chore(release): 4.0.1

Diff: https://github.com/socketio/socket.io/compare/4.0.0...4.0.1
fix(typings): add fallback to untyped event listener (#3834 )
2026-01-11 16:08:24 -05:00 · 2021-04-01 01:25:56 +02:00 · 2021-03-31 11:37:37 +02:00 · 2021-03-24 23:12:22 +01:00 · 2021-03-18 15:07:04 +01:00 · 2021-03-18 14:06:13 +01:00
99 changed files with 7268 additions and 31303 deletions
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -12,7 +12,7 @@ jobs:

    strategy:
      matrix:
-        node-version: [10.x, 12.x, 14.x]
+        node-version: [10.x, 12.x, 14.x, 15.x]

    steps:
      - uses: actions/checkout@v2
--- a/.replit
+++ b/.replit
@@ -0,0 +1,2 @@
+language = "nodejs"
+run = "npm start"
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,62 @@
+## [4.0.1](https://github.com/socketio/socket.io/compare/4.0.0...4.0.1) (2021-03-31)
+
+
+### Bug Fixes
+
+* **typings:** add fallback to untyped event listener ([#3834](https://github.com/socketio/socket.io/issues/3834)) ([a11152f](https://github.com/socketio/socket.io/commit/a11152f42b281df83409313962f60f230239c79e))
+* **typings:** update return type from emit ([#3843](https://github.com/socketio/socket.io/issues/3843)) ([1a72ae4](https://github.com/socketio/socket.io/commit/1a72ae4fe27a14cf60916f991a2c94da91d9e54a))
+
+
+# [4.0.0](https://github.com/socketio/socket.io/compare/3.1.2...4.0.0) (2021-03-10)
+
+
+### Bug Fixes
+
+* make io.to(...) immutable ([ac9e8ca](https://github.com/socketio/socket.io/commit/ac9e8ca6c71e00d4af45ee03f590fe56f3951186))
+
+
+### Features
+
+* add some utility methods ([b25495c](https://github.com/socketio/socket.io/commit/b25495c069031674da08e19aed68922c7c7a0e28))
+* add support for typed events ([#3822](https://github.com/socketio/socket.io/issues/3822)) ([0107510](https://github.com/socketio/socket.io/commit/0107510ba8a0f148c78029d8be8919b350feb633))
+* allow to exclude specific rooms when broadcasting ([#3789](https://github.com/socketio/socket.io/issues/3789)) ([7de2e87](https://github.com/socketio/socket.io/commit/7de2e87e888d849eb2dfc5e362af4c9e86044701))
+* allow to pass an array to io.to(...) ([085d1de](https://github.com/socketio/socket.io/commit/085d1de9df909651de8b313cc6f9f253374b702e))
+
+
+## [3.1.2](https://github.com/socketio/socket.io/compare/3.1.1...3.1.2) (2021-02-26)
+
+
+### Bug Fixes
+
+* ignore packets received after disconnection ([494c64e](https://github.com/socketio/socket.io/commit/494c64e44f645cbd24c645f1186d203789e84af0))
+
+
+## [3.1.1](https://github.com/socketio/socket.io/compare/3.1.0...3.1.1) (2021-02-03)
+
+
+### Bug Fixes
+
+* properly parse the CONNECT packet in v2 compatibility mode ([6f4bd7f](https://github.com/socketio/socket.io/commit/6f4bd7f8e7c41a075a8014565330a77c38b03a8d))
+* **typings:** add return types and general-case overload signatures ([#3776](https://github.com/socketio/socket.io/issues/3776)) ([9e8f288](https://github.com/socketio/socket.io/commit/9e8f288ca9f14f91064b8d3cce5946f7d23d407c))
+* **typings:** update the types of "query", "auth" and "headers" ([4f2e9a7](https://github.com/socketio/socket.io/commit/4f2e9a716d9835b550c8fd9a9b429ebf069c2895))
+
+
+# [3.1.0](https://github.com/socketio/socket.io/compare/3.0.5...3.1.0) (2021-01-15)
+
+
+### Features
+
+* confirm a weak but matching ETag ([#3485](https://github.com/socketio/socket.io/issues/3485)) ([161091d](https://github.com/socketio/socket.io/commit/161091dd4c9e1b1610ac3d45d964195e63d92b94))
+* **esm:** export the Namespace and Socket class ([#3699](https://github.com/socketio/socket.io/issues/3699)) ([233650c](https://github.com/socketio/socket.io/commit/233650c22209708b5fccc4349c38d2fa1b465d8f))
+* add support for Socket.IO v2 clients ([9925746](https://github.com/socketio/socket.io/commit/9925746c8ee3a6522bd640b5d586c83f04f2f1ba))
+* add room events ([155fa63](https://github.com/socketio/socket.io-adapter/commit/155fa6333a504036e99a33667dc0397f6aede25e))
+
+
+### Bug Fixes
+
+* allow integers as event names ([1c220dd](https://github.com/socketio/socket.io-parser/commit/1c220ddbf45ea4b44bc8dbf6f9ae245f672ba1b9))
+
+
 ## [3.0.5](https://github.com/socketio/socket.io/compare/3.0.4...3.0.5) (2021-01-05)


--- a/Readme.md
+++ b/Readme.md
@@ -1,6 +1,5 @@
-
 # socket.io
-
+[![Run on Repl.it](https://repl.it/badge/github/socketio/socket.io)](https://repl.it/github/socketio/socket.io)
 [![Backers on Open Collective](https://opencollective.com/socketio/backers/badge.svg)](#backers) [![Sponsors on Open Collective](https://opencollective.com/socketio/sponsors/badge.svg)](#sponsors)
 [![Build Status](https://github.com/socketio/socket.io/workflows/CI/badge.svg)](https://github.com/socketio/socket.io/actions)
 [![Dependency Status](https://david-dm.org/socketio/socket.io.svg)](https://david-dm.org/socketio/socket.io)
@@ -22,6 +21,8 @@ Some implementations in other languages are also available:
 - [C++](https://github.com/socketio/socket.io-client-cpp)
 - [Swift](https://github.com/socketio/socket.io-client-swift)
 - [Dart](https://github.com/rikulo/socket.io-client-dart)
+- [Python](https://github.com/miguelgrinberg/python-socketio)
+- [.Net](https://github.com/Quobject/SocketIoClientDotNet)

 Its main features are:

@@ -35,7 +36,7 @@ For this purpose, it relies on [Engine.IO](https://github.com/socketio/engine.io

 #### Auto-reconnection support

-Unless instructed otherwise a disconnected client will try to reconnect forever, until the server is available again. Please see the available reconnection options [here](https://github.com/socketio/socket.io-client/blob/master/docs/API.md#new-managerurl-options).
+Unless instructed otherwise a disconnected client will try to reconnect forever, until the server is available again. Please see the available reconnection options [here](https://socket.io/docs/v3/client-api/#new-Manager-url-options).

 #### Disconnection detection

@@ -84,7 +85,11 @@ This is a useful feature to send notifications to a group of users, or to a give
 ## Installation

 ```bash
+// with npm
 npm install socket.io
+
+// with yarn
+yarn add socket.io
 ```

 ## How to use
--- a/client-dist/socket.io.js
+++ b/client-dist/socket.io.js
--- a/client-dist/socket.io.js.map
+++ b/client-dist/socket.io.js.map
--- a/client-dist/socket.io.min.js
+++ b/client-dist/socket.io.min.js
--- a/client-dist/socket.io.min.js.map
+++ b/client-dist/socket.io.min.js.map
--- a/client-dist/socket.io.msgpack.min.js
+++ b/client-dist/socket.io.msgpack.min.js
--- a/client-dist/socket.io.msgpack.min.js.map
+++ b/client-dist/socket.io.msgpack.min.js.map
--- a/examples/.gitignore
+++ b/examples/.gitignore
@@ -0,0 +1 @@
+package-lock.json
--- a/examples/angular-todomvc/package-lock.json
+++ b/examples/angular-todomvc/package-lock.json
--- a/examples/angular-todomvc/package.json
+++ b/examples/angular-todomvc/package.json
@@ -21,8 +21,8 @@
    "@angular/platform-browser-dynamic": "~11.0.4",
    "@angular/router": "~11.0.4",
    "rxjs": "~6.6.0",
-    "socket.io": "^3.0.4",
-    "socket.io-client": "^3.0.4",
+    "socket.io": "^4.0.0",
+    "socket.io-client": "^4.0.0",
    "tslib": "^2.0.0",
    "zone.js": "~0.10.2"
  },
--- a/examples/angular-todomvc/server.ts
+++ b/examples/angular-todomvc/server.ts
@@ -1,4 +1,4 @@
-import { Server, Socket } from "socket.io";
+import { Server } from "socket.io";

 const io = new Server(8080, {
  cors: {
@@ -15,7 +15,7 @@ interface Todo {

 let todos: Array<Todo> = [];

-io.on("connect", (socket: Socket) => {
+io.on("connect", (socket) => {
    socket.emit("todos", todos);

    // note: we could also create a CRUD (create/read/update/delete) service for the todo list
--- a/examples/chat/package-lock.json
+++ b/examples/chat/package-lock.json
@@ -1,499 +0,0 @@
-{
-  "name": "socket.io-chat",
-  "version": "0.0.0",
-  "lockfileVersion": 1,
-  "requires": true,
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-      "requires": {
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-        "negotiator": "0.6.2"
-      }
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-}
--- a/examples/chat/package.json
+++ b/examples/chat/package.json
@@ -8,7 +8,7 @@
  "license": "BSD",
  "dependencies": {
    "express": "~4.17.1",
-    "socket.io": "^3.0.0"
+    "socket.io": "^4.0.0"
  },
  "scripts": {
    "start": "node index.js"
--- a/examples/chat/public/main.js
+++ b/examples/chat/public/main.js
@@ -1,36 +1,36 @@
 $(function() {
-  var FADE_TIME = 150; // ms
-  var TYPING_TIMER_LENGTH = 400; // ms
-  var COLORS = [
+  const FADE_TIME = 150; // ms
+  const TYPING_TIMER_LENGTH = 400; // ms
+  const COLORS = [
    '#e21400', '#91580f', '#f8a700', '#f78b00',
    '#58dc00', '#287b00', '#a8f07a', '#4ae8c4',
    '#3b88eb', '#3824aa', '#a700ff', '#d300e7'
  ];

  // Initialize variables
-  var $window = $(window);
-  var $usernameInput = $('.usernameInput'); // Input for username
-  var $messages = $('.messages'); // Messages area
-  var $inputMessage = $('.inputMessage'); // Input message input box
+  const $window = $(window);
+  const $usernameInput = $('.usernameInput'); // Input for username
+  const $messages = $('.messages');           // Messages area
+  const $inputMessage = $('.inputMessage');   // Input message input box

-  var $loginPage = $('.login.page'); // The login page
-  var $chatPage = $('.chat.page'); // The chatroom page
+  const $loginPage = $('.login.page');        // The login page
+  const $chatPage = $('.chat.page');          // The chatroom page
+
+  const socket = io();

  // Prompt for setting a username
-  var username;
-  var connected = false;
-  var typing = false;
-  var lastTypingTime;
-  var $currentInput = $usernameInput.focus();
-
-  var socket = io();
+  let username;
+  let connected = false;
+  let typing = false;
+  let lastTypingTime;
+  let $currentInput = $usernameInput.focus();

  const addParticipantsMessage = (data) => {
-    var message = '';
+    let message = '';
    if (data.numUsers === 1) {
-      message += "there's 1 participant";
+      message += `there's 1 participant`;
    } else {
-      message += "there are " + data.numUsers + " participants";
+      message += `there are ${data.numUsers} participants`;
    }
    log(message);
  }
@@ -53,45 +53,41 @@ $(function() {

  // Sends a chat message
  const sendMessage = () => {
-    var message = $inputMessage.val();
+    let message = $inputMessage.val();
    // Prevent markup from being injected into the message
    message = cleanInput(message);
    // if there is a non-empty message and a socket connection
    if (message && connected) {
      $inputMessage.val('');
-      addChatMessage({
-        username: username,
-        message: message
-      });
+      addChatMessage({ username, message });
      // tell server to execute 'new message' and send along one parameter
      socket.emit('new message', message);
    }
  }

  // Log a message
-    const log = (message, options) => {
-    var $el = $('<li>').addClass('log').text(message);
+  const log = (message, options) => {
+    const $el = $('<li>').addClass('log').text(message);
    addMessageElement($el, options);
  }

  // Adds the visual chat message to the message list
-  const addChatMessage = (data, options) => {
+  const addChatMessage = (data, options = {}) => {
    // Don't fade the message in if there is an 'X was typing'
-    var $typingMessages = getTypingMessages(data);
-    options = options || {};
+    const $typingMessages = getTypingMessages(data);
    if ($typingMessages.length !== 0) {
      options.fade = false;
      $typingMessages.remove();
    }

-    var $usernameDiv = $('<span class="username"/>')
+    const $usernameDiv = $('<span class="username"/>')
      .text(data.username)
      .css('color', getUsernameColor(data.username));
-    var $messageBodyDiv = $('<span class="messageBody">')
+    const $messageBodyDiv = $('<span class="messageBody">')
      .text(data.message);

-    var typingClass = data.typing ? 'typing' : '';
-    var $messageDiv = $('<li class="message"/>')
+    const typingClass = data.typing ? 'typing' : '';
+    const $messageDiv = $('<li class="message"/>')
      .data('username', data.username)
      .addClass(typingClass)
      .append($usernameDiv, $messageBodyDiv);
@@ -119,8 +115,7 @@ $(function() {
  // options.prepend - If the element should prepend
  //   all other messages (default = false)
  const addMessageElement = (el, options) => {
-    var $el = $(el);
-
+    const $el = $(el);
    // Setup default options
    if (!options) {
      options = {};
@@ -141,6 +136,7 @@ $(function() {
    } else {
      $messages.append($el);
    }
+
    $messages[0].scrollTop = $messages[0].scrollHeight;
  }

@@ -159,8 +155,8 @@ $(function() {
      lastTypingTime = (new Date()).getTime();

      setTimeout(() => {
-        var typingTimer = (new Date()).getTime();
-        var timeDiff = typingTimer - lastTypingTime;
+        const typingTimer = (new Date()).getTime();
+        const timeDiff = typingTimer - lastTypingTime;
        if (timeDiff >= TYPING_TIMER_LENGTH && typing) {
          socket.emit('stop typing');
          typing = false;
@@ -179,12 +175,12 @@ $(function() {
  // Gets the color of a username through our hash function
  const getUsernameColor = (username) => {
    // Compute hash code
-    var hash = 7;
-    for (var i = 0; i < username.length; i++) {
-       hash = username.charCodeAt(i) + (hash << 5) - hash;
+    let hash = 7;
+    for (let i = 0; i < username.length; i++) {
+      hash = username.charCodeAt(i) + (hash << 5) - hash;
    }
    // Calculate color
-    var index = Math.abs(hash % COLORS.length);
+    const index = Math.abs(hash % COLORS.length);
    return COLORS[index];
  }

@@ -229,7 +225,7 @@ $(function() {
  socket.on('login', (data) => {
    connected = true;
    // Display the welcome message
-    var message = "Welcome to Socket.IO Chat – ";
+    const message = 'Welcome to Socket.IO Chat – ';
    log(message, {
      prepend: true
    });
@@ -243,13 +239,13 @@ $(function() {

  // Whenever the server emits 'user joined', log it in the chat body
  socket.on('user joined', (data) => {
-    log(data.username + ' joined');
+    log(`${data.username} joined`);
    addParticipantsMessage(data);
  });

  // Whenever the server emits 'user left', log it in the chat body
  socket.on('user left', (data) => {
-    log(data.username + ' left');
+    log(`${data.username} left`);
    addParticipantsMessage(data);
    removeChatTyping(data);
  });
--- a/examples/cluster-haproxy/server/Dockerfile
+++ b/examples/cluster-haproxy/server/Dockerfile
@@ -1,4 +1,4 @@
-FROM mhart/alpine-node:6
+FROM node:14-alpine

 # Create app directory
 RUN mkdir -p /usr/src/app
@@ -6,7 +6,7 @@ WORKDIR /usr/src/app

 # Install app dependencies
 COPY package.json /usr/src/app/
-RUN npm install
+RUN npm install --prod

 # Bundle app source
 COPY . /usr/src/app
--- a/examples/cluster-haproxy/server/package.json
+++ b/examples/cluster-haproxy/server/package.json
@@ -8,8 +8,8 @@
  "license": "BSD",
  "dependencies": {
    "express": "4.13.4",
-    "socket.io": "^1.7.2",
-    "socket.io-redis": "^3.0.0"
+    "socket.io": "^4.0.0",
+    "socket.io-redis": "^6.0.1"
  },
  "scripts": {
    "start": "node index.js"
--- a/examples/cluster-httpd/httpd/httpd.conf
+++ b/examples/cluster-httpd/httpd/httpd.conf
@@ -3,6 +3,8 @@ Listen 80

 ServerName localhost

+LoadModule mpm_event_module             modules/mod_mpm_event.so
+
 LoadModule authn_file_module            modules/mod_authn_file.so
 LoadModule authn_core_module            modules/mod_authn_core.so
 LoadModule authz_host_module            modules/mod_authz_host.so
--- a/examples/cluster-httpd/server/Dockerfile
+++ b/examples/cluster-httpd/server/Dockerfile
@@ -6,7 +6,7 @@ WORKDIR /usr/src/app

