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 below.

Syntax

CommonJS

const httpServer = require("http").createServer();
const io = require("socket.io")(httpServer, {
  // ...
});

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

httpServer.listen(3000);

ES modules

Please see here for enabling ECMAScript modules in your Node.js project.

import { createServer } from "http";
import { Server } from "socket.io";

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

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

httpServer.listen(3000);

TypeScript

Socket.IO has now first-class support for TypeScript:

import { createServer } from "http";
import { Server, Socket } from "socket.io";

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

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

httpServer.listen(3000);

Initialization

Standalone

const options = { /* ... */ };
const io = require("socket.io")(options);

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

io.listen(3000);

You can also pass the port as the first argument:

const options = { /* ... */ };
const io = require("socket.io")(3000, options);

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

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

Attached to an existing HTTP server

With an HTTP server

const httpServer = require("http").createServer();
const options = { /* ... */ };
const io = require("socket.io")(httpServer, options);

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

httpServer.listen(3000);

With an HTTPS server

const fs = require("fs");
const httpServer = require("https").createServer({
  key: fs.readFileSync("/tmp/key.pem"),
  cert: fs.readFileSync("/tmp/cert.pem")
});
const options = { /* ... */ };
const io = require("socket.io")(httpServer, options);

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

httpServer.listen(3000);

Node.js documentation

With an HTTP/2 server

const fs = require("fs");
const httpServer = require("http2").createSecureServer({
  allowHTTP1: true,
  key: fs.readFileSync("/tmp/key.pem"),
  cert: fs.readFileSync("/tmp/cert.pem")
});
const options = { /* ... */ };
const io = require("socket.io")(httpServer, options);

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

httpServer.listen(3000);

Node.js documentation

With Express

const app = require("express")();
const httpServer = require("http").createServer(app);
const options = { /* ... */ };
const io = require("socket.io")(httpServer, options);

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

httpServer.listen(3000);
// WARNING !!! app.listen(3000); will not work here, as it creates a new HTTP server

More information here.

With Koa

const app = require("koa")();
const httpServer = require("http").createServer(app.callback());
const options = { /* ... */ };
const io = require("socket.io")(httpServer, options);

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

httpServer.listen(3000);

More information here.

Options

Socket.IO server options
- path
- serveClient
- adapter
- parser
- connectTimeout
Low-level engine options

Socket.IO server options

`path`

Default value: /socket.io/

It is the name of the path that is captured on the server side.

The server and the client values must match (unless you are using a path-rewriting proxy in between):

Server

const httpServer = require("http").createServer();
const io = require("socket.io")(httpServer, {
  path: "/my-custom-path/"
});

Client

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

const socket = io("https://example.com", {
  path: "/my-custom-path/"
});

`serveClient`

Default value: true

Whether to serve the client files. If true, the different bundles will be served at the following location:

<url>/socket.io/socket.io.js
<url>/socket.io/socket.io.min.js
<url>/socket.io/socket.io.msgpack.min.js

(including their associated source maps)

`adapter`

Default value: socket.io-adapter (in-memory adapter, whose source code can be found here)

The “Adapter” to use.

Example with the Redis adapter (the socket.io-redis package, more information here):

const httpServer = require("http").createServer();
const redisClient = require("redis").createClient();
const io = require("socket.io")(httpServer, {
  adapter: require("socket.io-redis")({
    pubClient: redisClient,
    subClient: redisClient.duplicate()
  })
});

`parser`

Default value: socket.io-parser

The parser to use. Please see the documentation here.

`connectTimeout`

Default value: 45000

The number of ms before disconnecting a client that has not successfully joined a namespace.

Low-level engine options

`pingTimeout`

Default value: 20000

This value is used in the heartbeat mechanism, which periodically checks if the connection is still alive between the server and the client.

The server sends a ping, and if the client does not answer with a pong within pingTimeout ms, the server considers that the connection is closed.

Similarly, if the client does not receive a ping from the server within pingInterval + pingTimeout ms, the client also considers that the connection is closed.

