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Socket.IO is a library that enables real-time, bidirectional, and event-based communication between the browser and the server. It consists of:

• a Node.js server: Source | API
• a Javascript client library for the browser (which can be also run from Node.js): Source | API

• Java: https://github.com/trinopoty/socket.io-server-java
• Python: https://github.com/miguelgrinberg/python-socketio

How does that work?

The client will try to establish a WebSocket connection if possible, and will fall back on HTTP long-polling if not.

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WebSocket is a communication protocol that provides a full-duplex and low-latency channel between the server and the browser. More information can be found here.

So, in the best-case scenario, provided that:

• the browser supports WebSocket (97% of all browsers in 2020)
• there is no element (proxy, firewall, …) preventing WebSocket connections between the client and the server

you can consider the Socket.IO client as a “slight” wrapper around the WebSocket API. Instead of writing:

const socket = new WebSocket(“ws://localhost:3000”);

socket.onopen = () => {
socket.send(“Hello!”);
};

socket.onmessage = (data) => {
console.log(data);
};

You will have, on the client-side:

const socket = io(“ws://localhost:3000”);
socket.on(“connect”, () => {
// either with send()
socket.send(“Hello”);
// or with emit() and custom event names
socket.emit(“salutations”, “Hello!”, { “mr”: “john” }, Uint8Array.from([1, 2, 3, 4]));
});
// handle the event sent with socket.send()
socket.on(“message”, data => {
console.log(data);
});
// handle the event sent with socket.emit()
socket.on(“greetings”, (elem1, elem2, elem3) => {
console.log(elem1, elem2, elem3);
});

The API on the server-side is similar, you also get a socket object which extends the Node.js EventEmitter class:

const io = require(“socket.io”)(3000);
io.on(“connection”, socket => {
// either with send()
socket.send(“Hello!”);
// or with emit() and custom event names
socket.emit(“greetings”, “Hey!”, { “ms”: “jane” }, Buffer.from([4, 3, 3, 1]));
// handle the event sent with socket.send()
socket.on(“message”, (data) => {
console.log(data);
});
// handle the event sent with socket.emit()
socket.on(“salutations”, (elem1, elem2, elem3) => {
console.log(elem1, elem2, elem3);
});
});

Socket.IO provides additional features over a plain WebSocket object, which are listed below.

But first, let’s detail what the Socket.IO library is not.

What Socket.IO is not

Socket.IO is NOT a WebSocket implementation. Although Socket.IO indeed uses WebSocket as a transport when possible, it adds additional metadata to each packet. That is why a WebSocket client will not be able to successfully connect to a Socket.IO server, and a Socket.IO client will not be able to connect to a plain WebSocket server either.

// WARNING: the client will NOT be able to connect!
const socket = io(“ws://echo.websocket.org”);

If you are looking for a plain WebSocket server, please take a look at ws or uWebSockets.js.

There are also talks to include a WebSocket server in the Node.js core.

On the client-side, you might be interested in the robust-websocket package.

Features

Here are the features provided by Socket.IO over plain WebSockets:

• reliability (fallback to HTTP long-polling in case the WebSocket connection cannot be established)
• automatic reconnection
• packet buffering
• acknowledgments
• broadcasting to all clients or to a subset of clients (what we call “Room”)
• multiplexing (what we call “Namespace”)

Please find more details about how it works here.

What is the use of Socket.io in node js?

Socket.IO enables real-time bidirectional event-based communication. It works on every platform, browser, or device, focusing equally on reliability and speed. Socket.IO is built on top of the WebSockets API (Client-side) and Node. js.