 # Install app dependencies
 COPY package.json /usr/src/app/
-RUN npm install
+RUN npm install --prod

 # Bundle app source
 COPY . /usr/src/app
--- a/examples/cluster-httpd/server/package.json
+++ b/examples/cluster-httpd/server/package.json
@@ -8,8 +8,8 @@
  "license": "BSD",
  "dependencies": {
    "express": "4.13.4",
-    "socket.io": "^1.7.2",
-    "socket.io-redis": "^3.0.0"
+    "socket.io": "^4.0.0",
+    "socket.io-redis": "^6.0.1"
  },
  "scripts": {
    "start": "node index.js"
--- a/examples/cluster-nginx/README.md
+++ b/examples/cluster-nginx/README.md
@@ -22,6 +22,16 @@ Each node connects to the redis backend, which will enable to broadcast to every
 $ docker-compose stop server-george
 ```

+A `client` container is included in the `docker-compose.yml` file, in order to test the routing.
+
+You can create additional `client` containers with:
+
+```
+$ docker-compose up -d --scale=client=10 client
+# and then
+$ docker-compose logs client
+```
+
 ## Features

 - Multiple users can join a chat room by each entering a unique username
--- a/examples/cluster-nginx/client/Dockerfile
+++ b/examples/cluster-nginx/client/Dockerfile
@@ -0,0 +1,15 @@
+FROM node:14-alpine
+
+# Create app directory
+RUN mkdir -p /usr/src/app
+WORKDIR /usr/src/app
+
+# Install app dependencies
+COPY package.json /usr/src/app/
+RUN npm install --prod
+
+# Bundle app source
+COPY . /usr/src/app
+
+EXPOSE 3000
+CMD [ "npm", "start" ]
--- a/examples/cluster-nginx/client/index.js
+++ b/examples/cluster-nginx/client/index.js
@@ -0,0 +1,13 @@
+const socket = require('socket.io-client')('ws://nginx');
+
+socket.on('connect', () => {
+  console.log('connected');
+});
+
+socket.on('my-name-is', (serverName) => {
+  console.log(`connected to ${serverName}`);
+});
+
+socket.on('disconnect', (reason) => {
+  console.log(`disconnected due to ${reason}`);
+});
--- a/examples/cluster-nginx/client/package.json
+++ b/examples/cluster-nginx/client/package.json
@@ -0,0 +1,15 @@
+{
+  "name": "socket.io-chat",
+  "version": "0.0.0",
+  "description": "A simple chat client using socket.io",
+  "main": "index.js",
+  "author": "Grant Timmerman",
+  "private": true,
+  "license": "MIT",
+  "dependencies": {
+    "socket.io-client": "^4.0.0"
+  },
+  "scripts": {
+    "start": "node index.js"
+  }
+}
--- a/examples/cluster-nginx/docker-compose.yml
+++ b/examples/cluster-nginx/docker-compose.yml
@@ -45,6 +45,11 @@ server-ringo:
  environment:
    - NAME=Ringo

+client:
+  build: ./client
+  links:
+    - nginx
+
 redis:
  image: redis:alpine
  expose:
--- a/examples/cluster-nginx/nginx/nginx.conf
+++ b/examples/cluster-nginx/nginx/nginx.conf
@@ -24,8 +24,12 @@ http {
  }

  upstream nodes {
-    # enable sticky session
-    ip_hash;
+    # enable sticky session with either "hash" (uses the complete IP address)
+    hash $remote_addr consistent;
+    # or "ip_hash" (uses the first three octets of the client IPv4 address, or the entire IPv6 address)
+    # ip_hash;
+    # or "sticky" (needs commercial subscription)
+    # sticky cookie srv_id expires=1h domain=.example.com path=/;

    server server-john:3000;
    server server-paul:3000;
--- a/examples/cluster-nginx/server/Dockerfile
+++ b/examples/cluster-nginx/server/Dockerfile
@@ -1,4 +1,4 @@
-FROM mhart/alpine-node:6
+FROM node:14-alpine

 # Create app directory
 RUN mkdir -p /usr/src/app
@@ -6,7 +6,7 @@ WORKDIR /usr/src/app

 # Install app dependencies
 COPY package.json /usr/src/app/
-RUN npm install
+RUN npm install --prod

 # Bundle app source
 COPY . /usr/src/app
--- a/examples/cluster-nginx/server/package.json
+++ b/examples/cluster-nginx/server/package.json
@@ -8,8 +8,8 @@
  "license": "MIT",
  "dependencies": {
    "express": "4.13.4",
-    "socket.io": "^1.7.2",
-    "socket.io-redis": "^3.0.0"
+    "socket.io": "^4.0.0",
+    "socket.io-redis": "^6.0.1"
  },
  "scripts": {
    "start": "node index.js"
--- a/examples/cluster-traefik/README.md
+++ b/examples/cluster-traefik/README.md
@@ -0,0 +1,22 @@
+
+# Socket.IO Chat with traefik & [redis](https://redis.io/)
+
+A simple chat demo for Socket.IO
+
+## How to use
+
+Install [Docker Compose](https://docs.docker.com/compose/install/), then:
+
+```
+$ docker-compose up -d
+```
+
+And then point your browser to `http://localhost:3000`.
+
+You can then scale the server to multiple instances:
+
+```
+$ docker-compose up -d --scale=server=7
+```
+
+The session stickiness, which is [required](https://socket.io/docs/v3/using-multiple-nodes/) when using multiple Socket.IO server instances, is achieved with a cookie. More information [here](https://doc.traefik.io/traefik/v2.0/routing/services/#sticky-sessions).
--- a/examples/cluster-traefik/docker-compose.yml
+++ b/examples/cluster-traefik/docker-compose.yml
@@ -0,0 +1,27 @@
+version: "3"
+
+services:
+  traefik:
+    image: traefik:2.4
+    volumes:
+      - ./traefik.yml:/etc/traefik/traefik.yml
+      - /var/run/docker.sock:/var/run/docker.sock
+    links:
+      - server
+    ports:
+     - "3000:80"
+     - "8080:8080"
+
+  server:
+    build: ./server
+    links:
+      - redis
+    labels:
+      - "traefik.http.routers.chat.rule=PathPrefix(`/`)"
+      - traefik.http.services.chat.loadBalancer.sticky.cookie.name=server_id
+      - traefik.http.services.chat.loadBalancer.sticky.cookie.httpOnly=true
+
+  redis:
+    image: redis:6-alpine
+    labels:
+      - traefik.enable=false
--- a/examples/cluster-traefik/server/Dockerfile
+++ b/examples/cluster-traefik/server/Dockerfile
@@ -0,0 +1,15 @@
+FROM node:14-alpine
+
+# Create app directory
+RUN mkdir -p /usr/src/app
+WORKDIR /usr/src/app
+
+# Install app dependencies
+COPY package.json /usr/src/app/
+RUN npm install --prod
+
+# Bundle app source
+COPY . /usr/src/app
+
+EXPOSE 3000
+CMD [ "npm", "start" ]
--- a/examples/cluster-traefik/server/index.js
+++ b/examples/cluster-traefik/server/index.js
@@ -0,0 +1,83 @@
+// Setup basic express server
+var express = require('express');
+var app = express();
+var server = require('http').createServer(app);
+var io = require('socket.io')(server);
+var redis = require('socket.io-redis');
+var port = process.env.PORT || 3000;
+var crypto = require('crypto');
+var serverName = crypto.randomBytes(3).toString('hex');
+
+io.adapter(redis({ host: 'redis', port: 6379 }));
+
+server.listen(port, function () {
+  console.log('Server listening at port %d', port);
+  console.log('Hello, I\'m %s, how can I help?', serverName);
+});
+
+// Routing
+app.use(express.static(__dirname + '/public'));
+
+// Chatroom
+
+var numUsers = 0;
+
+io.on('connection', function (socket) {
+  socket.emit('my-name-is', serverName);
+
+  var addedUser = false;
+
+  // when the client emits 'new message', this listens and executes
+  socket.on('new message', function (data) {
+    // we tell the client to execute 'new message'
+    socket.broadcast.emit('new message', {
+      username: socket.username,
+      message: data
+    });
+  });
+
+  // when the client emits 'add user', this listens and executes
+  socket.on('add user', function (username) {
+    if (addedUser) return;
+
+    // we store the username in the socket session for this client
+    socket.username = username;
+    ++numUsers;
+    addedUser = true;
+    socket.emit('login', {
+      numUsers: numUsers
+    });
+    // echo globally (all clients) that a person has connected
+    socket.broadcast.emit('user joined', {
+      username: socket.username,
+      numUsers: numUsers
+    });
+  });
+
+  // when the client emits 'typing', we broadcast it to others
+  socket.on('typing', function () {
+    socket.broadcast.emit('typing', {
+      username: socket.username
+    });
+  });
+
+  // when the client emits 'stop typing', we broadcast it to others
+  socket.on('stop typing', function () {
+    socket.broadcast.emit('stop typing', {
+      username: socket.username
+    });
+  });
+
+  // when the user disconnects.. perform this
+  socket.on('disconnect', function () {
+    if (addedUser) {
+      --numUsers;
+
+      // echo globally that this client has left
+      socket.broadcast.emit('user left', {
+        username: socket.username,
+        numUsers: numUsers
+      });
+    }
+  });
+});
--- a/examples/cluster-traefik/server/package.json
+++ b/examples/cluster-traefik/server/package.json
@@ -0,0 +1,17 @@
+{
+  "name": "socket.io-chat",
+  "version": "0.0.0",
+  "description": "A simple chat client using socket.io",
+  "main": "index.js",
+  "author": "Grant Timmerman",
+  "private": true,
+  "license": "MIT",
+  "dependencies": {
+    "express": "4.13.4",
+    "socket.io": "^4.0.0",
+    "socket.io-redis": "^6.0.1"
+  },
+  "scripts": {
+    "start": "node index.js"
+  }
+}
--- a/examples/cluster-traefik/server/public/index.html
+++ b/examples/cluster-traefik/server/public/index.html
@@ -0,0 +1,28 @@
+<!doctype html>
+<html lang="en">
+<head>
+  <meta charset="UTF-8">
+  <title>Socket.IO Chat Example</title>
+  <link rel="stylesheet" href="style.css">
+</head>
+<body>
+  <ul class="pages">
+    <li class="chat page">
+      <div class="chatArea">
+        <ul class="messages"></ul>
+      </div>
+      <input class="inputMessage" placeholder="Type here..."/>
+    </li>
+    <li class="login page">
+      <div class="form">
+        <h3 class="title">What's your nickname?</h3>
+        <input class="usernameInput" type="text" maxlength="14" />
+      </div>
+    </li>
+  </ul>
+
+  <script src="https://code.jquery.com/jquery-1.10.2.min.js"></script>
+  <script src="/socket.io/socket.io.js"></script>
+  <script src="/main.js"></script>
+</body>
+</html>
--- a/examples/cluster-traefik/server/public/main.js
+++ b/examples/cluster-traefik/server/public/main.js
@@ -0,0 +1,286 @@
+$(function() {
+  var FADE_TIME = 150; // ms
+  var TYPING_TIMER_LENGTH = 400; // ms
+  var COLORS = [
+    '#e21400', '#91580f', '#f8a700', '#f78b00',
+    '#58dc00', '#287b00', '#a8f07a', '#4ae8c4',
+    '#3b88eb', '#3824aa', '#a700ff', '#d300e7'
+  ];
+
+  // Initialize variables
+  var $window = $(window);
+  var $usernameInput = $('.usernameInput'); // Input for username
+  var $messages = $('.messages'); // Messages area
+  var $inputMessage = $('.inputMessage'); // Input message input box
+
+  var $loginPage = $('.login.page'); // The login page
+  var $chatPage = $('.chat.page'); // The chatroom page
+
+  // Prompt for setting a username
+  var username;
+  var connected = false;
+  var typing = false;
+  var lastTypingTime;
+  var $currentInput = $usernameInput.focus();
+
+  var socket = io();
+
+  function addParticipantsMessage (data) {
+    var message = '';
+    if (data.numUsers === 1) {
+      message += "there's 1 participant";
+    } else {
+      message += "there are " + data.numUsers + " participants";
+    }
+    log(message);
+  }
+
+  // Sets the client's username
+  function setUsername () {
+    username = cleanInput($usernameInput.val().trim());
+
+    // If the username is valid
+    if (username) {
+      $loginPage.fadeOut();
+      $chatPage.show();
+      $loginPage.off('click');
+      $currentInput = $inputMessage.focus();
+
+      // Tell the server your username
+      socket.emit('add user', username);
+    }
+  }
+
+  // Sends a chat message
+  function sendMessage () {
+    var message = $inputMessage.val();
+    // Prevent markup from being injected into the message
+    message = cleanInput(message);
+    // if there is a non-empty message and a socket connection
+    if (message && connected) {
+      $inputMessage.val('');
+      addChatMessage({
+        username: username,
+        message: message
+      });
+      // tell server to execute 'new message' and send along one parameter
+      socket.emit('new message', message);
+    }
+  }
+
+  // Log a message
+  function log (message, options) {
+    var $el = $('<li>').addClass('log').text(message);
+    addMessageElement($el, options);
+  }
+
+  // Adds the visual chat message to the message list
+  function addChatMessage (data, options) {
+    // Don't fade the message in if there is an 'X was typing'
+    var $typingMessages = getTypingMessages(data);
+    options = options || {};
+    if ($typingMessages.length !== 0) {
+      options.fade = false;
+      $typingMessages.remove();
+    }
+
+    var $usernameDiv = $('<span class="username"/>')
+      .text(data.username)
+      .css('color', getUsernameColor(data.username));
+    var $messageBodyDiv = $('<span class="messageBody">')
+      .text(data.message);
+
+    var typingClass = data.typing ? 'typing' : '';
+    var $messageDiv = $('<li class="message"/>')
+      .data('username', data.username)
+      .addClass(typingClass)
+      .append($usernameDiv, $messageBodyDiv);
+
+    addMessageElement($messageDiv, options);
+  }
+
+  // Adds the visual chat typing message
+  function addChatTyping (data) {
+    data.typing = true;
+    data.message = 'is typing';
+    addChatMessage(data);
+  }
+
+  // Removes the visual chat typing message
+  function removeChatTyping (data) {
+    getTypingMessages(data).fadeOut(function () {
+      $(this).remove();
+    });
+  }
+
+  // Adds a message element to the messages and scrolls to the bottom
+  // el - The element to add as a message
+  // options.fade - If the element should fade-in (default = true)
+  // options.prepend - If the element should prepend
+  //   all other messages (default = false)
+  function addMessageElement (el, options) {
+    var $el = $(el);
+
+    // Setup default options
+    if (!options) {
+      options = {};
+    }
+    if (typeof options.fade === 'undefined') {
+      options.fade = true;
+    }
+    if (typeof options.prepend === 'undefined') {
+      options.prepend = false;
+    }
+
+    // Apply options
+    if (options.fade) {
+      $el.hide().fadeIn(FADE_TIME);
+    }
+    if (options.prepend) {
+      $messages.prepend($el);
+    } else {
+      $messages.append($el);
+    }
+    $messages[0].scrollTop = $messages[0].scrollHeight;
+  }
+
+  // Prevents input from having injected markup
+  function cleanInput (input) {
+    return $('<div/>').text(input).text();
+  }
+
+  // Updates the typing event
+  function updateTyping () {
+    if (connected) {
+      if (!typing) {
+        typing = true;
+        socket.emit('typing');
+      }
+      lastTypingTime = (new Date()).getTime();
+
+      setTimeout(function () {
+        var typingTimer = (new Date()).getTime();
+        var timeDiff = typingTimer - lastTypingTime;
+        if (timeDiff >= TYPING_TIMER_LENGTH && typing) {
+          socket.emit('stop typing');
+          typing = false;
+        }
+      }, TYPING_TIMER_LENGTH);
+    }
+  }
+
+  // Gets the 'X is typing' messages of a user
+  function getTypingMessages (data) {
+    return $('.typing.message').filter(function (i) {
+      return $(this).data('username') === data.username;
+    });
+  }
+
+  // Gets the color of a username through our hash function
+  function getUsernameColor (username) {
+    // Compute hash code
+    var hash = 7;
+    for (var i = 0; i < username.length; i++) {
+       hash = username.charCodeAt(i) + (hash << 5) - hash;
+    }
+    // Calculate color
+    var index = Math.abs(hash % COLORS.length);
+    return COLORS[index];
+  }
+
+  // Keyboard events
+
+  $window.keydown(function (event) {
+    // Auto-focus the current input when a key is typed
+    if (!(event.ctrlKey || event.metaKey || event.altKey)) {
+      $currentInput.focus();
+    }
+    // When the client hits ENTER on their keyboard
+    if (event.which === 13) {
+      if (username) {
+        sendMessage();
+        socket.emit('stop typing');
+        typing = false;
+      } else {
+        setUsername();
+      }
+    }
+  });
+
+  $inputMessage.on('input', function() {
+    updateTyping();
+  });
+
+  // Click events
+
+  // Focus input when clicking anywhere on login page
+  $loginPage.click(function () {
+    $currentInput.focus();
+  });
+
+  // Focus input when clicking on the message input's border
+  $inputMessage.click(function () {
+    $inputMessage.focus();
+  });
+
+  // Socket events
+
+  // Whenever the server emits 'login', log the login message
+  socket.on('login', function (data) {
+    connected = true;
+    // Display the welcome message
+    var message = "Welcome to Socket.IO Chat – ";
+    log(message, {
+      prepend: true
+    });
+    addParticipantsMessage(data);
+  });
+
+  // Whenever the server emits 'new message', update the chat body
+  socket.on('new message', function (data) {
+    addChatMessage(data);
+  });
+
+  // Whenever the server emits 'user joined', log it in the chat body
+  socket.on('user joined', function (data) {
+    log(data.username + ' joined');
+    addParticipantsMessage(data);
+  });
+
+  // Whenever the server emits 'user left', log it in the chat body
+  socket.on('user left', function (data) {
+    log(data.username + ' left');
+    addParticipantsMessage(data);
+    removeChatTyping(data);
+  });
+
+  // Whenever the server emits 'typing', show the typing message
+  socket.on('typing', function (data) {
+    addChatTyping(data);
+  });
+
+  // Whenever the server emits 'stop typing', kill the typing message
+  socket.on('stop typing', function (data) {
+    removeChatTyping(data);
+  });
+
+  socket.on('disconnect', function () {
+    log('you have been disconnected');
+  });
+
+  socket.on('connect', function () {
+    if (username) {
+      log('you have been reconnected');
+      socket.emit('add user', username);
+    }
+  });
+
+  socket.io.on('reconnect_error', function () {
+    log('attempt to reconnect has failed');
+  });
+
+  socket.on('my-name-is', function (serverName) {
+    log('host is now ' + serverName);
+  })
+
+});
--- a/examples/cluster-traefik/server/public/style.css
+++ b/examples/cluster-traefik/server/public/style.css
@@ -0,0 +1,149 @@
+/* Fix user-agent */
+
+* {
+  box-sizing: border-box;
+}
+
+html {
+  font-weight: 300;
+  -webkit-font-smoothing: antialiased;
+}
+
+html, input {
+  font-family:
+    "HelveticaNeue-Light",
+    "Helvetica Neue Light",
+    "Helvetica Neue",
+    Helvetica,
+    Arial,
+    "Lucida Grande",
+    sans-serif;
+}
+
+html, body {
+  height: 100%;
+  margin: 0;
+  padding: 0;
+}
+
+ul {
+  list-style: none;
+  word-wrap: break-word;
+}
+
+/* Pages */
+
+.pages {
+  height: 100%;
+  margin: 0;
+  padding: 0;
+  width: 100%;
+}
+
+.page {
+  height: 100%;
+  position: absolute;
+  width: 100%;
+}
+
+/* Login Page */
+
+.login.page {
+  background-color: #000;
+}
+
+.login.page .form {
+  height: 100px;
+  margin-top: -100px;
+  position: absolute;
+
+  text-align: center;
+  top: 50%;
+  width: 100%;
+}
+
+.login.page .form .usernameInput {
+  background-color: transparent;
+  border: none;
+  border-bottom: 2px solid #fff;
+  outline: none;
+  padding-bottom: 15px;
+  text-align: center;
+  width: 400px;
+}
+
+.login.page .title {
+  font-size: 200%;
+}
+
+.login.page .usernameInput {
+  font-size: 200%;
+  letter-spacing: 3px;
+}
+
+.login.page .title, .login.page .usernameInput {
+  color: #fff;
+  font-weight: 100;
+}
+
+/* Chat page */
+
+.chat.page {
+  display: none;
+}
+
+/* Font */
+
+.messages {
+  font-size: 150%;
+}
+
+.inputMessage {
+  font-size: 100%;
+}
+
+.log {
+  color: gray;
+  font-size: 70%;
+  margin: 5px;
+  text-align: center;
+}
+
+/* Messages */
+
+.chatArea {
+  height: 100%;
+  padding-bottom: 60px;
+}
+
+.messages {
+  height: 100%;
+  margin: 0;
+  overflow-y: scroll;
+  padding: 10px 20px 10px 20px;
+}
+
+.message.typing .messageBody {
+  color: gray;
+}
+
+.username {
+  font-weight: 700;
+  overflow: hidden;
+  padding-right: 15px;
+  text-align: right;
+}
+
+/* Input */
+
+.inputMessage {
+  border: 10px solid #000;
+  bottom: 0;
+  height: 60px;
+  left: 0;
+  outline: none;
+  padding-left: 10px;
+  position: absolute;
+  right: 0;
+  width: 100%;
+}
--- a/examples/cluster-traefik/traefik.yml
+++ b/examples/cluster-traefik/traefik.yml
@@ -0,0 +1,10 @@
+
+api:
+  insecure: true
+
+entryPoints:
+  web:
+    address: ":80"
+
+providers:
+  docker: {}
--- a/examples/create-react-app-example/package.json
+++ b/examples/create-react-app-example/package.json
@@ -9,8 +9,8 @@
    "react": "^16.13.1",
    "react-dom": "^16.13.1",
    "react-scripts": "3.4.1",
-    "socket.io": "^2.3.0",
-    "socket.io-client": "^2.3.0"
+    "socket.io": "4",
+    "socket.io-client": "4"
  },
  "scripts": {
    "start": "react-scripts start",
--- a/examples/create-react-app-example/server.js
+++ b/examples/create-react-app-example/server.js
@@ -1,4 +1,8 @@
-const io = require('socket.io')();
+const io = require('socket.io')({
+  cors: {
+    origin: ['http://localhost:3000']
+  }
+});

 io.on('connection', socket => {
  console.log(`connect: ${socket.id}`);
--- a/examples/create-react-app-example/yarn.lock
+++ b/examples/create-react-app-example/yarn.lock
--- a/examples/custom-parsers/package.json
+++ b/examples/custom-parsers/package.json
@@ -12,10 +12,10 @@
    "component-emitter": "^1.2.1",
    "express": "^4.15.2",
    "schemapack": "^1.4.2",
-    "socket.io": "socketio/socket.io",
-    "socket.io-client": "socketio/socket.io-client",
-    "socket.io-json-parser": "^1.0.0",
-    "socket.io-msgpack-parser": "^1.0.0",
+    "socket.io": "^4.0.0",
+    "socket.io-client": "^4.0.0",
+    "socket.io-json-parser": "^3.0.0",
+    "socket.io-msgpack-parser": "^3.0.1",
    "webpack": "^2.4.1"
  }
 }
--- a/examples/custom-parsers/public/.gitignore
+++ b/examples/custom-parsers/public/.gitignore
@@ -0,0 +1 @@
+*.bundle.js
--- a/examples/custom-parsers/src/custom-parser.js
+++ b/examples/custom-parsers/src/custom-parser.js
@@ -40,12 +40,12 @@ const errorPacket = {
 };

 class Encoder {
-  encode (packet, callback) {
+  encode (packet) {
    switch (packet.type) {
      case TYPES.EVENT:
-        return callback([ this.pack(packet) ]);
+        return [ this.pack(packet) ];
      default:
-        return callback([ JSON.stringify(packet) ]);
+        return [ JSON.stringify(packet) ];
    }
  }
  pack (packet) {
--- a/examples/custom-parsers/src/server.js
+++ b/examples/custom-parsers/src/server.js
@@ -14,15 +14,22 @@ const msgpackParser = require('socket.io-msgpack-parser');
 const jsonParser = require('socket.io-json-parser');
 const customParser = require('./custom-parser');

-let server1 = io(3001, {});
+const cors = {
+  origin: ["http://localhost:3000"]
+}
+
+let server1 = io(3001, { cors });
 let server2 = io(3002, {
-  parser: msgpackParser
+  parser: msgpackParser,
+  cors
 });
 let server3 = io(3003, {
-  parser: jsonParser
+  parser: jsonParser,
+  cors
 });
 let server4 = io(3004, {
-  parser: customParser
+  parser: customParser,
+  cors
 });

 let string = [];
--- a/examples/es-modules/package-lock.json
+++ b/examples/es-modules/package-lock.json
@@ -1,217 +0,0 @@
-{
-  "name": "es-modules",
-  "version": "1.0.0",
-  "lockfileVersion": 1,
-  "requires": true,
-  "dependencies": {
-    "@types/component-emitter": {
-      "version": "1.2.10",
-      "resolved": "https://registry.npmjs.org/@types/component-emitter/-/component-emitter-1.2.10.tgz",
-      "integrity": "sha512-bsjleuRKWmGqajMerkzox19aGbscQX5rmmvvXl3wlIp5gMG1HgkiwPxsN5p070fBDKTNSPgojVbuY1+HWMbFhg=="
-    },
-    "accepts": {
-      "version": "1.3.7",
-      "resolved": "https://registry.npmjs.org/accepts/-/accepts-1.3.7.tgz",
-      "integrity": "sha512-Il80Qs2WjYlJIBNzNkK6KYqlVMTbZLXgHx2oT0pU/fjRHyEp+PEfEPY0R3WCwAGVOtauxh1hOxNgIf5bv7dQpA==",
-      "requires": {
-        "mime-types": "~2.1.24",
-        "negotiator": "0.6.2"
-      }
-    },
-    "backo2": {
-      "version": "1.0.2",
-      "resolved": "https://registry.npmjs.org/backo2/-/backo2-1.0.2.tgz",
-      "integrity": "sha1-MasayLEpNjRj41s+u2n038+6eUc="
-    },
-    "base64-arraybuffer": {
-      "version": "0.1.4",
-      "resolved": "https://registry.npmjs.org/base64-arraybuffer/-/base64-arraybuffer-0.1.4.tgz",
-      "integrity": "sha1-mBjHngWbE1X5fgQooBfIOOkLqBI="
-    },
-    "base64id": {
-      "version": "2.0.0",
-      "resolved": "https://registry.npmjs.org/base64id/-/base64id-2.0.0.tgz",
-      "integrity": "sha512-lGe34o6EHj9y3Kts9R4ZYs/Gr+6N7MCaMlIFA3F1R2O5/m7K06AxfSeO5530PEERE6/WyEg3lsuyw4GHlPZHog=="
-    },
-    "component-bind": {
-      "version": "1.0.0",
-      "resolved": "https://registry.npmjs.org/component-bind/-/component-bind-1.0.0.tgz",
-      "integrity": "sha1-AMYIq33Nk4l8AAllGx06jh5zu9E="
-    },
-    "component-emitter": {
-      "version": "1.3.0",
-      "resolved": "https://registry.npmjs.org/component-emitter/-/component-emitter-1.3.0.tgz",
-      "integrity": "sha512-Rd3se6QB+sO1TwqZjscQrurpEPIfO0/yYnSin6Q/rD3mOutHvUrCAhJub3r90uNb+SESBuE0QYoB90YdfatsRg=="