In both cases, the disconnection reason will be: ping timeout

socket.on("disconnect", (reason) => {
  console.log(reason); // "ping timeout"
});

Note: the default value might be a bit low if you need to send big files in your application. Please increase it if that’s the case:

const io = require("socket.io")(httpServer, {
  pingTimeout: 30000
});

`pingInterval`

Default value: 25000

See above.

`upgradeTimeout`

Default value: 10000

This is the delay in milliseconds before an uncompleted transport upgrade is cancelled.

`maxHttpBufferSize`

Default value: 1e6 (1 MB)

This defines how many bytes a single message can be, before closing the socket. You may increase or decrease this value depending on your needs.

const io = require("socket.io")(httpServer, {
  maxHttpBufferSize: 1e8
});

It matches the maxPayload option of the ws package.

`allowRequest`

Default: -

A function that receives a given handshake or upgrade request as its first parameter, and can decide whether to continue or not.

Example:

const io = require("socket.io")(httpServer, {
  allowRequest: (req, callback) => {
    const isOriginValid = check(req);
    callback(null, isOriginValid);
  }
});

`transports`

Default value: ["polling", "websocket"]

The low-level transports that are allowed on the server-side.

`allowUpgrades`

Default value: true

Whether to allow transport upgrades.

`perMessageDeflate`

History

Version	Changes
v3.0.0	The permessage-deflate extension is now disabled by default.
v1.4.0	First implementation.

Default value: false

Whether to enable the permessage-deflate extension for the WebSocket transport. This extension is known to add a significant overhead in terms of performance and memory consumption, so we suggest to only enable it if it is really needed.

Please note that if perMessageDeflate is set to false (which is the default), the compress flag used when emitting (socket.compress(true).emit(...)) will be ignored when the connection is established with WebSockets, as the permessage-deflate extension cannot be enabled on a per-message basis.

All options from the ws module are supported:

const io = require("socket.io")(httpServer, {
  perMessageDeflate: {
    threshold: 2048, // defaults to 1024

    zlibDeflateOptions: {
      chunkSize: 8 * 1024, // defaults to 16 * 1024
    },

    zlibInflateOptions: {
      windowBits: 14, // defaults to 15
      memLevel: 7, // defaults to 8
    },

    clientNoContextTakeover: true, // defaults to negotiated value.
    serverNoContextTakeover: true, // defaults to negotiated value.
    serverMaxWindowBits: 10, // defaults to negotiated value.

    concurrencyLimit: 20, // defaults to 10
  }
});

`httpCompression`

Added in v1.4.0

Default value: true

Whether to enable the compression for the HTTP long-polling transport.

Please note that if httpCompression is set to false, the compress flag used when emitting (socket.compress(true).emit(...)) will be ignored when the connection is established with HTTP long-polling requests.

All options from the Node.js zlib module are supported.

Example:

const io = require("socket.io")(httpServer, {
  httpCompression: {
    // Engine.IO options
    threshold: 2048, // defaults to 1024
    // Node.js zlib options
    chunkSize: 8 * 1024, // defaults to 16 * 1024
    windowBits: 14, // defaults to 15
    memLevel: 7, // defaults to 8
  }
});

`wsEngine`

Default value: "ws" (source code can be found here)

The WebSocket server implementation to use. Please see the documentation here.

Example:

const io = require("socket.io")(httpServer, {
  wsEngine: require("eiows").Server
});

`cors`

Default value: -

The list of options that will be forwarded to the cors module. More information can be found here.

Example:

const io = require("socket.io")(httpServer, {
  cors: {
    origin: ["https://example.com", "https://dev.example.com"],
    allowedHeaders: ["my-custom-header"],
    credentials: true
  }
});

`cookie`

Default value: -

The list of options that will be forwarded to the cookie module.

Please note that since Socket.IO v3, there is no cookie sent by default anymore (reference).

Example:

const io = require("socket.io")(httpServer, {
  cookie: {
    name: "my-cookie",
    httpOnly: true,
    sameSite: "strict",
    maxAge: 86400
  }
});

`allowEIO3`

Default value: false

Whether to enable compatibility with Socket.IO v2 clients.