WebSockets are an alternative to HTTP communication in Web Applications.

They offer a long lived, bidirectional communication channel between client and server.

Once established, the channel is kept open, offering a very fast connection with low latency and overhead.

Browser support for WebSockets

WebSockets are supported by all modern browsers.

Browser support for WebSockets

How WebSockets differ from HTTP

HTTP is a very different protocol, and also a different way of communicate.

HTTP is a request/response protocol: the server returns some data when the client requests it.

With WebSockets:

  • the server can send a message to the client without the client explicitly requesting something
  • the client and the server can talk to each other simultaneously
  • very little data overhead needs to be exchanged to send messages. This means a low latency communication.

WebSockets are great for real-time and long-lived communications.

HTTP is great for occasional data exchange and interactions initiated by the client.

HTTP is much simpler to implement, while WebSockets require a bit more overhead.

Secured WebSockets

Always use the secure, encrypted protocol for WebSockets, wss://.

ws:// refers to the unsafe WebSockets version (the http:// of WebSockets), and should be avoided for obvious reasons.

Create a new WebSockets connection

const url = 'wss://myserver.com/something'
const connection = new WebSocket(url)

connection is a WebSocket object.

When the connection is successfully established, the open event is fired.

Listen for it by assigning a callback function to the onopen property of the connection object:

connection.onopen = () => {
  //...
}

If there’s any error, the onerror function callback is fired:

connection.onerror = (error) => {
  console.log(`WebSocket error: ${error}`)
}

Sending data to the server using WebSockets

Once the connection is open, you can send data to the server.

You can do so conveniently inside the onopen callback function:

connection.onopen = () => {
  connection.send('hey')
}

Receiving data from the server using WebSockets

Listen with a callback function on onmessage, which is called when the message event is received:

connection.onmessage = (e) => {
  console.log(e.data)
}

Implement a server in Node.js

ws is a popular WebSockets library for Node.js.

We’ll use it to build a WebSockets server. It can also be used to implement a client, and use WebSockets to communicate between two backend services.

Easily install it using

yarn init
yarn add ws

The code you need to write is very little:

const WebSocket = require('ws')

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

wss.on('connection', (ws) => {
  ws.on('message', (message) => {
    console.log(`Received message => ${message}`)
  })
  ws.send('ho!')
})

This code creates a new server on port 8080 (the default port for WebSockets), and adds a callback function when a connection is established, sending ho! to the client, and logging the messages it receives.

See a live example on Glitch

Here is a live example of a WebSockets server: https://glitch.com/edit/#!/flavio-websockets-server-example

Here is a WebSockets client that interacts with the server: https://glitch.com/edit/#!/flavio-websockets-client-example