-    },
-    "cookie": {
-      "version": "0.4.1",
-      "resolved": "https://registry.npmjs.org/cookie/-/cookie-0.4.1.tgz",
-      "integrity": "sha512-ZwrFkGJxUR3EIoXtO+yVE69Eb7KlixbaeAWfBQB9vVsNn/o+Yw69gBWSSDK825hQNdN+wF8zELf3dFNl/kxkUA=="
-    },
-    "cors": {
-      "version": "2.8.5",
-      "resolved": "https://registry.npmjs.org/cors/-/cors-2.8.5.tgz",
-      "integrity": "sha512-KIHbLJqu73RGr/hnbrO9uBeixNGuvSQjul/jdFvS/KFSIH1hWVd1ng7zOHx+YrEfInLG7q4n6GHQ9cDtxv/P6g==",
-      "requires": {
-        "object-assign": "^4",
-        "vary": "^1"
-      }
-    },
-    "debug": {
-      "version": "4.1.1",
-      "resolved": "https://registry.npmjs.org/debug/-/debug-4.1.1.tgz",
-      "integrity": "sha512-pYAIzeRo8J6KPEaJ0VWOh5Pzkbw/RetuzehGM7QRRX5he4fPHx2rdKMB256ehJCkX+XRQm16eZLqLNS8RSZXZw==",
-      "requires": {
-        "ms": "^2.1.1"
-      }
-    },
-    "engine.io": {
-      "version": "4.0.1",
-      "resolved": "https://registry.npmjs.org/engine.io/-/engine.io-4.0.1.tgz",
-      "integrity": "sha512-6EaSBxasBUwxRdf6B68SEYpD3tcrG80J4YTzHl/D+9Q+vM0AMHZabfYcc2WdnvEaQxZjX/UZsa+UdGoM0qQQkQ==",
-      "requires": {
-        "accepts": "~1.3.4",
-        "base64id": "2.0.0",
-        "cookie": "~0.4.1",
-        "cors": "~2.8.5",
-        "debug": "~4.1.0",
-        "engine.io-parser": "~4.0.0",
-        "ws": "^7.1.2"
-      }
-    },
-    "engine.io-client": {
-      "version": "4.0.1",
-      "resolved": "https://registry.npmjs.org/engine.io-client/-/engine.io-client-4.0.1.tgz",
-      "integrity": "sha512-3XXfWrEutlf1vg5PlS805bD+AgZXhRIKYAG04f1iCGOs70dWEYlZGfCZUNwPwNx05lBCKs1lIeL3SkLB0P++xw==",
-      "requires": {
-        "base64-arraybuffer": "0.1.4",
-        "component-emitter": "~1.3.0",
-        "debug": "~4.1.0",
-        "engine.io-parser": "~4.0.1",
-        "has-cors": "1.1.0",
-        "parseqs": "0.0.6",
-        "parseuri": "0.0.6",
-        "ws": "~7.2.1",
-        "xmlhttprequest-ssl": "~1.5.4",
-        "yeast": "0.1.2"
-      },
-      "dependencies": {
-        "ws": {
-          "version": "7.2.5",
-          "resolved": "https://registry.npmjs.org/ws/-/ws-7.2.5.tgz",
-          "integrity": "sha512-C34cIU4+DB2vMyAbmEKossWq2ZQDr6QEyuuCzWrM9zfw1sGc0mYiJ0UnG9zzNykt49C2Fi34hvr2vssFQRS6EA=="
-        }
-      }
-    },
-    "engine.io-parser": {
-      "version": "4.0.1",
-      "resolved": "https://registry.npmjs.org/engine.io-parser/-/engine.io-parser-4.0.1.tgz",
-      "integrity": "sha512-v5aZK1hlckcJDGmHz3W8xvI3NUHYc9t8QtTbqdR5OaH3S9iJZilPubauOm+vLWOMMWzpE3hiq92l9lTAHamRCg=="
-    },
-    "has-cors": {
-      "version": "1.1.0",
-      "resolved": "https://registry.npmjs.org/has-cors/-/has-cors-1.1.0.tgz",
-      "integrity": "sha1-XkdHk/fqmEPRu5nCPu9J/xJv/zk="
-    },
-    "mime-db": {
-      "version": "1.44.0",
-      "resolved": "https://registry.npmjs.org/mime-db/-/mime-db-1.44.0.tgz",
-      "integrity": "sha512-/NOTfLrsPBVeH7YtFPgsVWveuL+4SjjYxaQ1xtM1KMFj7HdxlBlxeyNLzhyJVx7r4rZGJAZ/6lkKCitSc/Nmpg=="
-    },
-    "mime-types": {
-      "version": "2.1.27",
-      "resolved": "https://registry.npmjs.org/mime-types/-/mime-types-2.1.27.tgz",
-      "integrity": "sha512-JIhqnCasI9yD+SsmkquHBxTSEuZdQX5BuQnS2Vc7puQQQ+8yiP5AY5uWhpdv4YL4VM5c6iliiYWPgJ/nJQLp7w==",
-      "requires": {
-        "mime-db": "1.44.0"
-      }
-    },
-    "ms": {
-      "version": "2.1.2",
-      "resolved": "https://registry.npmjs.org/ms/-/ms-2.1.2.tgz",
-      "integrity": "sha512-sGkPx+VjMtmA6MX27oA4FBFELFCZZ4S4XqeGOXCv68tT+jb3vk/RyaKWP0PTKyWtmLSM0b+adUTEvbs1PEaH2w=="
-    },
-    "negotiator": {
-      "version": "0.6.2",
-      "resolved": "https://registry.npmjs.org/negotiator/-/negotiator-0.6.2.tgz",
-      "integrity": "sha512-hZXc7K2e+PgeI1eDBe/10Ard4ekbfrrqG8Ep+8Jmf4JID2bNg7NvCPOZN+kfF574pFQI7mum2AUqDidoKqcTOw=="
-    },
-    "object-assign": {
-      "version": "4.1.1",
-      "resolved": "https://registry.npmjs.org/object-assign/-/object-assign-4.1.1.tgz",
-      "integrity": "sha1-IQmtx5ZYh8/AXLvUQsrIv7s2CGM="
-    },
-    "parseqs": {
-      "version": "0.0.6",
-      "resolved": "https://registry.npmjs.org/parseqs/-/parseqs-0.0.6.tgz",
-      "integrity": "sha512-jeAGzMDbfSHHA091hr0r31eYfTig+29g3GKKE/PPbEQ65X0lmMwlEoqmhzu0iztID5uJpZsFlUPDP8ThPL7M8w=="
-    },
-    "parseuri": {
-      "version": "0.0.6",
-      "resolved": "https://registry.npmjs.org/parseuri/-/parseuri-0.0.6.tgz",
-      "integrity": "sha512-AUjen8sAkGgao7UyCX6Ahv0gIK2fABKmYjvP4xmy5JaKvcbTRueIqIPHLAfq30xJddqSE033IOMUSOMCcK3Sow=="
-    },
-    "socket.io": {
-      "version": "3.0.0",
-      "resolved": "https://registry.npmjs.org/socket.io/-/socket.io-3.0.0.tgz",
-      "integrity": "sha512-arLQtd+UoJ08NXBRBGUJDyQ9B+cc9WwD67hc5s1WQcs2DyAkYzI5HWg4U0CrFtK00kjyAWxBGhLwVbfOeMqz1A==",
-      "requires": {
-        "accepts": "~1.3.4",
-        "base64id": "~2.0.0",
-        "debug": "~4.1.0",
-        "engine.io": "~4.0.0",
-        "socket.io-adapter": "~2.0.3",
-        "socket.io-parser": "~4.0.1"
-      }
-    },
-    "socket.io-adapter": {
-      "version": "2.0.3",
-      "resolved": "https://registry.npmjs.org/socket.io-adapter/-/socket.io-adapter-2.0.3.tgz",
-      "integrity": "sha512-2wo4EXgxOGSFueqvHAdnmi5JLZzWqMArjuP4nqC26AtLh5PoCPsaRbRdah2xhcwTAMooZfjYiNVNkkmmSMaxOQ=="
-    },
-    "socket.io-client": {
-      "version": "3.0.0",
-      "resolved": "https://registry.npmjs.org/socket.io-client/-/socket.io-client-3.0.0.tgz",
-      "integrity": "sha512-NreA86JJSMMSyPomkpyWL+FeKB3wmsWbGbNvtoxPbodV7dEIWxwyOklYfhuTpJJDCpEFd55vET2/ZyYwJ66Bfg==",
-      "requires": {
-        "@types/component-emitter": "^1.2.10",
-        "backo2": "1.0.2",
-        "component-bind": "1.0.0",
-        "component-emitter": "~1.3.0",
-        "debug": "~4.1.0",
-        "engine.io-client": "~4.0.0",
-        "parseuri": "0.0.6",
-        "socket.io-parser": "~4.0.1"
-      }
-    },
-    "socket.io-parser": {
-      "version": "4.0.1",
-      "resolved": "https://registry.npmjs.org/socket.io-parser/-/socket.io-parser-4.0.1.tgz",
-      "integrity": "sha512-5JfNykYptCwU2lkOI0ieoePWm+6stEhkZ2UnLDjqnE1YEjUlXXLd1lpxPZ+g+h3rtaytwWkWrLQCaJULlGqjOg==",
-      "requires": {
-        "component-emitter": "~1.3.0",
-        "debug": "~4.1.0"
-      }
-    },
-    "vary": {
-      "version": "1.1.2",
-      "resolved": "https://registry.npmjs.org/vary/-/vary-1.1.2.tgz",
-      "integrity": "sha1-IpnwLG3tMNSllhsLn3RSShj2NPw="
-    },
-    "ws": {
-      "version": "7.4.0",
-      "resolved": "https://registry.npmjs.org/ws/-/ws-7.4.0.tgz",
-      "integrity": "sha512-kyFwXuV/5ymf+IXhS6f0+eAFvydbaBW3zjpT6hUdAh/hbVjTIB5EHBGi0bPoCLSK2wcuz3BrEkB9LrYv1Nm4NQ=="
-    },
-    "xmlhttprequest-ssl": {
-      "version": "1.5.5",
-      "resolved": "https://registry.npmjs.org/xmlhttprequest-ssl/-/xmlhttprequest-ssl-1.5.5.tgz",
-      "integrity": "sha1-wodrBhaKrcQOV9l+gRkayPQ5iz4="
-    },
-    "yeast": {
-      "version": "0.1.2",
-      "resolved": "https://registry.npmjs.org/yeast/-/yeast-0.1.2.tgz",
-      "integrity": "sha1-AI4G2AlDIMNy28L47XagymyKxBk="
-    }
-  }
-}
--- a/examples/es-modules/package.json
+++ b/examples/es-modules/package.json
@@ -9,7 +9,7 @@
    "node": ">=12.17.0"
  },
  "dependencies": {
-    "socket.io": "^3.0.0",
-    "socket.io-client": "^3.0.0"
+    "socket.io": "^4.0.0",
+    "socket.io-client": "^4.0.0"
  }
 }
--- a/examples/passport-example/index.js
+++ b/examples/passport-example/index.js
@@ -51,9 +51,9 @@ app.post(
 app.post("/logout", (req, res) => {
  console.log(`logout ${req.session.id}`);
  const socketId = req.session.socketId;
-  if (socketId && io.of("/").connected[socketId]) {
+  if (socketId && io.of("/").sockets.get(socketId)) {
    console.log(`forcefully closing socket ${socketId}`);
-    io.sockets.connected[socketId].disconnect(true);
+    io.of("/").sockets.get(socketId).disconnect(true);
  }
  req.logout();
  res.cookie("connect.sid", "", { expires: new Date() });
--- a/examples/passport-example/package-lock.json
+++ b/examples/passport-example/package-lock.json
@@ -1,783 +0,0 @@
-{
-  "name": "passport-example",
-  "version": "0.0.1",
-  "lockfileVersion": 1,
-  "requires": true,
-  "dependencies": {
-    "accepts": {
-      "version": "1.3.7",
-      "resolved": "https://registry.npmjs.org/accepts/-/accepts-1.3.7.tgz",
-      "integrity": "sha512-Il80Qs2WjYlJIBNzNkK6KYqlVMTbZLXgHx2oT0pU/fjRHyEp+PEfEPY0R3WCwAGVOtauxh1hOxNgIf5bv7dQpA==",
-      "requires": {
-        "mime-types": "~2.1.24",
-        "negotiator": "0.6.2"
-      }
-    },
-    "after": {
-      "version": "0.8.2",
-      "resolved": "https://registry.npmjs.org/after/-/after-0.8.2.tgz",
-      "integrity": "sha1-/ts5T58OAqqXaOcCvaI7UF+ufh8="
-    },
-    "array-flatten": {
-      "version": "1.1.1",
-      "resolved": "https://registry.npmjs.org/array-flatten/-/array-flatten-1.1.1.tgz",
-      "integrity": "sha1-ml9pkFGx5wczKPKgCJaLZOopVdI="
-    },
-    "arraybuffer.slice": {
-      "version": "0.0.7",
-      "resolved": "https://registry.npmjs.org/arraybuffer.slice/-/arraybuffer.slice-0.0.7.tgz",
-      "integrity": "sha512-wGUIVQXuehL5TCqQun8OW81jGzAWycqzFF8lFp+GOM5BXLYj3bKNsYC4daB7n6XjCqxQA/qgTJ+8ANR3acjrog=="
-    },
-    "async-limiter": {
-      "version": "1.0.1",
-      "resolved": "https://registry.npmjs.org/async-limiter/-/async-limiter-1.0.1.tgz",
-      "integrity": "sha512-csOlWGAcRFJaI6m+F2WKdnMKr4HhdhFVBk0H/QbJFMCr+uO2kwohwXQPxw/9OCxp05r5ghVBFSyioixx3gfkNQ=="
-    },
-    "backo2": {
-      "version": "1.0.2",
-      "resolved": "https://registry.npmjs.org/backo2/-/backo2-1.0.2.tgz",
-      "integrity": "sha1-MasayLEpNjRj41s+u2n038+6eUc="
-    },
-    "base64-arraybuffer": {
-      "version": "0.1.5",
-      "resolved": "https://registry.npmjs.org/base64-arraybuffer/-/base64-arraybuffer-0.1.5.tgz",
-      "integrity": "sha1-c5JncZI7Whl0etZmqlzUv5xunOg="
-    },
-    "base64id": {
-      "version": "2.0.0",
-      "resolved": "https://registry.npmjs.org/base64id/-/base64id-2.0.0.tgz",
-      "integrity": "sha512-lGe34o6EHj9y3Kts9R4ZYs/Gr+6N7MCaMlIFA3F1R2O5/m7K06AxfSeO5530PEERE6/WyEg3lsuyw4GHlPZHog=="
-    },
-    "better-assert": {
-      "version": "1.0.2",
-      "resolved": "https://registry.npmjs.org/better-assert/-/better-assert-1.0.2.tgz",
-      "integrity": "sha1-QIZrnhueC1W0gYlDEeaPr/rrxSI=",
-      "requires": {
-        "callsite": "1.0.0"
-      }
-    },
-    "blob": {
-      "version": "0.0.5",
-      "resolved": "https://registry.npmjs.org/blob/-/blob-0.0.5.tgz",
-      "integrity": "sha512-gaqbzQPqOoamawKg0LGVd7SzLgXS+JH61oWprSLH+P+abTczqJbhTR8CmJ2u9/bUYNmHTGJx/UEmn6doAvvuig=="
-    },
-    "body-parser": {
-      "version": "1.19.0",
-      "resolved": "https://registry.npmjs.org/body-parser/-/body-parser-1.19.0.tgz",
-      "integrity": "sha512-dhEPs72UPbDnAQJ9ZKMNTP6ptJaionhP5cBb541nXPlW60Jepo9RV/a4fX4XWW9CuFNK22krhrj1+rgzifNCsw==",
-      "requires": {
-        "bytes": "3.1.0",
-        "content-type": "~1.0.4",
-        "debug": "2.6.9",
-        "depd": "~1.1.2",
-        "http-errors": "1.7.2",
-        "iconv-lite": "0.4.24",
-        "on-finished": "~2.3.0",
-        "qs": "6.7.0",
-        "raw-body": "2.4.0",
-        "type-is": "~1.6.17"
-      }
-    },
-    "bytes": {
-      "version": "3.1.0",
-      "resolved": "https://registry.npmjs.org/bytes/-/bytes-3.1.0.tgz",
-      "integrity": "sha512-zauLjrfCG+xvoyaqLoV8bLVXXNGC4JqlxFCutSDWA6fJrTo2ZuvLYTqZ7aHBLZSMOopbzwv8f+wZcVzfVTI2Dg=="
-    },
-    "callsite": {
-      "version": "1.0.0",
-      "resolved": "https://registry.npmjs.org/callsite/-/callsite-1.0.0.tgz",
-      "integrity": "sha1-KAOY5dZkvXQDi28JBRU+borxvCA="
-    },
-    "component-bind": {
-      "version": "1.0.0",
-      "resolved": "https://registry.npmjs.org/component-bind/-/component-bind-1.0.0.tgz",
-      "integrity": "sha1-AMYIq33Nk4l8AAllGx06jh5zu9E="
-    },
-    "component-emitter": {
-      "version": "1.2.1",
-      "resolved": "https://registry.npmjs.org/component-emitter/-/component-emitter-1.2.1.tgz",
-      "integrity": "sha1-E3kY1teCg/ffemt8WmPhQOaUJeY="
-    },
-    "component-inherit": {
-      "version": "0.0.3",
-      "resolved": "https://registry.npmjs.org/component-inherit/-/component-inherit-0.0.3.tgz",
-      "integrity": "sha1-ZF/ErfWLcrZJ1crmUTVhnbJv8UM="
-    },
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--- a/examples/passport-example/package.json
+++ b/examples/passport-example/package.json
@@ -8,7 +8,7 @@
    "express-session": "~1.17.1",
    "passport": "~0.4.1",
    "passport-local": "~1.0.0",
-    "socket.io": "~2.3.0"
+    "socket.io": "^4.0.0"
  },
  "scripts": {
    "start": "node index.js"
--- a/examples/private-messaging/.gitignore
+++ b/examples/private-messaging/.gitignore
@@ -0,0 +1,24 @@
+.DS_Store
+node_modules
+/dist
+
+
+# local env files
+.env.local
+.env.*.local
+
+# Log files
+npm-debug.log*
+yarn-debug.log*
+yarn-error.log*
+pnpm-debug.log*
+
+# Editor directories and files
+.idea
+.vscode
+*.suo
+*.ntvs*
+*.njsproj
+*.sln
+*.sw?
+package-lock.json
--- a/examples/private-messaging/README.md
+++ b/examples/private-messaging/README.md
@@ -0,0 +1,23 @@
+# Private messaging with Socket.IO
+
+Please read the related guide:
+
+- [Part I](https://socket.io/get-started/private-messaging-part-1/): initial implementation
+- [Part II](https://socket.io/get-started/private-messaging-part-2/): persistent user ID
+- [Part III](https://socket.io/get-started/private-messaging-part-3/): persistent messages
+- [Part IV](https://socket.io/get-started/private-messaging-part-4/): scaling up
+
+## Running the frontend
+
+```
+npm install
+npm run serve
+```
+
+### Running the server
+
+```
+cd server
+npm install
+npm start
+```
--- a/examples/private-messaging/babel.config.js
+++ b/examples/private-messaging/babel.config.js
@@ -0,0 +1,5 @@
+module.exports = {
+  presets: [
+    '@vue/cli-plugin-babel/preset'
+  ]
+}
--- a/examples/private-messaging/package.json
+++ b/examples/private-messaging/package.json
@@ -0,0 +1,43 @@
+{
+  "name": "private-messaging",
+  "version": "0.1.0",
+  "private": true,
+  "scripts": {
+    "serve": "vue-cli-service serve",
+    "build": "vue-cli-service build",
+    "lint": "vue-cli-service lint"
+  },
+  "dependencies": {
+    "core-js": "^3.6.5",
+    "socket.io-client": "^4.0.0",
+    "vue": "^2.6.11"
+  },
+  "devDependencies": {
+    "@vue/cli-plugin-babel": "~4.5.0",
+    "@vue/cli-plugin-eslint": "~4.5.0",
+    "@vue/cli-service": "~4.5.0",
+    "babel-eslint": "^10.1.0",
+    "eslint": "^6.7.2",
+    "eslint-plugin-vue": "^6.2.2",
+    "vue-template-compiler": "^2.6.11"
+  },
+  "eslintConfig": {
+    "root": true,
+    "env": {
+      "node": true
+    },
+    "extends": [
+      "plugin:vue/essential",
+      "eslint:recommended"
+    ],
+    "parserOptions": {
+      "parser": "babel-eslint"
+    },
+    "rules": {}
+  },
+  "browserslist": [
+    "> 1%",
+    "last 2 versions",
+    "not dead"
+  ]
+}
--- a/examples/private-messaging/public/favicon.ico
+++ b/examples/private-messaging/public/favicon.ico
--- a/examples/private-messaging/public/fonts/Lato-Regular.ttf
+++ b/examples/private-messaging/public/fonts/Lato-Regular.ttf
--- a/examples/private-messaging/public/index.html
+++ b/examples/private-messaging/public/index.html
@@ -0,0 +1,17 @@
+<!DOCTYPE html>
+<html lang="">
+  <head>
+    <meta charset="utf-8">
+    <meta http-equiv="X-UA-Compatible" content="IE=edge">
+    <meta name="viewport" content="width=device-width,initial-scale=1.0">
+    <link rel="icon" href="<%= BASE_URL %>favicon.ico">
+    <title>Private messaging with Socket.IO</title>
+  </head>
+  <body>
+    <noscript>
+      <strong>We're sorry but this application doesn't work properly without JavaScript enabled. Please enable it to continue.</strong>
+    </noscript>
+    <div id="app"></div>
+    <!-- built files will be auto injected -->
+  </body>
+</html>
--- a/examples/private-messaging/server/cluster.js
+++ b/examples/private-messaging/server/cluster.js
@@ -0,0 +1,31 @@
+const cluster = require("cluster");
+const http = require("http");
+const { setupMaster } = require("@socket.io/sticky");
+
+const WORKERS_COUNT = 4;
+
+if (cluster.isMaster) {
+  console.log(`Master ${process.pid} is running`);
+
+  for (let i = 0; i < WORKERS_COUNT; i++) {
+    cluster.fork();
+  }
+
+  cluster.on("exit", (worker) => {
+    console.log(`Worker ${worker.process.pid} died`);
+    cluster.fork();
+  });
+
+  const httpServer = http.createServer();
+  setupMaster(httpServer, {
+    loadBalancingMethod: "least-connection", // either "random", "round-robin" or "least-connection"
+  });
+  const PORT = process.env.PORT || 3000;
+
+  httpServer.listen(PORT, () =>
+    console.log(`server listening at http://localhost:${PORT}`)
+  );
+} else {
+  console.log(`Worker ${process.pid} started`);
+  require("./index");
+}
--- a/examples/private-messaging/server/docker-compose.yml
+++ b/examples/private-messaging/server/docker-compose.yml
@@ -0,0 +1,7 @@
+version: "3"
+
+services:
+  redis:
+    image: redis:5
+    ports:
+      - "6379:6379"
--- a/examples/private-messaging/server/index.js
+++ b/examples/private-messaging/server/index.js
@@ -0,0 +1,125 @@
+const httpServer = require("http").createServer();
+const Redis = require("ioredis");
+const redisClient = new Redis();
+const io = require("socket.io")(httpServer, {
+  cors: {
+    origin: "http://localhost:8080",
+  },
+  adapter: require("socket.io-redis")({
+    pubClient: redisClient,
+    subClient: redisClient.duplicate(),
+  }),
+});
+
+const { setupWorker } = require("@socket.io/sticky");
+const crypto = require("crypto");
+const randomId = () => crypto.randomBytes(8).toString("hex");
+
+const { RedisSessionStore } = require("./sessionStore");
+const sessionStore = new RedisSessionStore(redisClient);
+
+const { RedisMessageStore } = require("./messageStore");
+const messageStore = new RedisMessageStore(redisClient);
+
+io.use(async (socket, next) => {
+  const sessionID = socket.handshake.auth.sessionID;
+  if (sessionID) {
+    const session = await sessionStore.findSession(sessionID);
+    if (session) {
+      socket.sessionID = sessionID;
+      socket.userID = session.userID;
+      socket.username = session.username;
+      return next();
+    }
+  }
+  const username = socket.handshake.auth.username;
+  if (!username) {
+    return next(new Error("invalid username"));
+  }
+  socket.sessionID = randomId();
+  socket.userID = randomId();
+  socket.username = username;
+  next();
+});
+
+io.on("connection", async (socket) => {
+  // persist session
+  sessionStore.saveSession(socket.sessionID, {
+    userID: socket.userID,
+    username: socket.username,
+    connected: true,
+  });
+
+  // emit session details
+  socket.emit("session", {
+    sessionID: socket.sessionID,
+    userID: socket.userID,
+  });
+
+  // join the "userID" room
+  socket.join(socket.userID);
+
+  // fetch existing users
+  const users = [];
+  const [messages, sessions] = await Promise.all([
+    messageStore.findMessagesForUser(socket.userID),
+    sessionStore.findAllSessions(),
+  ]);
+  const messagesPerUser = new Map();
+  messages.forEach((message) => {
+    const { from, to } = message;
+    const otherUser = socket.userID === from ? to : from;
+    if (messagesPerUser.has(otherUser)) {
+      messagesPerUser.get(otherUser).push(message);
+    } else {
+      messagesPerUser.set(otherUser, [message]);
+    }
+  });
+
+  sessions.forEach((session) => {
+    users.push({
+      userID: session.userID,
+      username: session.username,
+      connected: session.connected,
+      messages: messagesPerUser.get(session.userID) || [],
+    });
+  });
+  socket.emit("users", users);
+
+  // notify existing users
+  socket.broadcast.emit("user connected", {
+    userID: socket.userID,
+    username: socket.username,
+    connected: true,
+    messages: [],
+  });
+
+  // forward the private message to the right recipient (and to other tabs of the sender)
+  socket.on("private message", ({ content, to }) => {
+    const message = {
+      content,
+      from: socket.userID,
+      to,
+    };
+    socket.to(to).to(socket.userID).emit("private message", message);
+    messageStore.saveMessage(message);
+  });
+
+  // notify users upon disconnection
+  socket.on("disconnect", async () => {
+    const matchingSockets = await io.in(socket.userID).allSockets();
+    const isDisconnected = matchingSockets.size === 0;
+    if (isDisconnected) {
+      // notify other users
+      socket.broadcast.emit("user disconnected", socket.userID);
+      // update the connection status of the session
+      sessionStore.saveSession(socket.sessionID, {
+        userID: socket.userID,
+        username: socket.username,
+        connected: false,
+      });
+    }
+  });
+});
+
+setupWorker(io);
--- a/examples/private-messaging/server/messageStore.js
+++ b/examples/private-messaging/server/messageStore.js
@@ -0,0 +1,54 @@
+/* abstract */ class MessageStore {
+  saveMessage(message) {}
+  findMessagesForUser(userID) {}
+}
+
+class InMemoryMessageStore extends MessageStore {
+  constructor() {
+    super();
+    this.messages = [];
+  }
+
+  saveMessage(message) {
+    this.messages.push(message);
+  }
+
+  findMessagesForUser(userID) {
+    return this.messages.filter(
+      ({ from, to }) => from === userID || to === userID
+    );
+  }
+}
+
+const CONVERSATION_TTL = 24 * 60 * 60;
+
+class RedisMessageStore extends MessageStore {
+  constructor(redisClient) {
+    super();
+    this.redisClient = redisClient;
+  }
+
+  saveMessage(message) {
+    const value = JSON.stringify(message);
+    this.redisClient
+      .multi()
+      .rpush(`messages:${message.from}`, value)
+      .rpush(`messages:${message.to}`, value)
+      .expire(`messages:${message.from}`, CONVERSATION_TTL)
+      .expire(`messages:${message.to}`, CONVERSATION_TTL)
+      .exec();
+  }
+
+  findMessagesForUser(userID) {
+    return this.redisClient
+      .lrange(`messages:${userID}`, 0, -1)
+      .then((results) => {
+        return results.map((result) => JSON.parse(result));
+      });
+  }
+}
+
+module.exports = {
+  InMemoryMessageStore,
+  RedisMessageStore,
+};
--- a/examples/private-messaging/server/package.json
+++ b/examples/private-messaging/server/package.json
@@ -0,0 +1,17 @@
+{
+  "name": "server",
+  "version": "1.0.0",
+  "description": "",
+  "main": "index.js",
+  "scripts": {
+    "start": "node cluster.js"
+  },
+  "author": "Damien Arrachequesne <damien.arrachequesne@gmail.com>",
+  "license": "MIT",
+  "dependencies": {
+    "@socket.io/sticky": "^1.0.0",
+    "ioredis": "^4.22.0",
+    "socket.io": "^4.0.0",
+    "socket.io-redis": "^6.0.1"
+  }
+}
--- a/examples/private-messaging/server/sessionStore.js
+++ b/examples/private-messaging/server/sessionStore.js
@@ -0,0 +1,89 @@
+/* abstract */ class SessionStore {
+  findSession(id) {}
+  saveSession(id, session) {}
+  findAllSessions() {}
+}
+
+class InMemorySessionStore extends SessionStore {
+  constructor() {
+    super();
+    this.sessions = new Map();
+  }
+
+  findSession(id) {
+    return this.sessions.get(id);
+  }
+
+  saveSession(id, session) {
+    this.sessions.set(id, session);
+  }
+
+  findAllSessions() {
+    return [...this.sessions.values()];
+  }
+}
+
+const SESSION_TTL = 24 * 60 * 60;
+const mapSession = ([userID, username, connected]) =>
+  userID ? { userID, username, connected: connected === "true" } : undefined;
+
+class RedisSessionStore extends SessionStore {
+  constructor(redisClient) {
+    super();
+    this.redisClient = redisClient;
+  }
+
+  findSession(id) {
+    return this.redisClient
+      .hmget(`session:${id}`, "userID", "username", "connected")
+      .then(mapSession);
+  }
+
+  saveSession(id, { userID, username, connected }) {
+    this.redisClient
+      .multi()
+      .hset(
+        `session:${id}`,
+        "userID",
+        userID,
+        "username",
+        username,
+        "connected",
+        connected
+      )
+      .expire(`session:${id}`, SESSION_TTL)
+      .exec();
+  }
+
+  async findAllSessions() {
+    const keys = new Set();
+    let nextIndex = 0;
+    do {
+      const [nextIndexAsStr, results] = await this.redisClient.scan(
+        nextIndex,
+        "MATCH",
+        "session:*",
+        "COUNT",
+        "100"
+      );
+      nextIndex = parseInt(nextIndexAsStr, 10);
+      results.forEach((s) => keys.add(s));
+    } while (nextIndex !== 0);
+    const commands = [];
+    keys.forEach((key) => {
+      commands.push(["hmget", key, "userID", "username", "connected"]);
+    });
+    return this.redisClient
+      .multi(commands)
+      .exec()
