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Version: 4.x

Server Initialization

Once you have installed the Socket.IO server library, you can now init the server. The complete list of options can be found here.

tip

For TypeScript users, it is possible to provide type hints for the events. Please check this.

Initialization​

Standalone​

const { Server } = require("socket.io");

const io = new Server({ /* options */ });

io.on("connection", (socket) => {
  // ...
});

io.listen(3000);

You can also pass the port as the first argument:

const { Server } = require("socket.io");

const io = new Server(3000, { /* options */ });

io.on("connection", (socket) => {
  // ...
});

This implicitly starts a Node.js HTTP server, which can be accessed through io.httpServer.

With an HTTP server​

const { createServer } = require("http");
const { Server } = require("socket.io");

const httpServer = createServer();
const io = new Server(httpServer, { /* options */ });

io.on("connection", (socket) => {
  // ...
});

httpServer.listen(3000);

With an HTTPS server​

const { readFileSync } = require("fs");
const { createServer } = require("https");
const { Server } = require("socket.io");

const httpsServer = createServer({
  key: readFileSync("/path/to/my/key.pem"),
  cert: readFileSync("/path/to/my/cert.pem")
});

const io = new Server(httpsServer, { /* options */ });

io.on("connection", (socket) => {
  // ...
});

httpsServer.listen(3000);

See also: Node.js documentation

With client-certificate authentication:

Server

import { readFileSync } from "fs";
import { createServer } from "https";
import { Server } from "socket.io";

const httpsServer = createServer({
  key: readFileSync("/path/to/server-key.pem"),
  cert: readFileSync("/path/to/server-cert.pem"),
  requestCert: true,
  ca: [
    readFileSync("/path/to/client-cert.pem")
  ]
});

const io = new Server(httpsServer, { /* options */ });

io.engine.on("connection", (rawSocket) => {
  // if you need the certificate details (it is no longer available once the handshake is completed)
  rawSocket.peerCertificate = rawSocket.request.client.getPeerCertificate();
});

io.on("connection", (socket) => {
  console.log(socket.conn.peerCertificate);
  // ...
});

httpsServer.listen(3000);

Client

import { readFileSync } from "fs";
import { io } from "socket.io-client";

const socket = io("https://example.com", {
  key: readFileSync("/path/to/client-key.pem"),
  cert: readFileSync("/path/to/client-cert.pem"),
  ca: [
    readFileSync("/path/to/server-cert.pem")
  ]
});

With an HTTP/2 server​

const { readFileSync } = require("fs");
const { createSecureServer } = require("http2");
const { Server } = require("socket.io");

const httpServer = createSecureServer({
  allowHTTP1: true,
  key: readFileSync("/path/to/my/key.pem"),
  cert: readFileSync("/path/to/my/cert.pem")
});

const io = new Server(httpServer, { /* options */ });

io.on("connection", (socket) => {
  // ...
});

httpServer.listen(3000);

See also: Node.js documentation

With Express​

const express = require("express");
const { createServer } = require("http");
const { Server } = require("socket.io");

const app = express();
const httpServer = createServer(app);
const io = new Server(httpServer, { /* options */ });

io.on("connection", (socket) => {
  // ...
});

httpServer.listen(3000);
caution

Using app.listen(3000) will not work here, as it creates a new HTTP server.

More information here.

With Koa​

const Koa = require("koa");
const { createServer } = require("http");
const { Server } = require("socket.io");

const app = new Koa();
const httpServer = createServer(app.callback());
const io = new Server(httpServer, { /* options */ });

io.on("connection", (socket) => {
  // ...
});

httpServer.listen(3000);

More information here.

With Nest​

See the documentation here.

caution

NestJS v7 and below relies on Socket.IO v2, while NestJS v8 relies on Socket.IO v4. Please use a compatible client.

With Fastify​

You need to register the fastify-socket.io plugin:

const fastify = require("fastify");
const fastifyIO = require("fastify-socket.io");

const server = fastify();
server.register(fastifyIO);

server.get("/", (req, reply) => {
  server.io.emit("hello");
});

server.ready().then(() => {
  // we need to wait for the server to be ready, else `server.io` is undefined
  server.io.on("connection", (socket) => {
    // ...
  });
});

server.listen({ port: 3000 });

With µWebSockets.js​

import { App } from "uWebSockets.js";
import { Server } from "socket.io";

const app = App();
const io = new Server();

io.attachApp(app);

io.on("connection", (socket) => {
  // ...
});

app.listen(3000, (token) => {
  if (!token) {
    console.warn("port already in use");
  }
});

Reference: https://github.com/uNetworking/uWebSockets.js

Options​

The complete list of available options can be found here.