flow of data between the endpoints of the connection.

TCP communication is used in a wide range of web applications, from file transfers to secure internet browsing.

Example of TCP Implementation in Node.js

Basic TCP Server

To create a basic TCP server in Node.js, we can use the net module. The following example shows how to create a TCP server that listens on a specific port and handles incoming connections:

tcp.mjs

import net from 'node:net';

// Create TCP server
const server = net.createServer((socket) => {
  console.log('Client connected');

  // Event when receiving data from the client
  socket.on('data', (data) => {
    console.log('Data received:', data.toString());

    // Send data back to the client
    socket.write('Data received by the server: ' + data.toString());
  });

  // Event when the client connection is closed
  socket.on('close', () => {
    console.log('Client disconnected');
  });

  // Handle connection errors
  socket.on('error', (err) => {
    console.error('Connection error:', err);
  });
});

const port = 3000;
server.listen(port, () => {
  console.log(`TCP server started on port ${port}`);
});

In this example:

A TCP server is created that listens on port 3000.
When a client connects, a message is printed to the console.
When the server receives data from the client, it prints it to the console and sends a response to the client.
When a client disconnects, a message is printed to the console.

To test this TCP server, you will need a client that can connect and send data. You can use a tool like telnet in the command line or write a client in JavaScript to test it 👇

TCP Client in Node.js

To connect to a TCP server from a client in Node.js, we can also use the net module. The following example shows how to create a TCP client that connects to a server on a specific host and port:

client.mjs

import net from 'node:net';

const client = new net.Socket();

const port = 3000;
const host = 'localhost'; // Change to the host where the server is

client.connect(port, host, () => {
  console.log('Connected to TCP server');
  client.write('Hello from the TCP client');
});

client.on('data', (data) => {
  console.log('Data received from server:', data.toString());
  client.end();
});

client.on('close', () => {
  console.log('Connection closed');
});

client.on('error', (err) => {
  console.error('Connection error:', err);
});

This TCP client will connect to the server we created earlier and send a message. When the server receives the message, it will print it to the console and send a response back to the client, which will also be printed in the client’s console.

To test it,

Run the Node.js server
Run the Node.js client.
You should see that the client connects to the server, sends a message, receives a response from the server, and then disconnects.

Download the Code

All the code from this post is available for download on Github