+      .then((results) => {
+        return results
+          .map(([err, session]) => (err ? undefined : mapSession(session)))
+          .filter((v) => !!v);
+      });
+  }
+}
+module.exports = {
+  InMemorySessionStore,
+  RedisSessionStore,
+};
--- a/examples/private-messaging/src/App.vue
+++ b/examples/private-messaging/src/App.vue
@@ -0,0 +1,78 @@
+<template>
+  <div id="app">
+    <select-username
+      v-if="!usernameAlreadySelected"
+      @input="onUsernameSelection"
+    />
+    <chat v-else />
+  </div>
+</template>
+
+<script>
+import SelectUsername from "./components/SelectUsername";
+import Chat from "./components/Chat";
+import socket from "./socket";
+
+export default {
+  name: "App",
+  components: {
+    Chat,
+    SelectUsername,
+  },
+  data() {
+    return {
+      usernameAlreadySelected: false,
+    };
+  },
+  methods: {
+    onUsernameSelection(username) {
+      this.usernameAlreadySelected = true;
+      socket.auth = { username };
+      socket.connect();
+    },
+  },
+  created() {
+    const sessionID = localStorage.getItem("sessionID");
+
+    if (sessionID) {
+      this.usernameAlreadySelected = true;
+      socket.auth = { sessionID };
+      socket.connect();
+    }
+
+    socket.on("session", ({ sessionID, userID }) => {
+      // attach the session ID to the next reconnection attempts
+      socket.auth = { sessionID };
+      // store it in the localStorage
+      localStorage.setItem("sessionID", sessionID);
+      // save the ID of the user
+      socket.userID = userID;
+    });
+
+    socket.on("connect_error", (err) => {
+      if (err.message === "invalid username") {
+        this.usernameAlreadySelected = false;
+      }
+    });
+  },
+  destroyed() {
+    socket.off("connect_error");
+  },
+};
+</script>
+
+<style>
+body {
+  margin: 0;
+}
+
+@font-face {
+  font-family: Lato;
+  src: url("/fonts/Lato-Regular.ttf");
+}
+
+#app {
+  font-family: Lato, Arial, sans-serif;
+  font-size: 14px;
+}
+</style>
--- a/examples/private-messaging/src/components/Chat.vue
+++ b/examples/private-messaging/src/components/Chat.vue
@@ -0,0 +1,165 @@
+<template>
+  <div>
+    <div class="left-panel">
+      <user
+        v-for="user in users"
+        :key="user.userID"
+        :user="user"
+        :selected="selectedUser === user"
+        @select="onSelectUser(user)"
+      />
+    </div>
+    <message-panel
+      v-if="selectedUser"
+      :user="selectedUser"
+      @input="onMessage"
+      class="right-panel"
+    />
+  </div>
+</template>
+
+<script>
+import socket from "../socket";
+import User from "./User";
+import MessagePanel from "./MessagePanel";
+
+export default {
+  name: "Chat",
+  components: { User, MessagePanel },
+  data() {
+    return {
+      selectedUser: null,
+      users: [],
+    };
+  },
+  methods: {
+    onMessage(content) {
+      if (this.selectedUser) {
+        socket.emit("private message", {
+          content,
+          to: this.selectedUser.userID,
+        });
+        this.selectedUser.messages.push({
+          content,
+          fromSelf: true,
+        });
+      }
+    },
+    onSelectUser(user) {
+      this.selectedUser = user;
+      user.hasNewMessages = false;
+    },
+  },
+  created() {
+    socket.on("connect", () => {
+      this.users.forEach((user) => {
+        if (user.self) {
+          user.connected = true;
+        }
+      });
+    });
+
+    socket.on("disconnect", () => {
+      this.users.forEach((user) => {
+        if (user.self) {
+          user.connected = false;
+        }
+      });
+    });
+
+    const initReactiveProperties = (user) => {
+      user.hasNewMessages = false;
+    };
+
+    socket.on("users", (users) => {
+      users.forEach((user) => {
+        user.messages.forEach((message) => {
+          message.fromSelf = message.from === socket.userID;
+        });
+        for (let i = 0; i < this.users.length; i++) {
+          const existingUser = this.users[i];
+          if (existingUser.userID === user.userID) {
+            existingUser.connected = user.connected;
+            existingUser.messages = user.messages;
+            return;
+          }
+        }
+        user.self = user.userID === socket.userID;
+        initReactiveProperties(user);
+        this.users.push(user);
+      });
+      // put the current user first, and sort by username
+      this.users.sort((a, b) => {
+        if (a.self) return -1;
+        if (b.self) return 1;
+        if (a.username < b.username) return -1;
+        return a.username > b.username ? 1 : 0;
+      });
+    });
+
+    socket.on("user connected", (user) => {
+      for (let i = 0; i < this.users.length; i++) {
+        const existingUser = this.users[i];
+        if (existingUser.userID === user.userID) {
+          existingUser.connected = true;
+          return;
+        }
+      }
+      initReactiveProperties(user);
+      this.users.push(user);
+    });
+
+    socket.on("user disconnected", (id) => {
+      for (let i = 0; i < this.users.length; i++) {
+        const user = this.users[i];
+        if (user.userID === id) {
+          user.connected = false;
+          break;
+        }
+      }
+    });
+
+    socket.on("private message", ({ content, from, to }) => {
+      for (let i = 0; i < this.users.length; i++) {
+        const user = this.users[i];
+        const fromSelf = socket.userID === from;
+        if (user.userID === (fromSelf ? to : from)) {
+          user.messages.push({
+            content,
+            fromSelf,
+          });
+          if (user !== this.selectedUser) {
+            user.hasNewMessages = true;
+          }
+          break;
+        }
+      }
+    });
+  },
+  destroyed() {
+    socket.off("connect");
+    socket.off("disconnect");
+    socket.off("users");
+    socket.off("user connected");
+    socket.off("user disconnected");
+    socket.off("private message");
+  },
+};
+</script>
+
+<style scoped>
+.left-panel {
+  position: fixed;
+  left: 0;
+  top: 0;
+  bottom: 0;
+  width: 260px;
+  overflow-x: hidden;
+  background-color: #3f0e40;
+  color: white;
+}
+
+.right-panel {
+  margin-left: 260px;
+}
+</style>
--- a/examples/private-messaging/src/components/MessagePanel.vue
+++ b/examples/private-messaging/src/components/MessagePanel.vue
@@ -0,0 +1,101 @@
+<template>
+  <div>
+    <div class="header">
+      <status-icon :connected="user.connected" />{{ user.username }}
+    </div>
+
+    <ul class="messages">
+      <li
+        v-for="(message, index) in user.messages"
+        :key="index"
+        class="message"
+      >
+        <div v-if="displaySender(message, index)" class="sender">
+          {{ message.fromSelf ? "(yourself)" : user.username }}
+        </div>
+        {{ message.content }}
+      </li>
+    </ul>
+
+    <form @submit.prevent="onSubmit" class="form">
+      <textarea v-model="input" placeholder="Your message..." class="input" />
+      <button :disabled="!isValid" class="send-button">Send</button>
+    </form>
+  </div>
+</template>
+
+<script>
+import StatusIcon from "./StatusIcon";
+
+export default {
+  name: "MessagePanel",
+  components: {
+    StatusIcon,
+  },
+  props: {
+    user: Object,
+  },
+  data() {
+    return {
+      input: "",
+    };
+  },
+  methods: {
+    onSubmit() {
+      this.$emit("input", this.input);
+      this.input = "";
+    },
+    displaySender(message, index) {
+      return (
+        index === 0 ||
+        this.user.messages[index - 1].fromSelf !==
+          this.user.messages[index].fromSelf
+      );
+    },
+  },
+  computed: {
+    isValid() {
+      return this.input.length > 0;
+    },
+  },
+};
+</script>
+
+<style scoped>
+.header {
+  line-height: 40px;
+  padding: 10px 20px;
+  border-bottom: 1px solid #dddddd;
+}
+
+.messages {
+  margin: 0;
+  padding: 20px;
+}
+
+.message {
+  list-style: none;
+}
+
+.sender {
+  font-weight: bold;
+  margin-top: 5px;
+}
+
+.form {
+  padding: 10px;
+}
+
+.input {
+  width: 80%;
+  resize: none;
+  padding: 10px;
+  line-height: 1.5;
+  border-radius: 5px;
+  border: 1px solid #000;
+}
+
+.send-button {
+  vertical-align: top;
+}
+</style>
--- a/examples/private-messaging/src/components/SelectUsername.vue
+++ b/examples/private-messaging/src/components/SelectUsername.vue
@@ -0,0 +1,36 @@
+<template>
+  <div class="select-username">
+    <form @submit.prevent="onSubmit">
+      <input v-model="username" placeholder="Your username..." />
+      <button :disabled="!isValid">Send</button>
+    </form>
+  </div>
+</template>
+
+<script>
+export default {
+  name: "SelectUsername",
+  data() {
+    return {
+      username: "",
+    };
+  },
+  computed: {
+    isValid() {
+      return this.username.length > 2;
+    },
+  },
+  methods: {
+    onSubmit() {
+      this.$emit("input", this.username);
+    },
+  },
+};
+</script>
+
+<style scoped>
+.select-username {
+  width: 300px;
+  margin: 200px auto 0;
+}
+</style>
--- a/examples/private-messaging/src/components/StatusIcon.vue
+++ b/examples/private-messaging/src/components/StatusIcon.vue
@@ -0,0 +1,27 @@
+<template>
+  <i class="icon" :class="{ connected: connected }"></i>
+</template>
+
+<script>
+export default {
+  name: "StatusIcon",
+  props: {
+    connected: Boolean,
+  },
+};
+</script>
+
+<style scoped>
+.icon {
+  height: 8px;
+  width: 8px;
+  border-radius: 50%;
+  display: inline-block;
+  background-color: #e38968;
+  margin-right: 6px;
+}
+
+.icon.connected {
+  background-color: #86bb71;
+}
+</style>
--- a/examples/private-messaging/src/components/User.vue
+++ b/examples/private-messaging/src/components/User.vue
@@ -0,0 +1,63 @@
+<template>
+  <div class="user" @click="onClick" :class="{ selected: selected }">
+    <div class="description">
+      <div class="name">
+        {{ user.username }} {{ user.self ? " (yourself)" : "" }}
+      </div>
+      <div class="status">
+        <status-icon :connected="user.connected" />{{ status }}
+      </div>
+    </div>
+    <div v-if="user.hasNewMessages" class="new-messages">!</div>
+  </div>
+</template>
+
+<script>
+import StatusIcon from "./StatusIcon";
+export default {
+  name: "User",
+  components: { StatusIcon },
+  props: {
+    user: Object,
+    selected: Boolean,
+  },
+  methods: {
+    onClick() {
+      this.$emit("select");
+    },
+  },
+  computed: {
+    status() {
+      return this.user.connected ? "online" : "offline";
+    },
+  },
+};
+</script>
+
+<style scoped>
+.selected {
+  background-color: #1164a3;
+}
+
+.user {
+  padding: 10px;
+}
+
+.description {
+  display: inline-block;
+}
+
+.status {
+  color: #92959e;
+}
+
+.new-messages {
+  color: white;
+  background-color: red;
+  width: 20px;
+  border-radius: 5px;
+  text-align: center;
+  float: right;
+  margin-top: 10px;
+}
+</style>
--- a/examples/private-messaging/src/main.js
+++ b/examples/private-messaging/src/main.js
@@ -0,0 +1,8 @@
+import Vue from "vue";
+import App from "./App.vue";
+
+Vue.config.productionTip = false;
+
+new Vue({
+  render: (h) => h(App),
+}).$mount("#app");
--- a/examples/private-messaging/src/socket.js
+++ b/examples/private-messaging/src/socket.js
@@ -0,0 +1,10 @@
+import { io } from "socket.io-client";
+
+const URL = "http://localhost:3000";
+const socket = io(URL, { autoConnect: false });
+
+socket.onAny((event, ...args) => {
+  console.log(event, args);
+});
+
+export default socket;
--- a/examples/react-native/package-lock.json
+++ b/examples/react-native/package-lock.json
--- a/examples/react-native/package.json
+++ b/examples/react-native/package.json
@@ -13,8 +13,8 @@
    "react-dom": "~16.9.0",
    "react-native": "https://github.com/expo/react-native/archive/sdk-37.0.1.tar.gz",
    "react-native-web": "~0.11.7",
-    "socket.io": "^3.0.1",
-    "socket.io-client": "^3.0.1"
+    "socket.io": "^4.0.0",
+    "socket.io-client": "^4.0.0"
  },
  "devDependencies": {
    "babel-preset-expo": "~8.1.0",
--- a/examples/tweet-stream/index.js
+++ b/examples/tweet-stream/index.js
@@ -7,7 +7,11 @@ const twitter = new Twitter({
  token_secret: process.env.TWITTER_TOKEN_SECRET
 });

-const io = require('socket.io')(process.env.PORT || 3000);
+const io = require('socket.io')(process.env.PORT || 3000, {
+  cors: {
+    origin: true
+  }
+});

 twitter.track('socket.io');
 twitter.track('javascript');
--- a/examples/tweet-stream/package-lock.json
+++ b/examples/tweet-stream/package-lock.json
@@ -1,705 +0,0 @@
-{
-  "name": "socket.io-tweet-stream",
-  "version": "1.0.0",
-  "lockfileVersion": 1,
-  "requires": true,
-  "dependencies": {
-    "accepts": {
-      "version": "1.3.7",
-      "resolved": "https://registry.npmjs.org/accepts/-/accepts-1.3.7.tgz",
-      "integrity": "sha512-Il80Qs2WjYlJIBNzNkK6KYqlVMTbZLXgHx2oT0pU/fjRHyEp+PEfEPY0R3WCwAGVOtauxh1hOxNgIf5bv7dQpA==",
-      "requires": {
-        "mime-types": "~2.1.24",
-        "negotiator": "0.6.2"
-      }
-    },
-    "after": {
-      "version": "0.8.2",
-      "resolved": "https://registry.npmjs.org/after/-/after-0.8.2.tgz",
-      "integrity": "sha1-/ts5T58OAqqXaOcCvaI7UF+ufh8="
-    },
-    "ajv": {
-      "version": "6.12.0",
-      "resolved": "https://registry.npmjs.org/ajv/-/ajv-6.12.0.tgz",
-      "integrity": "sha512-D6gFiFA0RRLyUbvijN74DWAjXSFxWKaWP7mldxkVhyhAV3+SWA9HEJPHQ2c9soIeTFJqcSdFDGFgdqs1iUU2Hw==",
-      "requires": {
-        "fast-deep-equal": "^3.1.1",
-        "fast-json-stable-stringify": "^2.0.0",
-        "json-schema-traverse": "^0.4.1",
-        "uri-js": "^4.2.2"
-      }
-    },
-    "arraybuffer.slice": {
-      "version": "0.0.7",
-      "resolved": "https://registry.npmjs.org/arraybuffer.slice/-/arraybuffer.slice-0.0.7.tgz",
-      "integrity": "sha512-wGUIVQXuehL5TCqQun8OW81jGzAWycqzFF8lFp+GOM5BXLYj3bKNsYC4daB7n6XjCqxQA/qgTJ+8ANR3acjrog=="
-    },
-    "asn1": {
-      "version": "0.2.4",
-      "resolved": "https://registry.npmjs.org/asn1/-/asn1-0.2.4.tgz",
-      "integrity": "sha512-jxwzQpLQjSmWXgwaCZE9Nz+glAG01yF1QnWgbhGwHI5A6FRIEY6IVqtHhIepHqI7/kyEyQEagBC5mBEFlIYvdg==",
-      "requires": {
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--- a/examples/tweet-stream/package.json
+++ b/examples/tweet-stream/package.json
@@ -15,6 +15,6 @@
  "license": "MIT",
  "dependencies": {
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-    "socket.io": "~2.3.0"
+    "socket.io": "^4.0.0"
  }
 }
--- a/examples/typescript/package-lock.json
+++ b/examples/typescript/package-lock.json
@@ -1,295 +0,0 @@
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-      "integrity": "sha512-/NOTfLrsPBVeH7YtFPgsVWveuL+4SjjYxaQ1xtM1KMFj7HdxlBlxeyNLzhyJVx7r4rZGJAZ/6lkKCitSc/Nmpg=="
-    },
-    "mime-types": {
-      "version": "2.1.27",
-      "resolved": "https://registry.npmjs.org/mime-types/-/mime-types-2.1.27.tgz",
-      "integrity": "sha512-JIhqnCasI9yD+SsmkquHBxTSEuZdQX5BuQnS2Vc7puQQQ+8yiP5AY5uWhpdv4YL4VM5c6iliiYWPgJ/nJQLp7w==",
-      "requires": {
-        "mime-db": "1.44.0"
-      }
-    },
-    "ms": {
-      "version": "2.1.3",
-      "resolved": "https://registry.npmjs.org/ms/-/ms-2.1.3.tgz",
-      "integrity": "sha512-6FlzubTLZG3J2a/NVCAleEhjzq5oxgHyaCU9yYXvcLsvoVaHJq/s5xXI6/XXP6tz7R9xAOtHnSO/tXtF3WRTlA=="
-    },
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-      "resolved": "https://registry.npmjs.org/negotiator/-/negotiator-0.6.2.tgz",
-      "integrity": "sha512-hZXc7K2e+PgeI1eDBe/10Ard4ekbfrrqG8Ep+8Jmf4JID2bNg7NvCPOZN+kfF574pFQI7mum2AUqDidoKqcTOw=="
-    },
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-      "integrity": "sha1-IQmtx5ZYh8/AXLvUQsrIv7s2CGM="
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-      "integrity": "sha512-Vj1jUoO75WGc9txWd311ZJJqS9Dr8QtNJJ7gk2r7dcM/yGe9sit7qOijQl3GAwhpBOz/W8CwkD7R6yob07nLbA==",
-      "requires": {
-        "@types/cookie": "^0.4.0",
-        "@types/cors": "^2.8.8",
-        "@types/node": "^14.14.7",
-        "accepts": "~1.3.4",
-        "base64id": "~2.0.0",
-        "debug": "~4.1.0",
-        "engine.io": "~4.0.0",
-        "socket.io-adapter": "~2.0.3",
-        "socket.io-parser": "~4.0.1"
-      }
-    },
-    "socket.io-adapter": {
-      "version": "2.0.3",
-      "resolved": "https://registry.npmjs.org/socket.io-adapter/-/socket.io-adapter-2.0.3.tgz",
-      "integrity": "sha512-2wo4EXgxOGSFueqvHAdnmi5JLZzWqMArjuP4nqC26AtLh5PoCPsaRbRdah2xhcwTAMooZfjYiNVNkkmmSMaxOQ=="
-    },
-    "socket.io-client": {
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-      "resolved": "https://registry.npmjs.org/socket.io-client/-/socket.io-client-3.0.4.tgz",
-      "integrity": "sha512-qMvBuS+W9JIN2mkfAWDCxuIt+jpIKDf8C0604zEqx1JrPaPSS6cN0F3B2GYWC83TqBeVJXW66GFxWV3KD88n0Q==",
-      "requires": {
-        "@types/component-emitter": "^1.2.10",
-        "backo2": "1.0.2",
-        "component-bind": "1.0.0",
-        "component-emitter": "~1.3.0",
-        "debug": "~4.1.0",
-        "engine.io-client": "~4.0.0",
-        "parseuri": "0.0.6",
-        "socket.io-parser": "~4.0.1"
-      }
-    },
-    "socket.io-parser": {
-      "version": "4.0.2",
-      "resolved": "https://registry.npmjs.org/socket.io-parser/-/socket.io-parser-4.0.2.tgz",
-      "integrity": "sha512-Bs3IYHDivwf+bAAuW/8xwJgIiBNtlvnjYRc4PbXgniLmcP1BrakBoq/QhO24rgtgW7VZ7uAaswRGxutUnlAK7g==",
-      "requires": {
-        "@types/component-emitter": "^1.2.10",
-        "component-emitter": "~1.3.0",
-        "debug": "~4.1.0"
-      }
-    },
-    "source-map": {
-      "version": "0.6.1",
-      "resolved": "https://registry.npmjs.org/source-map/-/source-map-0.6.1.tgz",
-      "integrity": "sha512-UjgapumWlbMhkBgzT7Ykc5YXUT46F0iKu8SGXq0bcwP5dz/h0Plj6enJqjz1Zbq2l5WaqYnrVbwWOWMyF3F47g=="
-    },
-    "source-map-support": {
-      "version": "0.5.19",
-      "resolved": "https://registry.npmjs.org/source-map-support/-/source-map-support-0.5.19.tgz",
-      "integrity": "sha512-Wonm7zOCIJzBGQdB+thsPar0kYuCIzYvxZwlBa87yi/Mdjv7Tip2cyVbLj5o0cFPN4EVkuTwb3GDDyUx2DGnGw==",
-      "requires": {
-        "buffer-from": "^1.0.0",
-        "source-map": "^0.6.0"
-      }
-    },
-    "ts-node": {
-      "version": "9.0.0",
-      "resolved": "https://registry.npmjs.org/ts-node/-/ts-node-9.0.0.tgz",
-      "integrity": "sha512-/TqB4SnererCDR/vb4S/QvSZvzQMJN8daAslg7MeaiHvD8rDZsSfXmNeNumyZZzMned72Xoq/isQljYSt8Ynfg==",
-      "requires": {
-        "arg": "^4.1.0",
-        "diff": "^4.0.1",
-        "make-error": "^1.1.1",
-        "source-map-support": "^0.5.17",
-        "yn": "3.1.1"
-      }
-    },
-    "typescript": {
-      "version": "4.0.5",
-      "resolved": "https://registry.npmjs.org/typescript/-/typescript-4.0.5.tgz",
-      "integrity": "sha512-ywmr/VrTVCmNTJ6iV2LwIrfG1P+lv6luD8sUJs+2eI9NLGigaN+nUQc13iHqisq7bra9lnmUSYqbJvegraBOPQ=="
-    },
-    "vary": {
-      "version": "1.1.2",
-      "resolved": "https://registry.npmjs.org/vary/-/vary-1.1.2.tgz",
-      "integrity": "sha1-IpnwLG3tMNSllhsLn3RSShj2NPw="
-    },
-    "ws": {
-      "version": "7.4.2",
-      "resolved": "https://registry.npmjs.org/ws/-/ws-7.4.2.tgz",
-      "integrity": "sha512-T4tewALS3+qsrpGI/8dqNMLIVdq/g/85U98HPMa6F0m6xTbvhXU6RCQLqPH3+SlomNV/LdY6RXEbBpMH6EOJnA=="
-    },
-    "xmlhttprequest-ssl": {
-      "version": "1.5.5",
-      "resolved": "https://registry.npmjs.org/xmlhttprequest-ssl/-/xmlhttprequest-ssl-1.5.5.tgz",
-      "integrity": "sha1-wodrBhaKrcQOV9l+gRkayPQ5iz4="
-    },
-    "yeast": {
-      "version": "0.1.2",
-      "resolved": "https://registry.npmjs.org/yeast/-/yeast-0.1.2.tgz",
-      "integrity": "sha1-AI4G2AlDIMNy28L47XagymyKxBk="
-    },
-    "yn": {
-      "version": "3.1.1",
-      "resolved": "https://registry.npmjs.org/yn/-/yn-3.1.1.tgz",
-      "integrity": "sha512-Ux4ygGWsu2c7isFWe8Yu1YluJmqVhxqK2cLXNQA5AcC3QfbGNpM7fu0Y8b/z16pXLnFxZYvWhd3fhBY9DLmC6Q=="
-    }
-  }
-}
--- a/examples/typescript/package.json
+++ b/examples/typescript/package.json
@@ -11,8 +11,8 @@
  "author": "Damien Arrachequesne",
  "license": "MIT",
  "dependencies": {
-    "socket.io": "^3.0.4",
-    "socket.io-client": "^3.0.4",
+    "socket.io": "^4.0.0",
+    "socket.io-client": "^4.0.0",
    "ts-node": "^9.0.0",
    "typescript": "^4.0.5"
  }
--- a/examples/typescript/server.ts
+++ b/examples/typescript/server.ts
@@ -1,8 +1,8 @@
-import { Server, Socket } from "socket.io";
+import { Server } from "socket.io";

 const io = new Server(8080);

-io.on("connect", (socket: Socket) => {
+io.on("connect", (socket) => {
    console.log(`connect ${socket.id}`);

    socket.on("ping", (cb) => {
--- a/examples/webpack-build-server/lib/index.js
+++ b/examples/webpack-build-server/lib/index.js
@@ -1,8 +1,7 @@

 const server = require('http').createServer();
 const io = require('socket.io')(server, {
-  serveClient: false,
-  wsEngine: 'ws' // uws is not supported since it is a native module
+  serveClient: false
 });
 const port = process.env.PORT || 3000;

--- a/examples/webpack-build-server/package-lock.json
+++ b/examples/webpack-build-server/package-lock.json
--- a/examples/webpack-build-server/package.json
+++ b/examples/webpack-build-server/package.json
@@ -9,7 +9,7 @@
  "author": "Damien Arrachequesne",
  "license": "MIT",
  "devDependencies": {
-    "socket.io": "~2.3.0",
+    "socket.io": "^4.0.0",
    "webpack": "~4.43.0",
    "webpack-cli": "~3.3.11"
  }
--- a/examples/webpack-build-server/support/webpack.config.js
+++ b/examples/webpack-build-server/support/webpack.config.js
@@ -5,5 +5,6 @@ module.exports = {
  output: {
    path: require('path').join(__dirname, '../dist'),
    filename: 'server.js'
-  }
+  },
+  mode: 'production'
 };
--- a/examples/whiteboard/package-lock.json
+++ b/examples/whiteboard/package-lock.json
@@ -1,499 +0,0 @@
-{
-  "name": "whiteboard",
-  "version": "1.0.0",
-  "lockfileVersion": 1,
-  "requires": true,
-  "dependencies": {
-    "accepts": {
-      "version": "1.3.7",
-      "resolved": "https://registry.npmjs.org/accepts/-/accepts-1.3.7.tgz",
-      "integrity": "sha512-Il80Qs2WjYlJIBNzNkK6KYqlVMTbZLXgHx2oT0pU/fjRHyEp+PEfEPY0R3WCwAGVOtauxh1hOxNgIf5bv7dQpA==",
-      "requires": {
-        "mime-types": "~2.1.24",
-        "negotiator": "0.6.2"
-      }
-    },
-    "array-flatten": {
-      "version": "1.1.1",
-      "resolved": "https://registry.npmjs.org/array-flatten/-/array-flatten-1.1.1.tgz",
-      "integrity": "sha1-ml9pkFGx5wczKPKgCJaLZOopVdI="
-    },
-    "base64id": {
-      "version": "2.0.0",
-      "resolved": "https://registry.npmjs.org/base64id/-/base64id-2.0.0.tgz",
-      "integrity": "sha512-lGe34o6EHj9y3Kts9R4ZYs/Gr+6N7MCaMlIFA3F1R2O5/m7K06AxfSeO5530PEERE6/WyEg3lsuyw4GHlPZHog=="
-    },
-    "body-parser": {
-      "version": "1.19.0",
-      "resolved": "https://registry.npmjs.org/body-parser/-/body-parser-1.19.0.tgz",
-      "integrity": "sha512-dhEPs72UPbDnAQJ9ZKMNTP6ptJaionhP5cBb541nXPlW60Jepo9RV/a4fX4XWW9CuFNK22krhrj1+rgzifNCsw==",
-      "requires": {
-        "bytes": "3.1.0",
-        "content-type": "~1.0.4",
-        "debug": "2.6.9",
-        "depd": "~1.1.2",
-        "http-errors": "1.7.2",
-        "iconv-lite": "0.4.24",
-        "on-finished": "~2.3.0",
-        "qs": "6.7.0",
-        "raw-body": "2.4.0",
-        "type-is": "~1.6.17"
-      }
-    },
-    "bytes": {
-      "version": "3.1.0",
-      "resolved": "https://registry.npmjs.org/bytes/-/bytes-3.1.0.tgz",
-      "integrity": "sha512-zauLjrfCG+xvoyaqLoV8bLVXXNGC4JqlxFCutSDWA6fJrTo2ZuvLYTqZ7aHBLZSMOopbzwv8f+wZcVzfVTI2Dg=="
-    },
-    "component-emitter": {
-      "version": "1.3.0",
-      "resolved": "https://registry.npmjs.org/component-emitter/-/component-emitter-1.3.0.tgz",
-      "integrity": "sha512-Rd3se6QB+sO1TwqZjscQrurpEPIfO0/yYnSin6Q/rD3mOutHvUrCAhJub3r90uNb+SESBuE0QYoB90YdfatsRg=="
-    },
-    "content-disposition": {
-      "version": "0.5.3",
-      "resolved": "https://registry.npmjs.org/content-disposition/-/content-disposition-0.5.3.tgz",
-      "integrity": "sha512-ExO0774ikEObIAEV9kDo50o+79VCUdEB6n6lzKgGwupcVeRlhrj3qGAfwq8G6uBJjkqLrhT0qEYFcWng8z1z0g==",
-      "requires": {
-        "safe-buffer": "5.1.2"
-      }
-    },
-    "content-type": {
-      "version": "1.0.4",
-      "resolved": "https://registry.npmjs.org/content-type/-/content-type-1.0.4.tgz",
-      "integrity": "sha512-hIP3EEPs8tB9AT1L+NUqtwOAps4mk2Zob89MWXMHjHWg9milF/j4osnnQLXBCBFBk/tvIG/tUc9mOUJiPBhPXA=="
-    },
-    "cookie": {
-      "version": "0.4.0",
-      "resolved": "https://registry.npmjs.org/cookie/-/cookie-0.4.0.tgz",
-      "integrity": "sha512-+Hp8fLp57wnUSt0tY0tHEXh4voZRDnoIrZPqlo3DPiI4y9lwg/jqx+1Om94/W6ZaPDOUbnjOt/99w66zk+l1Xg=="
-    },
-    "cookie-signature": {
-      "version": "1.0.6",
-      "resolved": "https://registry.npmjs.org/cookie-signature/-/cookie-signature-1.0.6.tgz",
