Socket.io

Learn realtime communication with Socket.io.

What is Socket.io?

Socket.io is the most popular real-time communication library for Node.js. It provides two-way data flow between client and server. Chat apps, live notifications and multiplayer games are built with Socket.io.

Socket.io is not built in; you install it with npm install socket.io. On the server you import it with const socketIo = require("socket.io");. Socket.io uses WebSocket underneath, but if WebSocket is unavailable it falls back to long polling automatically. That makes browser compatibility very high. In this group you will learn the server side: socketIo(server), io.on, emit, rooms and namespaces.

Creating io with socketIo(server)

You must attach Socket.io to an HTTP server. First create a server with http.createServer, then obtain an io object with socketIo(server). This object manages all real-time events.

Example: const http = require("http"); const server = http.createServer(); const io = socketIo(server);. After this you listen for connecting clients with io.on("connection", ...). The socketIo(server) call binds the server and the Socket.io layer; the client side uses the Socket.io client library. Without an io object no Socket.io feature works.

Exercise Import Socket.io

Import the socket.io module with require. Print "Socket.io ready" with console.log.

Show example solution

const socketIo = require("socket.io");
console.log("Socket.io ready");

Test yourself

How is Socket.io imported?
```
const socketIo = require("socket.io")
```
- const socketIo = new Socket()
- import socket from "realtime"
- const socketIo = require("socket.io")
- const socketIo = require("http")
Show answer

const socketIo = require("socket.io") — Socket.io is imported with require('socket.io').

Complete the socketIo call

const io = socketIo(____);

Show answer

const io = socketIo(server);

Match each piece to its role

socketIo

Library
http.createServer

HTTP server
io

Socket.io instance
npm install

Installs the package

Next lesson: io.on

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