-      "integrity": "sha1-4wOogrNCzD7oylE6eZmXNNqzriw="
-    },
-    "cors": {
-      "version": "2.8.5",
-      "resolved": "https://registry.npmjs.org/cors/-/cors-2.8.5.tgz",
-      "integrity": "sha512-KIHbLJqu73RGr/hnbrO9uBeixNGuvSQjul/jdFvS/KFSIH1hWVd1ng7zOHx+YrEfInLG7q4n6GHQ9cDtxv/P6g==",
-      "requires": {
-        "object-assign": "^4",
-        "vary": "^1"
-      }
-    },
-    "debug": {
-      "version": "2.6.9",
-      "resolved": "https://registry.npmjs.org/debug/-/debug-2.6.9.tgz",
-      "integrity": "sha512-bC7ElrdJaJnPbAP+1EotYvqZsb3ecl5wi6Bfi6BJTUcNowp6cvspg0jXznRTKDjm/E7AdgFBVeAPVMNcKGsHMA==",
-      "requires": {
-        "ms": "2.0.0"
-      }
-    },
-    "depd": {
-      "version": "1.1.2",
-      "resolved": "https://registry.npmjs.org/depd/-/depd-1.1.2.tgz",
-      "integrity": "sha1-m81S4UwJd2PnSbJ0xDRu0uVgtak="
-    },
-    "destroy": {
-      "version": "1.0.4",
-      "resolved": "https://registry.npmjs.org/destroy/-/destroy-1.0.4.tgz",
-      "integrity": "sha1-l4hXRCxEdJ5CBmE+N5RiBYJqvYA="
-    },
-    "ee-first": {
-      "version": "1.1.1",
-      "resolved": "https://registry.npmjs.org/ee-first/-/ee-first-1.1.1.tgz",
-      "integrity": "sha1-WQxhFWsK4vTwJVcyoViyZrxWsh0="
-    },
-    "encodeurl": {
-      "version": "1.0.2",
-      "resolved": "https://registry.npmjs.org/encodeurl/-/encodeurl-1.0.2.tgz",
-      "integrity": "sha1-rT/0yG7C0CkyL1oCw6mmBslbP1k="
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-    "engine.io": {
-      "version": "4.0.1",
-      "resolved": "https://registry.npmjs.org/engine.io/-/engine.io-4.0.1.tgz",
-      "integrity": "sha512-6EaSBxasBUwxRdf6B68SEYpD3tcrG80J4YTzHl/D+9Q+vM0AMHZabfYcc2WdnvEaQxZjX/UZsa+UdGoM0qQQkQ==",
-      "requires": {
-        "accepts": "~1.3.4",
-        "base64id": "2.0.0",
-        "cookie": "~0.4.1",
-        "cors": "~2.8.5",
-        "debug": "~4.1.0",
-        "engine.io-parser": "~4.0.0",
-        "ws": "^7.1.2"
-      },
-      "dependencies": {
-        "cookie": {
-          "version": "0.4.1",
-          "resolved": "https://registry.npmjs.org/cookie/-/cookie-0.4.1.tgz",
-          "integrity": "sha512-ZwrFkGJxUR3EIoXtO+yVE69Eb7KlixbaeAWfBQB9vVsNn/o+Yw69gBWSSDK825hQNdN+wF8zELf3dFNl/kxkUA=="
-        },
-        "debug": {
-          "version": "4.1.1",
-          "resolved": "https://registry.npmjs.org/debug/-/debug-4.1.1.tgz",
-          "integrity": "sha512-pYAIzeRo8J6KPEaJ0VWOh5Pzkbw/RetuzehGM7QRRX5he4fPHx2rdKMB256ehJCkX+XRQm16eZLqLNS8RSZXZw==",
-          "requires": {
-            "ms": "^2.1.1"
-          }
-        },
-        "ms": {
-          "version": "2.1.2",
-          "resolved": "https://registry.npmjs.org/ms/-/ms-2.1.2.tgz",
-          "integrity": "sha512-sGkPx+VjMtmA6MX27oA4FBFELFCZZ4S4XqeGOXCv68tT+jb3vk/RyaKWP0PTKyWtmLSM0b+adUTEvbs1PEaH2w=="
-        }
-      }
-    },
-    "engine.io-parser": {
-      "version": "4.0.1",
-      "resolved": "https://registry.npmjs.org/engine.io-parser/-/engine.io-parser-4.0.1.tgz",
-      "integrity": "sha512-v5aZK1hlckcJDGmHz3W8xvI3NUHYc9t8QtTbqdR5OaH3S9iJZilPubauOm+vLWOMMWzpE3hiq92l9lTAHamRCg=="
-    },
-    "escape-html": {
-      "version": "1.0.3",
-      "resolved": "https://registry.npmjs.org/escape-html/-/escape-html-1.0.3.tgz",
-      "integrity": "sha1-Aljq5NPQwJdN4cFpGI7wBR0dGYg="
-    },
-    "etag": {
-      "version": "1.8.1",
-      "resolved": "https://registry.npmjs.org/etag/-/etag-1.8.1.tgz",
-      "integrity": "sha1-Qa4u62XvpiJorr/qg6x9eSmbCIc="
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-    "express": {
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-      "integrity": "sha512-mHJ9O79RqluphRrcw2X/GTh3k9tVv8YcoyY4Kkh4WDMUYKRZUq0h1o0w2rrrxBqM7VoeUVqgb27xlEMXTnYt4g==",
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-        "accepts": "~1.3.7",
-        "array-flatten": "1.1.1",
-        "body-parser": "1.19.0",
-        "content-disposition": "0.5.3",
-        "content-type": "~1.0.4",
-        "cookie": "0.4.0",
-        "cookie-signature": "1.0.6",
-        "debug": "2.6.9",
-        "depd": "~1.1.2",
-        "encodeurl": "~1.0.2",
-        "escape-html": "~1.0.3",
-        "etag": "~1.8.1",
-        "finalhandler": "~1.1.2",
-        "fresh": "0.5.2",
-        "merge-descriptors": "1.0.1",
-        "methods": "~1.1.2",
-        "on-finished": "~2.3.0",
-        "parseurl": "~1.3.3",
-        "path-to-regexp": "0.1.7",
-        "proxy-addr": "~2.0.5",
-        "qs": "6.7.0",
-        "range-parser": "~1.2.1",
-        "safe-buffer": "5.1.2",
-        "send": "0.17.1",
-        "serve-static": "1.14.1",
-        "setprototypeof": "1.1.1",
-        "statuses": "~1.5.0",
-        "type-is": "~1.6.18",
-        "utils-merge": "1.0.1",
-        "vary": "~1.1.2"
-      }
-    },
-    "finalhandler": {
-      "version": "1.1.2",
-      "resolved": "https://registry.npmjs.org/finalhandler/-/finalhandler-1.1.2.tgz",
-      "integrity": "sha512-aAWcW57uxVNrQZqFXjITpW3sIUQmHGG3qSb9mUah9MgMC4NeWhNOlNjXEYq3HjRAvL6arUviZGGJsBg6z0zsWA==",
-      "requires": {
-        "debug": "2.6.9",
-        "encodeurl": "~1.0.2",
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--- a/examples/whiteboard/package.json
+++ b/examples/whiteboard/package.json
@@ -9,7 +9,7 @@
  ],
  "dependencies": {
    "express": "~4.17.1",
-    "socket.io": "^3.0.0"
+    "socket.io": "^4.0.0"
  },
  "scripts": {
    "start": "node index"
--- a/lib/broadcast-operator.ts
+++ b/lib/broadcast-operator.ts
@@ -0,0 +1,315 @@
+import type { BroadcastFlags, Room, SocketId } from "socket.io-adapter";
+import { Handshake, RESERVED_EVENTS, Socket } from "./socket";
+import { PacketType } from "socket.io-parser";
+import type { Adapter } from "socket.io-adapter";
+import type {
+  EventParams,
+  EventNames,
+  EventsMap,
+  TypedEventBroadcaster,
+} from "./typed-events";
+
+export class BroadcastOperator<EmitEvents extends EventsMap>
+  implements TypedEventBroadcaster<EmitEvents> {
+  constructor(
+    private readonly adapter: Adapter,
+    private readonly rooms: Set<Room> = new Set<Room>(),
+    private readonly exceptRooms: Set<Room> = new Set<Room>(),
+    private readonly flags: BroadcastFlags = {}
+  ) {}
+
+  /**
+   * Targets a room when emitting.
+   *
+   * @param room
+   * @return a new BroadcastOperator instance
+   * @public
+   */
+  public to(room: Room | Room[]): BroadcastOperator<EmitEvents> {
+    const rooms = new Set(this.rooms);
+    if (Array.isArray(room)) {
+      room.forEach((r) => rooms.add(r));
+    } else {
+      rooms.add(room);
+    }
+    return new BroadcastOperator(
+      this.adapter,
+      rooms,
+      this.exceptRooms,
+      this.flags
+    );
+  }
+
+  /**
+   * Targets a room when emitting.
+   *
+   * @param room
+   * @return a new BroadcastOperator instance
+   * @public
+   */
+  public in(room: Room | Room[]): BroadcastOperator<EmitEvents> {
+    return this.to(room);
+  }
+
+  /**
+   * Excludes a room when emitting.
+   *
+   * @param room
+   * @return a new BroadcastOperator instance
+   * @public
+   */
+  public except(room: Room | Room[]): BroadcastOperator<EmitEvents> {
+    const exceptRooms = new Set(this.exceptRooms);
+    if (Array.isArray(room)) {
+      room.forEach((r) => exceptRooms.add(r));
+    } else {
+      exceptRooms.add(room);
+    }
+    return new BroadcastOperator(
+      this.adapter,
+      this.rooms,
+      exceptRooms,
+      this.flags
+    );
+  }
+
+  /**
+   * Sets the compress flag.
+   *
+   * @param compress - if `true`, compresses the sending data
+   * @return a new BroadcastOperator instance
+   * @public
+   */
+  public compress(compress: boolean): BroadcastOperator<EmitEvents> {
+    const flags = Object.assign({}, this.flags, { compress });
+    return new BroadcastOperator(
+      this.adapter,
+      this.rooms,
+      this.exceptRooms,
+      flags
+    );
+  }
+
+  /**
+   * Sets a modifier for a subsequent event emission that the event data may be lost if the client is not ready to
+   * receive messages (because of network slowness or other issues, or because they’re connected through long polling
+   * and is in the middle of a request-response cycle).
+   *
+   * @return a new BroadcastOperator instance
+   * @public
+   */
+  public get volatile(): BroadcastOperator<EmitEvents> {
+    const flags = Object.assign({}, this.flags, { volatile: true });
+    return new BroadcastOperator(
+      this.adapter,
+      this.rooms,
+      this.exceptRooms,
+      flags
+    );
+  }
+
+  /**
+   * Sets a modifier for a subsequent event emission that the event data will only be broadcast to the current node.
+   *
+   * @return a new BroadcastOperator instance
+   * @public
+   */
+  public get local(): BroadcastOperator<EmitEvents> {
+    const flags = Object.assign({}, this.flags, { local: true });
+    return new BroadcastOperator(
+      this.adapter,
+      this.rooms,
+      this.exceptRooms,
+      flags
+    );
+  }
+
+  /**
+   * Emits to all clients.
+   *
+   * @return Always true
+   * @public
+   */
+  public emit<Ev extends EventNames<EmitEvents>>(
+    ev: Ev,
+    ...args: EventParams<EmitEvents, Ev>
+  ): boolean {
+    if (RESERVED_EVENTS.has(ev)) {
+      throw new Error(`"${ev}" is a reserved event name`);
+    }
+    // set up packet object
+    const data = [ev, ...args];
+    const packet = {
+      type: PacketType.EVENT,
+      data: data,
+    };
+
+    if ("function" == typeof data[data.length - 1]) {
+      throw new Error("Callbacks are not supported when broadcasting");
+    }
+
+    this.adapter.broadcast(packet, {
+      rooms: this.rooms,
+      except: this.exceptRooms,
+      flags: this.flags,
+    });
+
+    return true;
+  }
+
+  /**
+   * Gets a list of clients.
+   *
+   * @public
+   */
+  public allSockets(): Promise<Set<SocketId>> {
+    if (!this.adapter) {
+      throw new Error(
+        "No adapter for this namespace, are you trying to get the list of clients of a dynamic namespace?"
+      );
+    }
+    return this.adapter.sockets(this.rooms);
+  }
+
+  /**
+   * Returns the matching socket instances
+   *
+   * @public
+   */
+  public fetchSockets(): Promise<RemoteSocket<EmitEvents>[]> {
+    return this.adapter
+      .fetchSockets({
+        rooms: this.rooms,
+        except: this.exceptRooms,
+      })
+      .then((sockets) => {
+        return sockets.map((socket) => {
+          if (socket instanceof Socket) {
+            // FIXME the TypeScript compiler complains about missing private properties
+            return (socket as unknown) as RemoteSocket<EmitEvents>;
+          } else {
+            return new RemoteSocket(this.adapter, socket as SocketDetails);
+          }
+        });
+      });
+  }
+
+  /**
+   * Makes the matching socket instances join the specified rooms
+   *
+   * @param room
+   * @public
+   */
+  public socketsJoin(room: Room | Room[]): void {
+    this.adapter.addSockets(
+      {
+        rooms: this.rooms,
+        except: this.exceptRooms,
+      },
+      Array.isArray(room) ? room : [room]
+    );
+  }
+
+  /**
+   * Makes the matching socket instances leave the specified rooms
+   *
+   * @param room
+   * @public
+   */
+  public socketsLeave(room: Room | Room[]): void {
+    this.adapter.delSockets(
+      {
+        rooms: this.rooms,
+        except: this.exceptRooms,
+      },
+      Array.isArray(room) ? room : [room]
+    );
+  }
+
+  /**
+   * Makes the matching socket instances disconnect
+   *
+   * @param close - whether to close the underlying connection
+   * @public
+   */
+  public disconnectSockets(close: boolean = false): void {
+    this.adapter.disconnectSockets(
+      {
+        rooms: this.rooms,
+        except: this.exceptRooms,
+      },
+      close
+    );
+  }
+}
+
+/**
+ * Format of the data when the Socket instance exists on another Socket.IO server
+ */
+interface SocketDetails {
+  id: SocketId;
+  handshake: Handshake;
+  rooms: Room[];
+  data: any;
+}
+
+/**
+ * Expose of subset of the attributes and methods of the Socket class
+ */
+export class RemoteSocket<EmitEvents extends EventsMap>
+  implements TypedEventBroadcaster<EmitEvents> {
+  public readonly id: SocketId;
+  public readonly handshake: Handshake;
+  public readonly rooms: Set<Room>;
+  public readonly data: any;
+
+  private readonly operator: BroadcastOperator<EmitEvents>;
+
+  constructor(adapter: Adapter, details: SocketDetails) {
+    this.id = details.id;
+    this.handshake = details.handshake;
+    this.rooms = new Set(details.rooms);
+    this.data = details.data;
+    this.operator = new BroadcastOperator(adapter, new Set([this.id]));
+  }
+
+  public emit<Ev extends EventNames<EmitEvents>>(
+    ev: Ev,
+    ...args: EventParams<EmitEvents, Ev>
+  ): boolean {
+    return this.operator.emit(ev, ...args);
+  }
+
+  /**
+   * Joins a room.
+   *
+   * @param {String|Array} room - room or array of rooms
+   * @public
+   */
+  public join(room: Room | Room[]): void {
+    return this.operator.socketsJoin(room);
+  }
+
+  /**
+   * Leaves a room.
+   *
+   * @param {String} room
+   * @public
+   */
+  public leave(room: Room): void {
+    return this.operator.socketsLeave(room);
+  }
+
+  /**
+   * Disconnects this client.
+   *
+   * @param {Boolean} close - if `true`, closes the underlying connection
+   * @return {Socket} self
+   *
+   * @public
+   */
+  public disconnect(close = false): this {
+    this.operator.disconnectSockets(close);
+    return this;
+  }
+}
--- a/lib/client.ts
+++ b/lib/client.ts
@@ -1,21 +1,26 @@
 import { Decoder, Encoder, Packet, PacketType } from "socket.io-parser";
 import debugModule = require("debug");
+import url = require("url");
 import type { IncomingMessage } from "http";
 import type { Namespace, Server } from "./index";
+import type { EventsMap } from "./typed-events";
 import type { Socket } from "./socket";
 import type { SocketId } from "socket.io-adapter";

 const debug = debugModule("socket.io:client");

-export class Client {
+export class Client<
+  ListenEvents extends EventsMap,
+  EmitEvents extends EventsMap
+> {
  public readonly conn;

  private readonly id: string;
-  private readonly server: Server;
+  private readonly server: Server<ListenEvents, EmitEvents>;
  private readonly encoder: Encoder;
  private readonly decoder: Decoder;
-  private sockets: Map<SocketId, Socket> = new Map();
-  private nsps: Map<string, Socket> = new Map();
+  private sockets: Map<SocketId, Socket<ListenEvents, EmitEvents>> = new Map();
+  private nsps: Map<string, Socket<ListenEvents, EmitEvents>> = new Map();
  private connectTimeout?: NodeJS.Timeout;

  /**
@@ -25,7 +30,7 @@ export class Client {
   * @param conn
   * @package
   */
-  constructor(server: Server, conn: Socket) {
+  constructor(server: Server<ListenEvents, EmitEvents>, conn: any) {
    this.server = server;
    this.conn = conn;
    this.encoder = server.encoder;
@@ -77,7 +82,7 @@ export class Client {
   * @param {Object} auth - the auth parameters
   * @private
   */
-  private connect(name: string, auth: object = {}) {
+  private connect(name: string, auth: object = {}): void {
    if (this.server._nsps.has(name)) {
      debug("connecting to namespace %s", name);
      return this.doConnect(name, auth);
@@ -86,7 +91,7 @@ export class Client {
    this.server._checkNamespace(
      name,
      auth,
-      (dynamicNspName: Namespace | false) => {
+      (dynamicNspName: Namespace<ListenEvents, EmitEvents> | false) => {
        if (dynamicNspName) {
          debug("dynamic namespace %s was created", dynamicNspName);
          this.doConnect(name, auth);
@@ -112,7 +117,7 @@ export class Client {
   *
   * @private
   */
-  private doConnect(name: string, auth: object) {
+  private doConnect(name: string, auth: object): void {
    const nsp = this.server.of(name);

    const socket = nsp._add(this, auth, () => {
@@ -131,7 +136,7 @@ export class Client {
   *
   * @private
   */
-  _disconnect() {
+  _disconnect(): void {
    for (const socket of this.sockets.values()) {
      socket.disconnect();
    }
@@ -144,7 +149,7 @@ export class Client {
   *
   * @private
   */
-  _remove(socket: Socket) {
+  _remove(socket: Socket<ListenEvents, EmitEvents>): void {
    if (this.sockets.has(socket.id)) {
      const nsp = this.sockets.get(socket.id)!.nsp.name;
      this.sockets.delete(socket.id);
@@ -159,8 +164,8 @@ export class Client {
   *
   * @private
   */
-  private close() {
-    if ("open" == this.conn.readyState) {
+  private close(): void {
+    if ("open" === this.conn.readyState) {
      debug("forcing transport close");
      this.conn.close();
      this.onclose("forced server close");
@@ -174,19 +179,20 @@ export class Client {
   * @param {Object} opts
   * @private
   */
-  _packet(packet, opts?) {
+  _packet(packet: Packet, opts?: any): void {
    opts = opts || {};
    const self = this;

    // this writes to the actual connection
-    function writeToEngine(encodedPackets) {
+    function writeToEngine(encodedPackets: any) {
+      // TODO clarify this.
      if (opts.volatile && !self.conn.transport.writable) return;
      for (let i = 0; i < encodedPackets.length; i++) {
        self.conn.write(encodedPackets[i], { compress: opts.compress });
      }
    }

-    if ("open" == this.conn.readyState) {
+    if ("open" === this.conn.readyState) {
      debug("writing packet %j", packet);
      if (!opts.preEncoded) {
        // not broadcasting, need to encode
@@ -205,7 +211,7 @@ export class Client {
   *
   * @private
   */
-  private ondata(data) {
+  private ondata(data): void {
    // try/catch is needed for protocol violations (GH-1880)
    try {
      this.decoder.add(data);
@@ -219,9 +225,14 @@ export class Client {
   *
   * @private
   */
-  private ondecoded(packet: Packet) {
-    if (PacketType.CONNECT == packet.type) {
-      this.connect(packet.nsp, packet.data);
+  private ondecoded(packet: Packet): void {
+    if (PacketType.CONNECT === packet.type) {
+      if (this.conn.protocol === 3) {
+        const parsed = url.parse(packet.nsp, true);
+        this.connect(parsed.pathname!, parsed.query);
+      } else {
+        this.connect(packet.nsp, packet.data);
+      }
    } else {
      const socket = this.nsps.get(packet.nsp);
      if (socket) {
@@ -240,7 +251,7 @@ export class Client {
   * @param {Object} err object
   * @private
   */
-  private onerror(err) {
+  private onerror(err): void {
    for (const socket of this.sockets.values()) {
      socket._onerror(err);
    }
@@ -253,7 +264,7 @@ export class Client {
   * @param reason
   * @private
   */
-  private onclose(reason: string) {
+  private onclose(reason: string): void {
    debug("client close with reason %s", reason);

    // ignore a potential subsequent `close` event
@@ -272,7 +283,7 @@ export class Client {
   * Cleans up event listeners.
   * @private
   */
-  private destroy() {
+  private destroy(): void {
    this.conn.removeListener("data", this.ondata);
    this.conn.removeListener("error", this.onerror);
    this.conn.removeListener("close", this.onclose);
--- a/lib/index.ts
+++ b/lib/index.ts
@@ -7,7 +7,11 @@ import path = require("path");
 import engine = require("engine.io");
 import { Client } from "./client";
 import { EventEmitter } from "events";
-import { ExtendedError, Namespace } from "./namespace";
+import {
+  ExtendedError,
+  Namespace,
+  NamespaceReservedEventsMap,
+} from "./namespace";
 import { ParentNamespace } from "./parent-namespace";
 import { Adapter, Room, SocketId } from "socket.io-adapter";
 import * as parser from "socket.io-parser";
@@ -16,6 +20,13 @@ import debugModule from "debug";
 import { Socket } from "./socket";
 import type { CookieSerializeOptions } from "cookie";
 import type { CorsOptions } from "cors";
+import type { BroadcastOperator, RemoteSocket } from "./broadcast-operator";
+import {
+  EventsMap,
+  DefaultEventsMap,
+  EventParams,
+  StrictEventEmitter,
+} from "./typed-events";

 const debug = debugModule("socket.io:server");

@@ -25,14 +36,14 @@ const dotMapRegex = /\.map/;
 type Transport = "polling" | "websocket";
 type ParentNspNameMatchFn = (
  name: string,
-  query: object,
+  auth: { [key: string]: any },
  fn: (err: Error | null, success: boolean) => void
 ) => void;

 interface EngineOptions {
  /**
   * how many ms without a pong packet to consider the connection closed
-   * @default 5000
+   * @default 20000
   */
  pingTimeout: number;
  /**
@@ -82,10 +93,12 @@ interface EngineOptions {
  httpCompression: boolean | object;
  /**
   * what WebSocket server implementation to use. Specified module must
-   * conform to the ws interface (see ws module api docs). Default value is ws.
-   * An alternative c++ addon is also available by installing uws module.
+   * conform to the ws interface (see ws module api docs).
+   * An alternative c++ addon is also available by installing eiows module.
+   *
+   * @default `require("ws").Server`
   */
-  wsEngine: string;
+  wsEngine: Function;
  /**
   * an optional packet which will be concatenated to the handshake packet emitted by Engine.IO.
   */
@@ -100,6 +113,11 @@ interface EngineOptions {
   * the options that will be forwarded to the cors module
   */
  cors: CorsOptions;
+  /**
+   * whether to enable compatibility with Socket.IO v2 clients
+   * @default false
+   */
+  allowEIO3: boolean;
 }

 interface AttachOptions {
@@ -150,8 +168,15 @@ interface ServerOptions extends EngineAttachOptions {
  connectTimeout: number;
 }

-export class Server extends EventEmitter {
-  public readonly sockets: Namespace;
+export class Server<
+  ListenEvents extends EventsMap = DefaultEventsMap,
+  EmitEvents extends EventsMap = ListenEvents
+> extends StrictEventEmitter<
+  {},
+  EmitEvents,
+  NamespaceReservedEventsMap<ListenEvents, EmitEvents>
+> {
+  public readonly sockets: Namespace<ListenEvents, EmitEvents>;

  /** @private */
  readonly _parser: typeof parser;
@@ -161,8 +186,11 @@ export class Server extends EventEmitter {
  /**
   * @private
   */
-  _nsps: Map<string, Namespace> = new Map();
-  private parentNsps: Map<ParentNspNameMatchFn, ParentNamespace> = new Map();
+  _nsps: Map<string, Namespace<ListenEvents, EmitEvents>> = new Map();
+  private parentNsps: Map<
+    ParentNspNameMatchFn,
+    ParentNamespace<ListenEvents, EmitEvents>
+  > = new Map();
  private _adapter?: typeof Adapter;
  private _serveClient: boolean;
  private opts: Partial<EngineOptions>;
@@ -186,6 +214,10 @@ export class Server extends EventEmitter {
   */
  constructor(opts?: Partial<ServerOptions>);
  constructor(srv?: http.Server | number, opts?: Partial<ServerOptions>);
+  constructor(
+    srv: undefined | Partial<ServerOptions> | http.Server | number,
+    opts?: Partial<ServerOptions>
+  );
  constructor(
    srv: undefined | Partial<ServerOptions> | http.Server | number,
    opts: Partial<ServerOptions> = {}
@@ -217,9 +249,10 @@ export class Server extends EventEmitter {
   * @return self when setting or value when getting
   * @public
   */
-  public serveClient(v: boolean): Server;
+  public serveClient(v: boolean): this;
  public serveClient(): boolean;
-  public serveClient(v?: boolean): Server | boolean {
+  public serveClient(v?: boolean): this | boolean;
+  public serveClient(v?: boolean): this | boolean {
    if (!arguments.length) return this._serveClient;
    this._serveClient = v!;
    return this;
@@ -229,16 +262,16 @@ export class Server extends EventEmitter {
   * Executes the middleware for an incoming namespace not already created on the server.
   *
   * @param name - name of incoming namespace
-   * @param {Object} auth - the auth parameters
+   * @param auth - the auth parameters
   * @param fn - callback
   *
   * @private
   */
  _checkNamespace(
    name: string,
-    auth: object,
-    fn: (nsp: Namespace | false) => void
-  ) {
+    auth: { [key: string]: any },
+    fn: (nsp: Namespace<ListenEvents, EmitEvents> | false) => void
+  ): void {
    if (this.parentNsps.size === 0) return fn(false);

    const keysIterator = this.parentNsps.keys();
@@ -267,9 +300,10 @@ export class Server extends EventEmitter {
   * @return {Server|String} self when setting or value when getting
   * @public
   */
-  public path(v: string): Server;
+  public path(v: string): this;
  public path(): string;
-  public path(v?: string): Server | string {
+  public path(v?: string): this | string;
+  public path(v?: string): this | string {
    if (!arguments.length) return this._path;

    this._path = v!.replace(/\/$/, "");
@@ -288,9 +322,10 @@ export class Server extends EventEmitter {
   * @param v
   * @public
   */
-  public connectTimeout(v: number): Server;
+  public connectTimeout(v: number): this;
  public connectTimeout(): number;
-  public connectTimeout(v?: number): Server | number {
+  public connectTimeout(v?: number): this | number;
+  public connectTimeout(v?: number): this | number {
    if (v === undefined) return this._connectTimeout;
    this._connectTimeout = v;
    return this;
@@ -304,8 +339,9 @@ export class Server extends EventEmitter {
   * @public
   */
  public adapter(): typeof Adapter | undefined;
-  public adapter(v: typeof Adapter): Server;
-  public adapter(v?: typeof Adapter): typeof Adapter | undefined | Server {
+  public adapter(v: typeof Adapter): this;
+  public adapter(v?: typeof Adapter): typeof Adapter | undefined | this;
+  public adapter(v?: typeof Adapter): typeof Adapter | undefined | this {
    if (!arguments.length) return this._adapter;
    this._adapter = v;
    for (const nsp of this._nsps.values()) {
@@ -325,7 +361,7 @@ export class Server extends EventEmitter {
  public listen(
    srv: http.Server | number,
    opts: Partial<ServerOptions> = {}
-  ): Server {
+  ): this {
    return this.attach(srv, opts);
  }

@@ -340,7 +376,7 @@ export class Server extends EventEmitter {
  public attach(
    srv: http.Server | number,
    opts: Partial<ServerOptions> = {}
-  ): Server {
+  ): this {
    if ("function" == typeof srv) {
      const msg =
        "You are trying to attach socket.io to an express " +
@@ -380,7 +416,10 @@ export class Server extends EventEmitter {
   * @param opts - options passed to engine.io
   * @private
   */
-  private initEngine(srv: http.Server, opts: Partial<EngineAttachOptions>) {
+  private initEngine(
+    srv: http.Server,
+    opts: Partial<EngineAttachOptions>
+  ): void {
    // initialize engine
    debug("creating engine.io instance with opts %j", opts);
    this.eio = engine.attach(srv, opts);
@@ -401,7 +440,7 @@ export class Server extends EventEmitter {
   * @param srv http server
   * @private
   */
-  private attachServe(srv: http.Server) {
+  private attachServe(srv: http.Server): void {
    debug("attaching client serving req handler");

    const evs = srv.listeners("request").slice(0);
@@ -424,7 +463,7 @@ export class Server extends EventEmitter {
   * @param res
   * @private
   */
-  private serve(req: http.IncomingMessage, res: http.ServerResponse) {
+  private serve(req: http.IncomingMessage, res: http.ServerResponse): void {
    const filename = req.url!.replace(this._path, "");
    const isMap = dotMapRegex.test(filename);
    const type = isMap ? "map" : "source";
@@ -432,10 +471,11 @@ export class Server extends EventEmitter {
    // Per the standard, ETags must be quoted:
    // https://tools.ietf.org/html/rfc7232#section-2.3
    const expectedEtag = '"' + clientVersion + '"';
+    const weakEtag = "W/" + expectedEtag;

    const etag = req.headers["if-none-match"];
    if (etag) {
-      if (expectedEtag == etag) {
+      if (expectedEtag === etag || weakEtag === etag) {
        debug("serve client %s 304", type);
        res.writeHead(304);
        res.end();
@@ -468,7 +508,7 @@ export class Server extends EventEmitter {
    filename: string,
    req: http.IncomingMessage,
    res: http.ServerResponse
-  ) {
+  ): void {
    const readStream = createReadStream(
      path.join(__dirname, "../client-dist/", filename)
    );
@@ -507,7 +547,7 @@ export class Server extends EventEmitter {
   * @return self
   * @public
   */
-  public bind(engine): Server {
+  public bind(engine): this {
    this.engine = engine;
    this.engine.on("connection", this.onconnection.bind(this));
    return this;
@@ -520,9 +560,13 @@ export class Server extends EventEmitter {
   * @return self
   * @private
   */
-  private onconnection(conn): Server {
+  private onconnection(conn): this {
    debug("incoming connection with id %s", conn.id);
-    new Client(this, conn);
+    const client = new Client(this, conn);
+    if (conn.protocol === 3) {
+      // @ts-ignore
+      client.connect("/");
+    }
    return this;
  }

@@ -530,13 +574,13 @@ export class Server extends EventEmitter {
   * Looks up a namespace.
   *
   * @param {String|RegExp|Function} name nsp name
-   * @param [fn] optional, nsp `connection` ev handler
+   * @param fn optional, nsp `connection` ev handler
   * @public
   */
  public of(
    name: string | RegExp | ParentNspNameMatchFn,
-    fn?: (socket: Socket) => void
-  ) {
+    fn?: (socket: Socket<ListenEvents, EmitEvents>) => void
+  ): Namespace<ListenEvents, EmitEvents> {
    if (typeof name === "function" || name instanceof RegExp) {
      const parentNsp = new ParentNamespace(this);
      debug("initializing parent namespace %s", parentNsp.name);
@@ -594,8 +638,11 @@ export class Server extends EventEmitter {
   * @public
   */
  public use(
-    fn: (socket: Socket, next: (err?: ExtendedError) => void) => void
-  ): Server {
+    fn: (
+      socket: Socket<ListenEvents, EmitEvents>,
+      next: (err?: ExtendedError) => void
+    ) => void
+  ): this {
    this.sockets.use(fn);
    return this;
  }
@@ -603,24 +650,34 @@ export class Server extends EventEmitter {
  /**
   * Targets a room when emitting.
   *
-   * @param name
+   * @param room
   * @return self
   * @public
   */
-  public to(name: Room): Server {
-    this.sockets.to(name);
-    return this;
+  public to(room: Room | Room[]): BroadcastOperator<EmitEvents> {
+    return this.sockets.to(room);
  }

  /**
   * Targets a room when emitting.
   *
+   * @param room
+   * @return self
+   * @public
+   */
+  public in(room: Room | Room[]): BroadcastOperator<EmitEvents> {
+    return this.sockets.in(room);
+  }
+
+  /**
+   * Excludes a room when emitting.
+   *
   * @param name
   * @return self
   * @public
   */
-  public in(name: Room): Server {
-    this.sockets.in(name);
+  public except(name: Room | Room[]): Server<ListenEvents, EmitEvents> {
+    this.sockets.except(name);
    return this;
  }

@@ -630,7 +687,7 @@ export class Server extends EventEmitter {
   * @return self
   * @public
   */
-  public send(...args: readonly any[]): Server {
+  public send(...args: EventParams<EmitEvents, "message">): this {
    this.sockets.emit("message", ...args);
    return this;
  }
@@ -641,7 +698,7 @@ export class Server extends EventEmitter {
   * @return self
   * @public
   */
-  public write(...args: readonly any[]): Server {
+  public write(...args: EventParams<EmitEvents, "message">): this {
    this.sockets.emit("message", ...args);
    return this;
  }
@@ -662,9 +719,8 @@ export class Server extends EventEmitter {
   * @return self
   * @public
   */
-  public compress(compress: boolean): Server {
-    this.sockets.compress(compress);
-    return this;
+  public compress(compress: boolean): BroadcastOperator<EmitEvents> {
+    return this.sockets.compress(compress);
  }

  /**
@@ -675,9 +731,8 @@ export class Server extends EventEmitter {
   * @return self
   * @public
   */
-  public get volatile(): Server {
-    this.sockets.volatile;
-    return this;
+  public get volatile(): BroadcastOperator<EmitEvents> {
+    return this.sockets.volatile;
  }

  /**
@@ -686,9 +741,47 @@ export class Server extends EventEmitter {
   * @return self
   * @public
   */
-  public get local(): Server {
-    this.sockets.local;
-    return this;
+  public get local(): BroadcastOperator<EmitEvents> {
+    return this.sockets.local;
+  }
+
+  /**
+   * Returns the matching socket instances
+   *
+   * @public
+   */
+  public fetchSockets(): Promise<RemoteSocket<EmitEvents>[]> {
+    return this.sockets.fetchSockets();
+  }
+
+  /**
+   * Makes the matching socket instances join the specified rooms
+   *
+   * @param room
+   * @public
+   */
+  public socketsJoin(room: Room | Room[]): void {
+    return this.sockets.socketsJoin(room);
+  }
+
+  /**
+   * Makes the matching socket instances leave the specified rooms
+   *
+   * @param room
+   * @public
+   */
+  public socketsLeave(room: Room | Room[]): void {
+    return this.sockets.socketsLeave(room);
+  }
+
+  /**
+   * Makes the matching socket instances disconnect
+   *
+   * @param close - whether to close the underlying connection
+   * @public
+   */
+  public disconnectSockets(close: boolean = false): void {
+    return this.sockets.disconnectSockets(close);
  }
 }

@@ -711,4 +804,4 @@ emitterMethods.forEach(function (fn) {
 module.exports = (srv?, opts?) => new Server(srv, opts);
 module.exports.Server = Server;

-export { Socket, ServerOptions, Namespace };
+export { Socket, ServerOptions, Namespace, BroadcastOperator, RemoteSocket };
--- a/lib/namespace.ts
+++ b/lib/namespace.ts
@@ -1,10 +1,16 @@
-import { Socket, RESERVED_EVENTS } from "./socket";
+import { Socket } from "./socket";
 import type { Server } from "./index";
+import {
+  EventParams,
+  EventNames,
+  EventsMap,
+  StrictEventEmitter,
+  DefaultEventsMap,
+} from "./typed-events";
 import type { Client } from "./client";
-import { EventEmitter } from "events";
-import { PacketType } from "socket.io-parser";
 import debugModule from "debug";
 import type { Adapter, Room, SocketId } from "socket.io-adapter";
+import { BroadcastOperator, RemoteSocket } from "./broadcast-operator";

 const debug = debugModule("socket.io:namespace");

@@ -12,26 +18,41 @@ export interface ExtendedError extends Error {
  data?: any;
 }

-export class Namespace extends EventEmitter {
+export interface NamespaceReservedEventsMap<
+  ListenEvents extends EventsMap,
+  EmitEvents extends EventsMap
+> {
+  connect: (socket: Socket<ListenEvents, EmitEvents>) => void;
+  connection: (socket: Socket<ListenEvents, EmitEvents>) => void;
+}
+
+export class Namespace<
+  ListenEvents extends EventsMap = DefaultEventsMap,
+  EmitEvents extends EventsMap = ListenEvents
+> extends StrictEventEmitter<
+  {},
+  EmitEvents,
+  NamespaceReservedEventsMap<ListenEvents, EmitEvents>
+> {
  public readonly name: string;
-  public readonly sockets: Map<SocketId, Socket> = new Map();
+  public readonly sockets: Map<
+    SocketId,
+    Socket<ListenEvents, EmitEvents>
+  > = new Map();

  public adapter: Adapter;

  /** @private */
-  readonly server: Server;
+  readonly server: Server<ListenEvents, EmitEvents>;

  /** @private */
  _fns: Array<
-    (socket: Socket, next: (err?: ExtendedError) => void) => void
+    (
+      socket: Socket<ListenEvents, EmitEvents>,
+      next: (err?: ExtendedError) => void
+    ) => void
  > = [];

-  /** @private */
-  _rooms: Set<Room> = new Set();
-
-  /** @private */
-  _flags: any = {};
-
  /** @private */
  _ids: number = 0;

@@ -41,7 +62,7 @@ export class Namespace extends EventEmitter {
   * @param server instance
   * @param name
   */
-  constructor(server: Server, name: string) {
+  constructor(server: Server<ListenEvents, EmitEvents>, name: string) {
    super();
    this.server = server;
    this.name = name;
@@ -66,8 +87,11 @@ export class Namespace extends EventEmitter {
   * @public
   */
  public use(
-    fn: (socket: Socket, next: (err?: ExtendedError) => void) => void
-  ): Namespace {
+    fn: (
+      socket: Socket<ListenEvents, EmitEvents>,
+      next: (err?: ExtendedError) => void
+    ) => void
+  ): this {
    this._fns.push(fn);
    return this;
  }
@@ -79,7 +103,10 @@ export class Namespace extends EventEmitter {
   * @param fn - last fn call in the middleware
   * @private
   */
-  private run(socket: Socket, fn: (err: ExtendedError | null) => void) {
+  private run(
+    socket: Socket<ListenEvents, EmitEvents>,
+    fn: (err: ExtendedError | null) => void
+  ) {
    const fns = this._fns.slice(0);
    if (!fns.length) return fn(null);

@@ -102,25 +129,34 @@ export class Namespace extends EventEmitter {
  /**
   * Targets a room when emitting.
   *
-   * @param name
+   * @param room
   * @return self
   * @public
   */
-  public to(name: Room): Namespace {
-    this._rooms.add(name);
-    return this;
+  public to(room: Room | Room[]): BroadcastOperator<EmitEvents> {
+    return new BroadcastOperator(this.adapter).to(room);
  }

  /**
   * Targets a room when emitting.
   *
-   * @param name
+   * @param room
   * @return self
   * @public
   */
-  public in(name: Room): Namespace {
-    this._rooms.add(name);
-    return this;
+  public in(room: Room | Room[]): BroadcastOperator<EmitEvents> {
+    return new BroadcastOperator(this.adapter).in(room);
+  }
+
+  /**
+   * Excludes a room when emitting.
+   *
+   * @param room
+   * @return self
+   * @public
+   */
+  public except(room: Room | Room[]): BroadcastOperator<EmitEvents> {
+    return new BroadcastOperator(this.adapter).except(room);
  }

  /**
@@ -129,17 +165,26 @@ export class Namespace extends EventEmitter {
   * @return {Socket}
   * @private
   */
-  _add(client: Client, query, fn?: () => void): Socket {
+  _add(
+    client: Client<ListenEvents, EmitEvents>,
+    query,
+    fn?: () => void
+  ): Socket<ListenEvents, EmitEvents> {
    debug("adding socket to nsp %s", this.name);
    const socket = new Socket(this, client, query);
    this.run(socket, (err) => {
      process.nextTick(() => {
        if ("open" == client.conn.readyState) {
-          if (err)
-            return socket._error({
-              message: err.message,
-              data: err.data,
-            });
+          if (err) {
+            if (client.conn.protocol === 3) {
+              return socket._error(err.data || err.message);
+            } else {
+              return socket._error({
+                message: err.message,
+                data: err.data,
+              });
+            }
+          }

          // track socket
          this.sockets.set(socket.id, socket);
@@ -152,8 +197,8 @@ export class Namespace extends EventEmitter {
          if (fn) fn();

          // fire user-set events
-          super.emit("connect", socket);
-          super.emit("connection", socket);
+          this.emitReserved("connect", socket);
+          this.emitReserved("connection", socket);
        } else {
          debug("next called after client was closed - ignoring socket");
        }
@@ -167,7 +212,7 @@ export class Namespace extends EventEmitter {
   *
   * @private
   */
-  _remove(socket: Socket): void {
+  _remove(socket: Socket<ListenEvents, EmitEvents>): void {
    if (this.sockets.has(socket.id)) {
      this.sockets.delete(socket.id);
    } else {
@@ -181,34 +226,11 @@ export class Namespace extends EventEmitter {
   * @return Always true
   * @public
   */
-  public emit(ev: string | Symbol, ...args: any[]): true {
-    if (RESERVED_EVENTS.has(ev)) {
-      throw new Error(`"${ev}" is a reserved event name`);
-    }
-    // set up packet object
-    args.unshift(ev);
-    const packet = {
-      type: PacketType.EVENT,
-      data: args,
-    };
-
-    if ("function" == typeof args[args.length - 1]) {
-      throw new Error("Callbacks are not supported when broadcasting");
-    }
-
-    const rooms = new Set(this._rooms);
-    const flags = Object.assign({}, this._flags);
-
-    // reset flags
-    this._rooms.clear();
-    this._flags = {};
-
-    this.adapter.broadcast(packet, {
-      rooms: rooms,
-      flags: flags,
-    });
-
-    return true;
+  public emit<Ev extends EventNames<EmitEvents>>(
+    ev: Ev,
+    ...args: EventParams<EmitEvents, Ev>
+  ): boolean {
+    return new BroadcastOperator<EmitEvents>(this.adapter).emit(ev, ...args);
  }

  /**
@@ -217,7 +239,7 @@ export class Namespace extends EventEmitter {
   * @return self
   * @public
   */
-  public send(...args: readonly any[]): Namespace {
+  public send(...args: EventParams<EmitEvents, "message">): this {
    this.emit("message", ...args);
    return this;
  }
@@ -228,7 +250,7 @@ export class Namespace extends EventEmitter {
   * @return self
   * @public
   */
-  public write(...args: readonly any[]): Namespace {
+  public write(...args: EventParams<EmitEvents, "message">): this {
    this.emit("message", ...args);
    return this;
  }
@@ -240,14 +262,7 @@ export class Namespace extends EventEmitter {
   * @public
   */
  public allSockets(): Promise<Set<SocketId>> {
-    if (!this.adapter) {
-      throw new Error(
-        "No adapter for this namespace, are you trying to get the list of clients of a dynamic namespace?"
-      );
-    }
-    const rooms = new Set(this._rooms);
-    this._rooms.clear();
-    return this.adapter.sockets(rooms);
+    return new BroadcastOperator(this.adapter).allSockets();
  }

  /**
@@ -257,9 +272,8 @@ export class Namespace extends EventEmitter {
   * @return self
   * @public
   */
-  public compress(compress: boolean): Namespace {
-    this._flags.compress = compress;
-    return this;
+  public compress(compress: boolean): BroadcastOperator<EmitEvents> {
+    return new BroadcastOperator(this.adapter).compress(compress);
  }

  /**
@@ -270,9 +284,8 @@ export class Namespace extends EventEmitter {
   * @return self
   * @public
   */
-  public get volatile(): Namespace {
-    this._flags.volatile = true;
-    return this;
+  public get volatile(): BroadcastOperator<EmitEvents> {
+    return new BroadcastOperator(this.adapter).volatile;
  }

  /**
@@ -281,8 +294,46 @@ export class Namespace extends EventEmitter {
   * @return self
   * @public
   */
-  public get local(): Namespace {
-    this._flags.local = true;
-    return this;
+  public get local(): BroadcastOperator<EmitEvents> {
+    return new BroadcastOperator(this.adapter).local;
+  }
+
+  /**
+   * Returns the matching socket instances
+   *
+   * @public
+   */
+  public fetchSockets(): Promise<RemoteSocket<EmitEvents>[]> {
+    return new BroadcastOperator(this.adapter).fetchSockets();
+  }
+
+  /**
+   * Makes the matching socket instances join the specified rooms
+   *
+   * @param room
+   * @public
+   */
+  public socketsJoin(room: Room | Room[]): void {
+    return new BroadcastOperator(this.adapter).socketsJoin(room);
+  }
+
+  /**
+   * Makes the matching socket instances leave the specified rooms
+   *
+   * @param room
+   * @public
+   */
+  public socketsLeave(room: Room | Room[]): void {
+    return new BroadcastOperator(this.adapter).socketsLeave(room);
+  }
+
+  /**
+   * Makes the matching socket instances disconnect
+   *
+   * @param close - whether to close the underlying connection
+   * @public
+   */
+  public disconnectSockets(close: boolean = false): void {
+    return new BroadcastOperator(this.adapter).disconnectSockets(close);
  }
 }
--- a/lib/parent-namespace.ts
+++ b/lib/parent-namespace.ts
@@ -1,11 +1,21 @@
 import { Namespace } from "./namespace";
 import type { Server } from "./index";
+import type {
+  EventParams,
+  EventNames,
+  EventsMap,
+  DefaultEventsMap,
+} from "./typed-events";
+import type { BroadcastOptions } from "socket.io-adapter";

-export class ParentNamespace extends Namespace {
+export class ParentNamespace<
+  ListenEvents extends EventsMap = DefaultEventsMap,
+  EmitEvents extends EventsMap = ListenEvents
+> extends Namespace<ListenEvents, EmitEvents> {
  private static count: number = 0;
-  private children: Set<Namespace> = new Set();
+  private children: Set<Namespace<ListenEvents, EmitEvents>> = new Set();

-  constructor(server: Server) {
+  constructor(server: Server<ListenEvents, EmitEvents>) {
    super(server, "/_" + ParentNamespace.count++);
  }

@@ -13,29 +23,34 @@ export class ParentNamespace extends Namespace {
   * @private
   */
  _initAdapter(): void {
-    /* no-op */
+    const broadcast = (packet: any, opts: BroadcastOptions) => {
+      this.children.forEach((nsp) => {
+        nsp.adapter.broadcast(packet, opts);
+      });
+    };
+    // @ts-ignore FIXME is there a way to declare an inner class in TypeScript?
+    this.adapter = { broadcast };
  }

-  public emit(ev: string | Symbol, ...args: [...any]): true {
+  public emit<Ev extends EventNames<EmitEvents>>(
+    ev: Ev,
+    ...args: EventParams<EmitEvents, Ev>
+  ): boolean {
    this.children.forEach((nsp) => {
-      nsp._rooms = this._rooms;
-      nsp._flags = this._flags;
      nsp.emit(ev, ...args);
    });
-    this._rooms.clear();
-    this._flags = {};

    return true;
  }

-  createChild(name: string): Namespace {
+  createChild(name: string): Namespace<ListenEvents, EmitEvents> {
    const namespace = new Namespace(this.server, name);
    namespace._fns = this._fns.slice(0);
    this.listeners("connect").forEach((listener) =>
-      namespace.on("connect", listener as (...args: any[]) => void)
+      namespace.on("connect", listener)
    );
    this.listeners("connection").forEach((listener) =>
-      namespace.on("connection", listener as (...args: any[]) => void)
+      namespace.on("connection", listener)
    );
    this.children.add(namespace);
    this.server._nsps.set(name, namespace);
--- a/lib/socket.ts
+++ b/lib/socket.ts
@@ -1,11 +1,17 @@
-import { EventEmitter } from "events";
-import { PacketType } from "socket.io-parser";
+import { Packet, PacketType } from "socket.io-parser";
 import url = require("url");
 import debugModule from "debug";
 import type { Server } from "./index";
+import {
+  EventParams,
+  EventNames,
+  EventsMap,
+  StrictEventEmitter,
+  DefaultEventsMap,
+} from "./typed-events";
 import type { Client } from "./client";
-import type { Namespace } from "./namespace";
-import type { IncomingMessage } from "http";
+import type { Namespace, NamespaceReservedEventsMap } from "./namespace";
+import type { IncomingMessage, IncomingHttpHeaders } from "http";
 import type {
  Adapter,
  BroadcastFlags,
@@ -13,18 +19,44 @@ import type {
  SocketId,
 } from "socket.io-adapter";
 import base64id from "base64id";
+import type { ParsedUrlQuery } from "querystring";
+import { BroadcastOperator } from "./broadcast-operator";

 const debug = debugModule("socket.io:socket");

-export const RESERVED_EVENTS = new Set(<const>[
+type ClientReservedEvents = "connect_error";
+
+export interface SocketReservedEventsMap {
+  disconnect: (reason: string) => void;
+  disconnecting: (reason: string) => void;
+  error: (err: Error) => void;
+}
+
+// EventEmitter reserved events: https://nodejs.org/api/events.html#events_event_newlistener
+export interface EventEmitterReservedEventsMap {
+  newListener: (
+    eventName: string | Symbol,
+    listener: (...args: any[]) => void
+  ) => void;
+  removeListener: (
+    eventName: string | Symbol,
+    listener: (...args: any[]) => void
+  ) => void;
+}
+
+export const RESERVED_EVENTS: ReadonlySet<string | Symbol> = new Set<
+  | ClientReservedEvents
+  | keyof NamespaceReservedEventsMap<never, never>
+  | keyof SocketReservedEventsMap
+  | keyof EventEmitterReservedEventsMap
+>(<const>[
  "connect",
  "connect_error",
  "disconnect",
  "disconnecting",
-  // EventEmitter reserved events: https://nodejs.org/api/events.html#events_event_newlistener
  "newListener",
  "removeListener",
-]) as ReadonlySet<string | Symbol>;
+]);

 /**
 * The handshake details
@@ -33,7 +65,7 @@ export interface Handshake {
  /**
   * The headers sent as part of the handshake
   */
-  headers: object;
+  headers: IncomingHttpHeaders;

  /**
   * The date of creation (as string)
@@ -68,29 +100,39 @@ export interface Handshake {
  /**
   * The query object
   */
-  query: object;
+  query: ParsedUrlQuery;

  /**
   * The auth object
   */
-  auth: object;
+  auth: { [key: string]: any };
 }

-export class Socket extends EventEmitter {
+export class Socket<
+  ListenEvents extends EventsMap = DefaultEventsMap,
+  EmitEvents extends EventsMap = ListenEvents
+> extends StrictEventEmitter<
+  ListenEvents,
+  EmitEvents,
+  SocketReservedEventsMap
+> {
  public readonly id: SocketId;
  public readonly handshake: Handshake;
+  /**
+   * Additional information that can be attached to the Socket instance and which will be used in the fetchSockets method
+   */
+  public data: any = {};

  public connected: boolean;
  public disconnected: boolean;

-  private readonly server: Server;
+  private readonly server: Server<ListenEvents, EmitEvents>;
  private readonly adapter: Adapter;
  private acks: Map<number, () => void> = new Map();
  private fns: Array<
-    (event: Array<any>, next: (err: Error) => void) => void
+    (event: Array<any>, next: (err?: Error) => void) => void
  > = [];
  private flags: BroadcastFlags = {};
-  private _rooms: Set<Room> = new Set();
  private _anyListeners?: Array<(...args: any[]) => void>;

  /**
@@ -101,11 +143,20 @@ export class Socket extends EventEmitter {
   * @param {Object} auth
   * @package
   */
-  constructor(readonly nsp: Namespace, readonly client: Client, auth: object) {
+  constructor(
+    readonly nsp: Namespace<ListenEvents, EmitEvents>,
+    readonly client: Client<ListenEvents, EmitEvents>,
+    auth: object
+  ) {
    super();
    this.server = nsp.server;
    this.adapter = this.nsp.adapter;
-    this.id = base64id.generateId(); // don't reuse the Engine.IO id because it's sensitive information
+    if (client.conn.protocol === 3) {
+      // @ts-ignore
+      this.id = nsp.name !== "/" ? nsp.name + "#" + client.id : client.id;
+    } else {
+      this.id = base64id.generateId(); // don't reuse the Engine.IO id because it's sensitive information
+    }
    this.connected = true;
    this.disconnected = false;
    this.handshake = this.buildHandshake(auth);
@@ -137,69 +188,65 @@ export class Socket extends EventEmitter {
   * @return Always returns `true`.
   * @public
   */
-  public emit(ev: string, ...args: any[]): boolean {
+  public emit<Ev extends EventNames<EmitEvents>>(
+    ev: Ev,
+    ...args: EventParams<EmitEvents, Ev>
+  ): boolean {
    if (RESERVED_EVENTS.has(ev)) {
      throw new Error(`"${ev}" is a reserved event name`);
    }
-    args.unshift(ev);
+    const data: any[] = [ev, ...args];
    const packet: any = {
      type: PacketType.EVENT,
-      data: args,
+      data: data,
    };

    // access last argument to see if it's an ACK callback
-    if (typeof args[args.length - 1] === "function") {
-      if (this._rooms.size || this.flags.broadcast) {
-        throw new Error("Callbacks are not supported when broadcasting");
-      }
-
+    if (typeof data[data.length - 1] === "function") {
      debug("emitting packet with ack id %d", this.nsp._ids);
-      this.acks.set(this.nsp._ids, args.pop());
+      this.acks.set(this.nsp._ids, data.pop());
      packet.id = this.nsp._ids++;
    }

-    const rooms = new Set(this._rooms);
    const flags = Object.assign({}, this.flags);
-
-    // reset flags
-    this._rooms.clear();
    this.flags = {};

-    if (rooms.size || flags.broadcast) {
-      this.adapter.broadcast(packet, {
-        except: new Set([this.id]),
-        rooms: rooms,
-        flags: flags,
-      });
-    } else {
-      // dispatch packet
-      this.packet(packet, flags);
-    }
+    this.packet(packet, flags);
+
    return true;
  }

  /**
   * Targets a room when broadcasting.
   *
-   * @param name
+   * @param room
   * @return self
   * @public
   */
-  public to(name: Room): Socket {
-    this._rooms.add(name);
-    return this;
+  public to(room: Room | Room[]): BroadcastOperator<EmitEvents> {
+    return this.newBroadcastOperator().to(room);
  }

  /**
   * Targets a room when broadcasting.
   *
-   * @param name
+   * @param room
   * @return self
   * @public
   */
-  public in(name: Room): Socket {
-    this._rooms.add(name);
-    return this;
+  public in(room: Room | Room[]): BroadcastOperator<EmitEvents> {
+    return this.newBroadcastOperator().in(room);
+  }
+
+  /**
+   * Excludes a room when broadcasting.
+   *
+   * @param room
+   * @return self
+   * @public
+   */
+  public except(room: Room | Room[]): BroadcastOperator<EmitEvents> {
+    return this.newBroadcastOperator().except(room);
  }

  /**
@@ -208,7 +255,7 @@ export class Socket extends EventEmitter {
   * @return self
   * @public
   */
-  public send(...args: readonly any[]): Socket {
+  public send(...args: EventParams<EmitEvents, "message">): this {
    this.emit("message", ...args);
    return this;
  }
@@ -219,7 +266,7 @@ export class Socket extends EventEmitter {
   * @return self
   * @public
   */
-  public write(...args: readonly any[]): Socket {
+  public write(...args: EventParams<EmitEvents, "message">): this {
    this.emit("message", ...args);
    return this;
  }
@@ -231,10 +278,13 @@ export class Socket extends EventEmitter {
   * @param {Object} opts - options
   * @private
   */
-  private packet(packet, opts: any = {}) {
+  private packet(
+    packet: Omit<Packet, "nsp"> & Partial<Pick<Packet, "nsp">>,
+    opts: any = {}
+  ): void {
    packet.nsp = this.nsp.name;
    opts.compress = false !== opts.compress;
-    this.client._packet(packet, opts);
+    this.client._packet(packet as Packet, opts);
  }

  /**
@@ -286,7 +336,11 @@ export class Socket extends EventEmitter {
  _onconnect(): void {
    debug("socket connected - writing packet");
    this.join(this.id);
-    this.packet({ type: PacketType.CONNECT, data: { sid: this.id } });
+    if (this.conn.protocol === 3) {
+      this.packet({ type: PacketType.CONNECT });
+    } else {
+      this.packet({ type: PacketType.CONNECT, data: { sid: this.id } });
+    }
  }

  /**
@@ -295,7 +349,7 @@ export class Socket extends EventEmitter {
   * @param {Object} packet
   * @private
   */
-  _onpacket(packet) {
+  _onpacket(packet: Packet): void {
    debug("got packet %j", packet);
    switch (packet.type) {
      case PacketType.EVENT:
@@ -326,10 +380,10 @@ export class Socket extends EventEmitter {
  /**
   * Called upon event packet.
   *
-   * @param {Object} packet - packet object
+   * @param {Packet} packet - packet object
   * @private
   */
-  private onevent(packet): void {
+  private onevent(packet: Packet): void {
    const args = packet.data || [];
    debug("emitting event %j", args);

@@ -353,7 +407,7 @@ export class Socket extends EventEmitter {
   * @param {Number} id - packet id
   * @private
   */
-  private ack(id: number) {
+  private ack(id: number): () => void {
    const self = this;
    let sent = false;
    return function () {
@@ -377,12 +431,12 @@ export class Socket extends EventEmitter {
   *
   * @private
   */
-  private onack(packet): void {
-    const ack = this.acks.get(packet.id);
+  private onack(packet: Packet): void {
+    const ack = this.acks.get(packet.id!);
    if ("function" == typeof ack) {
      debug("calling ack %s with %j", packet.id, packet.data);
      ack.apply(this, packet.data);
-      this.acks.delete(packet.id);
+      this.acks.delete(packet.id!);
    } else {
      debug("bad ack %s", packet.id);
    }
@@ -403,9 +457,9 @@ export class Socket extends EventEmitter {
   *
   * @private
   */
-  _onerror(err): void {
+  _onerror(err: Error): void {
    if (this.listeners("error").length) {
-      super.emit("error", err);
+      this.emitReserved("error", err);
    } else {
      console.error("Missing error handler on `socket`.");
      console.error(err.stack);
@@ -420,16 +474,16 @@ export class Socket extends EventEmitter {
   *
   * @private
   */
-  _onclose(reason: string): Socket | undefined {
+  _onclose(reason: string): this | undefined {
    if (!this.connected) return this;
    debug("closing socket - reason %s", reason);
-    super.emit("disconnecting", reason);
+    this.emitReserved("disconnecting", reason);
    this.leaveAll();
    this.nsp._remove(this);
    this.client._remove(this);
    this.connected = false;
    this.disconnected = true;
-    super.emit("disconnect", reason);
+    this.emitReserved("disconnect", reason);
    return;
  }

@@ -440,7 +494,7 @@ export class Socket extends EventEmitter {
   *
   * @private
   */
-  _error(err) {
+  _error(err): void {
    this.packet({ type: PacketType.CONNECT_ERROR, data: err });
  }

@@ -452,7 +506,7 @@ export class Socket extends EventEmitter {
   *
   * @public
   */
-  public disconnect(close = false): Socket {
+  public disconnect(close = false): this {
    if (!this.connected) return this;
    if (close) {
      this.client._disconnect();
@@ -470,7 +524,7 @@ export class Socket extends EventEmitter {
   * @return {Socket} self
   * @public
   */
-  public compress(compress: boolean): Socket {
+  public compress(compress: boolean): this {
    this.flags.compress = compress;
    return this;
  }
@@ -483,7 +537,7 @@ export class Socket extends EventEmitter {
   * @return {Socket} self
   * @public
   */
-  public get volatile(): Socket {
+  public get volatile(): this {
    this.flags.volatile = true;
    return this;
  }
@@ -495,9 +549,8 @@ export class Socket extends EventEmitter {
   * @return {Socket} self
   * @public
   */
-  public get broadcast(): Socket {
-    this.flags.broadcast = true;
-    return this;
+  public get broadcast(): BroadcastOperator<EmitEvents> {
+    return this.newBroadcastOperator();
  }

  /**
@@ -506,9 +559,8 @@ export class Socket extends EventEmitter {
   * @return {Socket} self
   * @public
   */
-  public get local(): Socket {
-    this.flags.local = true;
-    return this;
+  public get local(): BroadcastOperator<EmitEvents> {
+    return this.newBroadcastOperator().local;
  }

  /**
@@ -517,14 +569,18 @@ export class Socket extends EventEmitter {
   * @param {Array} event - event that will get emitted
   * @private
   */
-  private dispatch(event): void {
+  private dispatch(event: [eventName: string, ...args: any[]]): void {
    debug("dispatching an event %j", event);
    this.run(event, (err) => {
      process.nextTick(() => {
        if (err) {
          return this._onerror(err);
        }
-        super.emit.apply(this, event);
+        if (this.connected) {
+          super.emitUntyped.apply(this, event);
+        } else {
+          debug("ignore packet received after disconnection");
+        }
      });
    });
  }
@@ -537,8 +593,8 @@ export class Socket extends EventEmitter {
   * @public
   */
  public use(
-    fn: (event: Array<any>, next: (err: Error) => void) => void
-  ): Socket {
+    fn: (event: Array<any>, next: (err?: Error) => void) => void
+  ): this {
    this.fns.push(fn);
    return this;
  }
@@ -550,11 +606,14 @@ export class Socket extends EventEmitter {
   * @param {Function} fn - last fn call in the middleware
   * @private
   */
-  private run(event: Array<any>, fn: (err: Error | null) => void) {
+  private run(
+    event: [eventName: string, ...args: any[]],
+    fn: (err: Error | null) => void
+  ): void {
    const fns = this.fns.slice(0);
    if (!fns.length) return fn(null);

-    function run(i) {
+    function run(i: number) {
      fns[i](event, function (err) {
        // upon error, short-circuit
        if (err) return fn(err);
@@ -602,7 +661,7 @@ export class Socket extends EventEmitter {
   * @param listener
   * @public
   */
-  public onAny(listener: (...args: any[]) => void): Socket {
+  public onAny(listener: (...args: any[]) => void): this {
    this._anyListeners = this._anyListeners || [];
    this._anyListeners.push(listener);
    return this;
@@ -615,7 +674,7 @@ export class Socket extends EventEmitter {
   * @param listener
   * @public
   */
-  public prependAny(listener: (...args: any[]) => void): Socket {
+  public prependAny(listener: (...args: any[]) => void): this {
    this._anyListeners = this._anyListeners || [];
    this._anyListeners.unshift(listener);
    return this;
@@ -627,7 +686,7 @@ export class Socket extends EventEmitter {
   * @param listener
   * @public
   */
-  public offAny(listener?: (...args: any[]) => void): Socket {
+  public offAny(listener?: (...args: any[]) => void): this {
    if (!this._anyListeners) {
      return this;
    }
@@ -654,4 +713,15 @@ export class Socket extends EventEmitter {
  public listenersAny() {
    return this._anyListeners || [];
  }
+
+  private newBroadcastOperator(): BroadcastOperator<EmitEvents> {
+    const flags = Object.assign({}, this.flags);
+    this.flags = {};
+    return new BroadcastOperator(
+      this.adapter,
+      new Set<Room>(),
+      new Set<Room>([this.id]),
+      flags
+    );
+  }
 }
--- a/lib/typed-events.ts
+++ b/lib/typed-events.ts
@@ -0,0 +1,179 @@
+import { EventEmitter } from "events";
+
+/**
+ * An events map is an interface that maps event names to their value, which
+ * represents the type of the `on` listener.
+ */
+export interface EventsMap {
+  [event: string]: any;
+}
+
+/**
+ * The default events map, used if no EventsMap is given. Using this EventsMap
+ * is equivalent to accepting all event names, and any data.
+ */
+export interface DefaultEventsMap {
+  [event: string]: (...args: any[]) => void;
+}
+
+/**
+ * Returns a union type containing all the keys of an event map.
+ */
+export type EventNames<Map extends EventsMap> = keyof Map & (string | symbol);
+
+/** The tuple type representing the parameters of an event listener */
+export type EventParams<
+  Map extends EventsMap,
+  Ev extends EventNames<Map>
+> = Parameters<Map[Ev]>;
+
+/**
+ * The event names that are either in ReservedEvents or in UserEvents
+ */
+export type ReservedOrUserEventNames<
+  ReservedEventsMap extends EventsMap,
+  UserEvents extends EventsMap
+> = EventNames<ReservedEventsMap> | EventNames<UserEvents>;
+
+/**
+ * Type of a listener of a user event or a reserved event. If `Ev` is in
+ * `ReservedEvents`, the reserved event listener is returned.
+ */
+export type ReservedOrUserListener<
+  ReservedEvents extends EventsMap,
+  UserEvents extends EventsMap,
+  Ev extends ReservedOrUserEventNames<ReservedEvents, UserEvents>
+> = FallbackToUntypedListener<
+  Ev extends EventNames<ReservedEvents>
+    ? ReservedEvents[Ev]
+    : Ev extends EventNames<UserEvents>
+    ? UserEvents[Ev]
+    : never
+>;
+
+/**
+ * Returns an untyped listener type if `T` is `never`; otherwise, returns `T`.
+ *
+ * This is a hack to mitigate https://github.com/socketio/socket.io/issues/3833.
+ * Needed because of https://github.com/microsoft/TypeScript/issues/41778
+ */
+type FallbackToUntypedListener<T> = [T] extends [never]
+  ? (...args: any[]) => void
+  : T;
+
+/**
+ * Interface for classes that aren't `EventEmitter`s, but still expose a
+ * strictly typed `emit` method.
+ */
+export interface TypedEventBroadcaster<EmitEvents extends EventsMap> {
+  emit<Ev extends EventNames<EmitEvents>>(
+    ev: Ev,
+    ...args: EventParams<EmitEvents, Ev>
+  ): boolean;
+}
+
+/**
+ * Strictly typed version of an `EventEmitter`. A `TypedEventEmitter` takes type
+ * parameters for mappings of event names to event data types, and strictly
+ * types method calls to the `EventEmitter` according to these event maps.
+ *
+ * @typeParam ListenEvents - `EventsMap` of user-defined events that can be
+ * listened to with `on` or `once`
+ * @typeParam EmitEvents - `EventsMap` of user-defined events that can be
+ * emitted with `emit`
+ * @typeParam ReservedEvents - `EventsMap` of reserved events, that can be
+ * emitted by socket.io with `emitReserved`, and can be listened to with
+ * `listen`.
+ */
+export abstract class StrictEventEmitter<
+    ListenEvents extends EventsMap,
+    EmitEvents extends EventsMap,
+    ReservedEvents extends EventsMap = {}
+  >
+  extends EventEmitter
+  implements TypedEventBroadcaster<EmitEvents> {
+  /**
+   * Adds the `listener` function as an event listener for `ev`.
+   *
+   * @param ev Name of the event
+   * @param listener Callback function
+   */
+  on<Ev extends ReservedOrUserEventNames<ReservedEvents, ListenEvents>>(
+    ev: Ev,
+    listener: ReservedOrUserListener<ReservedEvents, ListenEvents, Ev>
+  ): this {
+    return super.on(ev, listener);
+  }
+
+  /**
+   * Adds a one-time `listener` function as an event listener for `ev`.
+   *
+   * @param ev Name of the event
+   * @param listener Callback function
+   */
+  once<Ev extends ReservedOrUserEventNames<ReservedEvents, ListenEvents>>(
+    ev: Ev,
+    listener: ReservedOrUserListener<ReservedEvents, ListenEvents, Ev>
+  ): this {
+    return super.once(ev, listener);
+  }
+
+  /**
+   * Emits an event.
+   *
+   * @param ev Name of the event
+   * @param args Values to send to listeners of this event
+   */
+  emit<Ev extends EventNames<EmitEvents>>(
+    ev: Ev,
+    ...args: EventParams<EmitEvents, Ev>
+  ): boolean {
+    return super.emit(ev, ...args);
+  }
+
+  /**
+   * Emits a reserved event.
+   *
+   * This method is `protected`, so that only a class extending
+   * `StrictEventEmitter` can emit its own reserved events.
+   *
+   * @param ev Reserved event name
+   * @param args Arguments to emit along with the event
+   */
+  protected emitReserved<Ev extends EventNames<ReservedEvents>>(
+    ev: Ev,
+    ...args: EventParams<ReservedEvents, Ev>
+  ): boolean {
+    return super.emit(ev, ...args);
+  }
+
+  /**
+   * Emits an event.
+   *
+   * This method is `protected`, so that only a class extending
+   * `StrictEventEmitter` can get around the strict typing. This is useful for
+   * calling `emit.apply`, which can be called as `emitUntyped.apply`.
+   *
+   * @param ev Event name
+   * @param args Arguments to emit along with the event
+   */
+  protected emitUntyped(ev: string, ...args: any[]): boolean {
+    return super.emit(ev, ...args);
+  }
+
+  /**
+   * Returns the listeners listening to an event.
+   *
+   * @param event Event name
+   * @returns Array of listeners subscribed to `event`
+   */
+  listeners<Ev extends ReservedOrUserEventNames<ReservedEvents, ListenEvents>>(
+    event: Ev
+  ): ReservedOrUserListener<ReservedEvents, ListenEvents, Ev>[] {
+    return super.listeners(event) as ReservedOrUserListener<
+      ReservedEvents,
+      ListenEvents,
+      Ev
+    >[];
+  }
+}
--- a/package-lock.json
+++ b/package-lock.json
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
  "name": "socket.io",
-  "version": "3.0.5",
+  "version": "4.0.1",
  "description": "node.js realtime framework server",
  "keywords": [
    "realtime",
@@ -37,7 +37,9 @@
  },
  "scripts": {
    "compile": "rimraf ./dist && tsc",
-    "test": "npm run format:check && npm run compile && nyc mocha --require ts-node/register --reporter spec --slow 200 --bail --timeout 10000 test/socket.io.ts",
+    "test": "npm run format:check && npm run compile && npm run test:types && npm run test:unit",
+    "test:types": "tsd",
+    "test:unit": "nyc mocha --require ts-node/register --reporter spec --slow 200 --bail --timeout 10000 test/socket.io.ts",
    "format:check": "prettier --check \"lib/**/*.ts\" \"test/**/*.ts\"",
    "format:fix": "prettier --write \"lib/**/*.ts\" \"test/**/*.ts\"",
    "prepack": "npm run compile"
@@ -45,12 +47,12 @@
  "dependencies": {
    "@types/cookie": "^0.4.0",
    "@types/cors": "^2.8.8",
-    "@types/node": "^14.14.10",
+    "@types/node": ">=10.0.0",
    "accepts": "~1.3.4",
    "base64id": "~2.0.0",
    "debug": "~4.3.1",
-    "engine.io": "~4.0.6",
-    "socket.io-adapter": "~2.0.3",
+    "engine.io": "~5.0.0",
+    "socket.io-adapter": "~2.2.0",
    "socket.io-parser": "~4.0.3"
  },
  "devDependencies": {
@@ -63,10 +65,12 @@
    "nyc": "^15.1.0",
    "prettier": "^2.2.0",
    "rimraf": "^3.0.2",
-    "socket.io-client": "3.0.5",
+    "socket.io-client": "4.0.1",
+    "socket.io-client-v2": "npm:socket.io-client@^2.4.0",
    "superagent": "^6.1.0",
    "supertest": "^6.0.1",
    "ts-node": "^9.0.0",
+    "tsd": "^0.14.0",
    "typescript": "^4.1.2"
  },
  "contributors": [
@@ -89,5 +93,8 @@
  ],
  "engines": {
    "node": ">=10.0.0"
+  },
+  "tsd": {
+    "directory": "test"
  }
 }
--- a/test/socket.io.test-d.ts
+++ b/test/socket.io.test-d.ts
@@ -0,0 +1,232 @@
+"use strict";
+import { Server, Socket } from "..";
+import type { DefaultEventsMap } from "../lib/typed-events";
+import { createServer } from "http";
+import { expectError, expectType } from "tsd";
+
+// This file is run by tsd, not mocha.
+
+describe("server", () => {
+  describe("no event map", () => {
+    describe("on", () => {
+      it("infers correct types for listener parameters of reserved events", (done) => {
+        const srv = createServer();
+        const sio = new Server(srv);
+        srv.listen(() => {
+          sio.on("connection", (s) => {
+            expectType<Socket<DefaultEventsMap, DefaultEventsMap>>(s);
+            s.on("disconnect", (reason) => {
+              expectType<string>(reason);
+            });
+            s.on("disconnecting", (reason) => {
+              expectType<string>(reason);
+            });
+          });
+          sio.on("connect", (s) => {
+            expectType<Socket<DefaultEventsMap, DefaultEventsMap>>(s);
+          });
+          done();
+        });
+      });
+
+      it("infers 'any' for listener parameters of other events", (done) => {
+        const srv = createServer();
+        const sio = new Server(srv);
+        srv.listen(() => {
+          sio.on("connection", (s) => {
+            s.on("random", (a, b, c) => {
+              expectType<any>(a);
+              expectType<any>(b);
+              expectType<any>(c);
+              done();
+            });
+            s.emit("random", 1, "2", [3]);
+          });
+        });
+      });
+
+      it("infers 'any' for listener parameters of other events using enums", () => {
+        const srv = createServer();
+        const sio = new Server(srv);
+        srv.listen(() => {
+          sio.on("connection", (socket) => {
+            expectType<Socket<DefaultEventsMap, DefaultEventsMap>>(socket);
+          });
+
+          enum Events {
+            CONNECTION = "connection",
+            TEST = "test",
+          }
+
+          sio.on(Events.CONNECTION, (socket) => {
+            // TODO(#3833): Make this expect `Socket<DefaultEventsMap, DefaultEventsMap>`
+            expectType<any>(socket);
+
+            socket.on("test", (a, b, c) => {
+              expectType<any>(a);
+              expectType<any>(b);
+              expectType<any>(c);
+            });
+
+            socket.on(Events.TEST, (a, b, c) => {
+              expectType<any>(a);
+              expectType<any>(b);
+              expectType<any>(c);
+            });
+          });
+        });
+      });
+    });
+
+    describe("emit", () => {
+      it("accepts any parameters", () => {
+        const srv = createServer();
+        const sio = new Server(srv);
+        srv.listen(() => {
+          sio.on("connection", (s) => {
+            s.emit("random", 1, "2", [3]);
+            s.emit("no parameters");
+          });
+        });
+      });
+    });
+  });
+
+  describe("single event map", () => {
+    interface BidirectionalEvents {
+      random: (a: number, b: string, c: number[]) => void;
+    }
+
+    describe("on", () => {
+      it("infers correct types for listener parameters", (done) => {
+        const srv = createServer();
+        const sio = new Server<BidirectionalEvents>(srv);
+        expectType<Server<BidirectionalEvents, BidirectionalEvents>>(sio);
+        srv.listen(() => {
+          sio.on("connection", (s) => {
+            expectType<Socket<BidirectionalEvents, BidirectionalEvents>>(s);
+            s.on("random", (a, b, c) => {
+              expectType<number>(a);
+              expectType<string>(b);
+              expectType<number[]>(c);
+              done();
+            });
+          });
+        });
+      });
+
+      it("does not accept arguments of wrong types", (done) => {
+        const srv = createServer();
+        const sio = new Server<BidirectionalEvents>(srv);
+        expectError(sio.on("random", (a, b, c) => {}));
+        srv.listen(() => {
+          expectError(sio.on("wrong name", (s) => {}));
+          sio.on("connection", (s) => {
+            s.on("random", (a, b, c) => {});
+            expectError(s.on("random"));
+            expectError(s.on("random", (a, b, c, d) => {}));
+            expectError(s.on(2, 3));
+          });
+        });
+      });
+    });
+
+    describe("emit", () => {
+      it("accepts arguments of the correct types", () => {
+        const srv = createServer();
+        const sio = new Server<BidirectionalEvents>(srv);
+        srv.listen(() => {
+          sio.on("connection", (s) => {
+            s.emit("random", 1, "2", [3]);
+          });
+        });
+      });
+
+      it("does not accept arguments of the wrong types", () => {
+        const srv = createServer();
+        const sio = new Server<BidirectionalEvents>(srv);
+        srv.listen(() => {
+          sio.on("connection", (s) => {
+            expectError(s.emit("noParameter", 2));
+            expectError(s.emit("oneParameter"));
+            expectError(s.emit("random"));
+            expectError(s.emit("oneParameter", 2, 3));
+            expectError(s.emit("random", (a, b, c) => {}));
+            expectError(s.emit("wrong name", () => {}));
+            expectError(s.emit("complicated name with spaces", 2));
+          });
+        });
+      });
+    });
+  });
+
+  describe("listen and emit event maps", () => {
+    interface ClientToServerEvents {
+      helloFromClient: (message: string) => void;
+    }
+
+    interface ServerToClientEvents {
+      helloFromServer: (message: string, x: number) => void;
+    }
+
+    describe("on", () => {
+      it("infers correct types for listener parameters", (done) => {
+        const srv = createServer();
+        const sio = new Server<ClientToServerEvents, ServerToClientEvents>(srv);
+        expectType<Server<ClientToServerEvents, ServerToClientEvents>>(sio);
+        srv.listen(() => {
+          sio.on("connection", (s) => {
+            expectType<Socket<ClientToServerEvents, ServerToClientEvents>>(s);
+            s.on("helloFromClient", (message) => {
+              expectType<string>(message);
+              done();
+            });
+          });
+        });
+      });
+
+      it("does not accept emit events", (done) => {
+        const srv = createServer();
+        const sio = new Server<ClientToServerEvents, ServerToClientEvents>(srv);
+        srv.listen(() => {
+          sio.on("connection", (s) => {
+            expectError(
+              s.on("helloFromServer", (message, number) => {
+                done();
+              })
+            );
+          });
+        });
+      });
+    });
+
+    describe("emit", () => {
+      it("accepts arguments of the correct types", (done) => {
+        const srv = createServer();
+        const sio = new Server<ClientToServerEvents, ServerToClientEvents>(srv);
+        srv.listen(() => {
+          sio.on("connection", (s) => {
+            s.emit("helloFromServer", "hi", 10);
+            done();
+          });
+        });
+      });
+
+      it("does not accept arguments of wrong types", (done) => {
+        const srv = createServer();
+        const sio = new Server<ClientToServerEvents, ServerToClientEvents>(srv);
+        srv.listen(() => {
+          sio.on("connection", (s) => {
+            expectError(s.emit("helloFromClient", "hi"));
+            expectError(s.emit("helloFromServer", "hi", 10, "10"));
+            expectError(s.emit("helloFromServer", "hi", "10"));
+            expectError(s.emit("helloFromServer", 0, 0));
+            expectError(s.emit("wrong name", 10));
+            expectError(s.emit("wrong name"));
+            done();
+          });
+        });
+      });
+    });
+  });
+});
--- a/test/socket.io.ts
+++ b/test/socket.io.ts
@@ -8,14 +8,15 @@ import { exec } from "child_process";
 import request from "supertest";
 import expect from "expect.js";
 import type { AddressInfo } from "net";
-
-const ioc = require("socket.io-client");
+import * as io_v2 from "socket.io-client-v2";
+import type { SocketId } from "socket.io-adapter";
+import { io as ioc, Socket as ClientSocket } from "socket.io-client";

 import "./support/util";
-import exp = require("constants");
+import "./utility-methods";

 // Creates a socket.io client for the given server
-function client(srv, nsp?: string | object, opts?: object) {
+function client(srv, nsp?: string | object, opts?: object): ClientSocket {
  if ("object" == typeof nsp) {
    opts = nsp;
    nsp = undefined;
@@ -26,6 +27,18 @@ function client(srv, nsp?: string | object, opts?: object) {
  return ioc(url, opts);
 }

+const success = (sio, clientSocket, done) => {
+  sio.close();
+  clientSocket.close();
+  done();
+};
+
+const waitFor = (emitter, event) => {
+  return new Promise((resolve) => {
+    emitter.once(event, resolve);
+  });
+};
+
 describe("socket.io", () => {
  it("should be the same version as client", () => {
    const version = require("../package").version;
@@ -116,6 +129,19 @@ describe("socket.io", () => {
          });
      });

+      it("should handle 304", (done) => {
+        const srv = createServer();
+        new Server(srv);
+        request(srv)
+          .get("/socket.io/socket.io.js")
+          .set("If-None-Match", 'W/"' + clientVersion + '"')
+          .end((err, res) => {
+            if (err) return done(err);
+            expect(res.statusCode).to.be(304);
+            done();
+          });
+      });
+
      it("should not serve static files", (done) => {
        const srv = createServer();
        new Server(srv, { serveClient: false });
@@ -202,7 +228,7 @@ describe("socket.io", () => {
      });
      request
        .options("http://localhost:54013/socket.io/default/")
-        .query({ transport: "polling" })
+        .query({ transport: "polling", EIO: 4 })
        .set("Origin", "http://localhost:54023")
        .end((err, res) => {
          expect(res.status).to.be(204);
@@ -230,7 +256,7 @@ describe("socket.io", () => {
      });
      request
        .get("http://localhost:54014/socket.io/default/")
-        .query({ transport: "polling" })
+        .query({ transport: "polling", EIO: 4 })
        .set("Origin", "http://localhost:54024")
        .end((err, res) => {
          expect(res.status).to.be(200);
@@ -250,7 +276,7 @@ describe("socket.io", () => {

      request
        .get("http://localhost:54022/socket.io/default/")
-        .query({ transport: "polling" })
+        .query({ transport: "polling", EIO: 4 })
        .end((err, res) => {
          expect(res.status).to.be(200);
          done();
@@ -264,7 +290,7 @@ describe("socket.io", () => {
      request
        .get("http://localhost:54023/socket.io/default/")
        .set("origin", "http://foo.example")
-        .query({ transport: "polling" })
+        .query({ transport: "polling", EIO: 4 })
        .end((err, res) => {
          expect(res.status).to.be(403);
          done();
@@ -307,7 +333,9 @@ describe("socket.io", () => {
      const net = require("net");
      const server = net.createServer();

-      const clientSocket = ioc("ws://0.0.0.0:" + PORT, { reconnection: false });
+      const clientSocket = ioc("ws://0.0.0.0:" + PORT, {
+        reconnection: false,
+      });

      clientSocket.on("disconnect", () => {
        expect(Object.keys(sio._nsps["/"].sockets).length).to.equal(0);
@@ -363,10 +391,22 @@ describe("socket.io", () => {
      expect(sio.write).to.be.a("function");
      expect(sio.allSockets).to.be.a("function");
      expect(sio.compress).to.be.a("function");
-      expect(sio.volatile).to.be(sio);
-      expect(sio.local).to.be(sio);
-      expect(sio.sockets._flags).to.eql({ volatile: true, local: true });
-      delete sio.sockets._flags;
+    });
+
+    it("should return an immutable broadcast operator", () => {
+      const sio = new Server();
+      const operator = sio.local.to(["room1", "room2"]).except("room3");
+      operator.compress(true).emit("hello");
+      operator.volatile.emit("hello");
+      operator.to("room4").emit("hello");
+      operator.except("room5").emit("hello");
+      sio.to("room6").emit("hello");
+      // @ts-ignore
+      expect(operator.rooms).to.contain("room1", "room2");
+      // @ts-ignore
+      expect(operator.exceptRooms).to.contain("room3");
+      // @ts-ignore
+      expect(operator.flags).to.eql({ local: true });
    });

    it("should automatically connect", (done) => {
@@ -567,7 +607,7 @@ describe("socket.io", () => {
      const srv = createServer();
      const sio = new Server(srv);
      const chatSids: string[] = [];
-      let otherSid = null;
+      let otherSid: SocketId | null = null;
      srv.listen(() => {
        const c1 = client(srv, "/chat");
        const c2 = client(srv, "/chat", { forceNew: true });
@@ -594,9 +634,9 @@ describe("socket.io", () => {
    it("should find all clients in a namespace room", (done) => {
      const srv = createServer();
      const sio = new Server(srv);
-      let chatFooSid = null;
-      let chatBarSid = null;
-      let otherSid = null;
+      let chatFooSid: SocketId | null = null;
+      let chatBarSid: SocketId | null = null;
+      let otherSid: SocketId | null = null;
      srv.listen(() => {
        const c1 = client(srv, "/chat");
        const c2 = client(srv, "/chat", { forceNew: true });
@@ -633,9 +673,9 @@ describe("socket.io", () => {
    it("should find all clients across namespace rooms", (done) => {
      const srv = createServer();
      const sio = new Server(srv);
-      let chatFooSid = null;
-      let chatBarSid = null;
-      let otherSid = null;
+      let chatFooSid: SocketId | null = null;
+      let chatBarSid: SocketId | null = null;
+      let otherSid: SocketId | null = null;
      srv.listen(() => {
        const c1 = client(srv, "/chat");
        const c2 = client(srv, "/chat", { forceNew: true });
@@ -798,6 +838,57 @@ describe("socket.io", () => {
      });
    });

+    it("should exclude a specific socket when emitting", (done) => {
+      const srv = createServer();
+      const sio = new Server(srv);
+
+      const nsp = sio.of("/nsp");
+
+      srv.listen(() => {
+        const socket1 = client(srv, "/nsp");
+        const socket2 = client(srv, "/nsp");
+
+        socket2.on("a", () => {
+          done(new Error("not"));
+        });
+        socket1.on("a", () => {
+          done();
+        });
+
+        socket2.on("connect", () => {
+          nsp.except(socket2.id).emit("a");
+        });
+      });
+    });
+
+    it("should exclude a specific room when emitting", (done) => {
+      const srv = createServer();
+      const sio = new Server(srv);
+
+      const nsp = sio.of("/nsp");
+
+      srv.listen(() => {
+        const socket1 = client(srv, "/nsp");
+        const socket2 = client(srv, "/nsp");
+
+        socket1.on("a", () => {
+          done();
+        });
+        socket2.on("a", () => {
+          done(new Error("not"));
+        });
+
+        nsp.on("connection", (socket) => {
+          socket.on("broadcast", () => {
+            socket.join("room1");
+            nsp.except("room1").emit("a");
+          });
+        });
+
+        socket2.emit("broadcast");
+      });
+    });
+
    describe("dynamic namespaces", () => {
      it("should allow connections to dynamic namespaces with a regex", (done) => {
        const srv = createServer();
@@ -864,7 +955,7 @@ describe("socket.io", () => {
      srv.listen(() => {
        const clientSocket = client(srv, { reconnection: false });
        clientSocket.on("connect", function init() {
-          clientSocket.removeListener("connect", init);
+          clientSocket.off("connect", init);
          clientSocket.io.engine.close();

          clientSocket.connect();
@@ -953,7 +1044,7 @@ describe("socket.io", () => {
              done();
            });
          });
-          s.client.ondata(null);
+          (s as any).client.ondata(null);
        });
      });
    });
@@ -1762,6 +1853,33 @@ describe("socket.io", () => {
      });
    });

+    it("should ignore a packet received after disconnection", (done) => {
+      const srv = createServer();
+      const sio = new Server(srv);
+
+      srv.listen(() => {
+        const clientSocket = client(srv);
+
+        const success = () => {
+          clientSocket.close();
+          sio.close();
+          done();
+        };
+
+        sio.on("connection", (socket) => {
+          socket.on("test", () => {
+            done(new Error("should not happen"));
+          });
+          socket.on("disconnect", success);
+        });
+
+        clientSocket.on("connect", () => {
+          clientSocket.emit("test", Buffer.alloc(10));
+          clientSocket.disconnect();
+        });
+      });
+    });
+
    describe("onAny", () => {
      it("should call listener", (done) => {
        const srv = createServer();
@@ -2114,7 +2232,7 @@ describe("socket.io", () => {
          expect(s.rooms).to.contain(s.id, "a", "b", "c");
          s.leave("b");
          expect(s.rooms).to.contain(s.id, "a", "c");
-          s.leaveAll();
+          (s as any).leaveAll();
          expect(s.rooms.size).to.eql(0);
          done();
        });
@@ -2150,7 +2268,7 @@ describe("socket.io", () => {
          expect(s.rooms).to.contain(s.id, "a", "b");
          s.leave("unknown");
          expect(s.rooms).to.contain(s.id, "a", "b");
-          s.leaveAll();
+          (s as any).leaveAll();
          expect(s.rooms.size).to.eql(0);
          done();
        });
@@ -2170,6 +2288,106 @@ describe("socket.io", () => {
        });
      });
    });
+
+    it("should exclude specific sockets when broadcasting", (done) => {
+      const srv = createServer();
+      const sio = new Server(srv);
+
+      srv.listen(() => {
+        const socket1 = client(srv, { multiplex: false });
+        const socket2 = client(srv, { multiplex: false });
+        const socket3 = client(srv, { multiplex: false });
+
+        socket2.on("a", () => {
+          done(new Error("not"));
+        });
+        socket3.on("a", () => {
+          done(new Error("not"));
+        });
+        socket1.on("a", () => {
+          done();
+        });
+
+        sio.on("connection", (socket) => {
+          socket.on("exclude", (id) => {
+            socket.broadcast.except(id).emit("a");
+          });
+        });
+
+        socket2.on("connect", () => {
+          socket3.emit("exclude", socket2.id);
+        });
+      });
+    });
+
+    it("should exclude a specific room when broadcasting", (done) => {
+      const srv = createServer();
+      const sio = new Server(srv);
+
+      srv.listen(() => {
+        const socket1 = client(srv, { multiplex: false });
+        const socket2 = client(srv, { multiplex: false });
+        const socket3 = client(srv, { multiplex: false });
+
+        socket2.on("a", () => {
+          done(new Error("not"));
+        });
+        socket3.on("a", () => {
+          done(new Error("not"));
+        });
+        socket1.on("a", () => {
+          done();
+        });
+
+        sio.on("connection", (socket) => {
+          socket.on("join", (room, cb) => {
+            socket.join(room);
+            cb();
+          });
+          socket.on("broadcast", () => {
+            socket.broadcast.except("room1").emit("a");
+          });
+        });
+
+        socket2.emit("join", "room1", () => {
+          socket3.emit("broadcast");
+        });
+      });
+    });
+
+    it("should return an immutable broadcast operator", (done) => {
+      const srv = createServer();
+      const sio = new Server(srv);
+
+      srv.listen(() => {
+        const clientSocket = client(srv, { multiplex: false });
+
+        sio.on("connection", (socket: Socket) => {
+          const operator = socket.local
+            .compress(false)
+            .to(["room1", "room2"])
+            .except("room3");
+          operator.compress(true).emit("hello");
+          operator.volatile.emit("hello");
+          operator.to("room4").emit("hello");
+          operator.except("room5").emit("hello");
+          socket.emit("hello");
+          socket.to("room6").emit("hello");
+          // @ts-ignore
+          expect(operator.rooms).to.contain("room1", "room2");
+          // @ts-ignore
+          expect(operator.rooms).to.not.contain("room4", "room5", "room6");
+          // @ts-ignore
+          expect(operator.exceptRooms).to.contain("room3");
+          // @ts-ignore
+          expect(operator.flags).to.eql({ local: true, compress: false });
+
+          clientSocket.close();
+          sio.close();
+          done();
+        });
+      });
+    });
  });

  describe("middleware", () => {
@@ -2237,6 +2455,7 @@ describe("socket.io", () => {
        socket.on("connect_error", (err) => {
          expect(err).to.be.an(Error);
          expect(err.message).to.eql("Authentication error");
+          // @ts-ignore
          expect(err.data).to.eql({ a: "b", c: 3 });
          done();
        });
@@ -2253,7 +2472,7 @@ describe("socket.io", () => {
      srv.listen(() => {
        const socket = client(srv);
        sio.on("connection", (socket) => {
-          expect(socket.name).to.be("guillermo");
+          expect((socket as any).name).to.be("guillermo");
          done();
        });
      });
@@ -2430,4 +2649,74 @@ describe("socket.io", () => {
      });
    });
  });
+
+  describe("v2 compatibility", () => {
+    it("should connect if `allowEIO3` is true", (done) => {
+      const srv = createServer();
+      const sio = new Server(srv, {
+        allowEIO3: true,
+      });
+
+      srv.listen(async () => {
+        const port = (srv.address() as AddressInfo).port;
+        const clientSocket = io_v2.connect(`http://localhost:${port}`, {
+          multiplex: false,
+        });
+
+        const [socket]: Array<any> = await Promise.all([
+          waitFor(sio, "connection"),
+          waitFor(clientSocket, "connect"),
+        ]);
+
+        expect(socket.id).to.eql(clientSocket.id);
+        success(sio, clientSocket, done);
+      });
+    });
+
+    it("should be able to connect to a namespace with a query", (done) => {
+      const srv = createServer();
+      const sio = new Server(srv, {
+        allowEIO3: true,
+      });
+
+      srv.listen(async () => {
+        const port = (srv.address() as AddressInfo).port;
+        const clientSocket = io_v2.connect(
+          `http://localhost:${port}/the-namespace`,
+          {
+            multiplex: false,
+          }
+        );
+        clientSocket.query = { test: "123" };
+
+        const [socket]: Array<any> = await Promise.all([
+          waitFor(sio.of("/the-namespace"), "connection"),
+          waitFor(clientSocket, "connect"),
+        ]);
+
+        expect(socket.handshake.auth).to.eql({ test: "123" });
+        success(sio, clientSocket, done);
+      });
+    });
+
+    it("should not connect if `allowEIO3` is false (default)", (done) => {
+      const srv = createServer();
+      const sio = new Server(srv);
+
+      srv.listen(() => {
+        const port = (srv.address() as AddressInfo).port;
+        const clientSocket = io_v2.connect(`http://localhost:${port}`, {
+          multiplex: false,
+        });
+
+        clientSocket.on("connect", () => {
+          done(new Error("should not happen"));
+        });
+
+        clientSocket.on("connect_error", () => {
+          success(sio, clientSocket, done);
+        });
+      });
+    });
+  });
 });
--- a/test/utility-methods.ts
+++ b/test/utility-methods.ts
@@ -0,0 +1,176 @@
+import { createServer } from "http";
+import { Server, Socket } from "..";
+import { io as ioc, Socket as ClientSocket } from "socket.io-client";
+import { Adapter, BroadcastOptions } from "socket.io-adapter";
+import expect from "expect.js";
+import type { AddressInfo } from "net";
+
+import "./support/util";
+
+const SOCKETS_COUNT = 3;
+
+const createPartialDone = (
+  count: number,
+  done: () => void,
+  callback?: () => void
+) => {
+  let i = 0;
+  return () => {
+    i++;
+    if (i === count) {
+      done();
+      if (callback) {
+        callback();
+      }
+    }
+  };
+};
+
+class DummyAdapter extends Adapter {
+  fetchSockets(opts: BroadcastOptions): Promise<any[]> {
+    return Promise.resolve([
+      {
+        id: "42",
+        handshake: {
+          headers: {
+            accept: "*/*",
+          },
+          query: {
+            transport: "polling",
+            EIO: "4",
+          },
+        },
+        rooms: ["42", "room1"],
+        data: {
+          username: "john",
+        },
+      },
+    ]);
+  }
+}
+
+describe("socket.io", () => {
+  let io: Server, clientSockets: ClientSocket[], serverSockets: Socket[];
+  beforeEach((done) => {
+    const srv = createServer();
+    io = new Server(srv);
+    srv.listen(() => {
+      const port = (srv.address() as AddressInfo).port;
+
+      clientSockets = [];
+      for (let i = 0; i < SOCKETS_COUNT; i++) {
+        clientSockets.push(ioc(`http://localhost:${port}`));
+      }
+
+      serverSockets = [];
+      io.on("connection", (socket: Socket) => {
+        serverSockets.push(socket);
+        if (serverSockets.length === SOCKETS_COUNT) {
+          done();
+        }
+      });
+    });
+  });
+
+  afterEach(() => {
+    io.close();
+    clientSockets.forEach((socket) => socket.disconnect());
+  });
+
+  describe("utility methods", () => {
+    describe("fetchSockets", () => {
+      it("returns all socket instances", async () => {
+        const sockets = await io.fetchSockets();
+        expect(sockets.length).to.eql(3);
+      });
+
+      it("returns all socket instances in the given room", async () => {
+        serverSockets[0].join(["room1", "room2"]);
+        serverSockets[1].join("room1");
+        serverSockets[2].join("room2");
+        const sockets = await io.in("room1").fetchSockets();
+        expect(sockets.length).to.eql(2);
+      });
+
+      it("works with a custom adapter", async () => {
+        io.adapter(DummyAdapter);
+        const sockets = await io.fetchSockets();
+        expect(sockets.length).to.eql(1);
+        const remoteSocket = sockets[0];
+        expect(remoteSocket.id).to.eql("42");
+        expect(remoteSocket.rooms).to.contain("42", "room1");
+        expect(remoteSocket.data).to.eql({ username: "john" });
+      });
+    });
+
+    describe("socketsJoin", () => {
+      it("makes all socket instances join the given room", () => {
+        io.socketsJoin("room1");
+        serverSockets.forEach((socket) => {
+          expect(socket.rooms).to.contain("room1");
+        });
+      });
+
+      it("makes all socket instances in a room join the given room", () => {
+        serverSockets[0].join(["room1", "room2"]);
+        serverSockets[1].join("room1");
+        serverSockets[2].join("room2");
+        io.in("room1").socketsJoin("room3");
+        expect(serverSockets[0].rooms).to.contain("room3");
+        expect(serverSockets[1].rooms).to.contain("room3");
+        expect(serverSockets[2].rooms).to.not.contain("room3");
+      });
+    });
+
+    describe("socketsLeave", () => {
+      it("makes all socket instances leave the given room", () => {
+        serverSockets[0].join(["room1", "room2"]);
+        serverSockets[1].join("room1");
+        serverSockets[2].join("room2");
+        io.socketsLeave("room1");
+        expect(serverSockets[0].rooms).to.contain("room2");
+        expect(serverSockets[0].rooms).to.not.contain("room1");
+        expect(serverSockets[1].rooms).to.not.contain("room1");
+      });
+
+      it("makes all socket instances in a room leave the given room", () => {
+        serverSockets[0].join(["room1", "room2"]);
+        serverSockets[1].join("room1");
+        serverSockets[2].join("room2");
+        io.in("room2").socketsLeave("room1");
+        expect(serverSockets[0].rooms).to.contain("room2");
+        expect(serverSockets[0].rooms).to.not.contain("room1");
+        expect(serverSockets[1].rooms).to.contain("room1");
+      });
+    });
+
+    describe("disconnectSockets", () => {
+      it("makes all socket instances disconnect", (done) => {
+        io.disconnectSockets(true);
+
+        const partialDone = createPartialDone(3, done);
+
+        clientSockets[0].on("disconnect", partialDone);
+        clientSockets[1].on("disconnect", partialDone);
+        clientSockets[2].on("disconnect", partialDone);
+      });
+
+      it("makes all socket instances in a room disconnect", (done) => {
+        serverSockets[0].join(["room1", "room2"]);
+        serverSockets[1].join("room1");
+        serverSockets[2].join("room2");
+        io.in("room2").disconnectSockets(true);
+
+        const partialDone = createPartialDone(2, done, () => {
+          clientSockets[1].off("disconnect");
+        });
+
+        clientSockets[0].on("disconnect", partialDone);
+        clientSockets[1].on("disconnect", () => {
+          done(new Error("should not happen"));
+        });
+        clientSockets[2].on("disconnect", partialDone);
+      });
+    });
+  });
+});
--- a/wrapper.mjs
+++ b/wrapper.mjs
@@ -1,3 +1,3 @@
 import io from "./dist/index.js";

-export const Server = io.Server;
+export const {Server, Namespace, Socket} = io;
Author	SHA1	Message	Date
Damien Arrachequesne	1faa7e3aea	chore(release): 4.0.1 Diff: https://github.com/socketio/socket.io/compare/4.0.0...4.0.1	2021-04-01 01:25:56 +02:00
Maxime Kjaer	a11152f42b	fix(typings): add fallback to untyped event listener (#3834 ) Related: https://github.com/socketio/socket.io/issues/3833	2021-03-31 11:37:37 +02:00
Maxime Kjaer	259f29720b	docs(examples): remove unnecessary type annotations (#3855 ) Typed events in Socket.IO 4.0 remove the need for writing type annotations in callbacks of reserved events.	2021-03-24 23:12:22 +01:00
Damien Arrachequesne	b4ae8d2e19	docs(examples): update all examples to Socket.IO v4	2021-03-18 15:07:04 +01:00
Adán Toscano	64be1c9985	docs(examples): fix chat example (#3787 )	2021-03-18 14:06:13 +01:00
Solomon English	1a72ae4fe2	fix(typings): update return type from emit (#3843 ) ``` (channel ? io.to(channel) : io).emit("stuff", message); ``` would no longer compile. Related: https://github.com/socketio/socket.io/issues/3844	2021-03-18 11:36:34 +01:00
Damien Arrachequesne	5eaeffc8e2	chore(release): 4.0.0 Diff: https://github.com/socketio/socket.io/compare/3.1.2...4.0.0	2021-03-10 12:43:53 +01:00
Damien Arrachequesne	1b6d6de4ed	chore: include Engine.IO v5 Release notes: https://github.com/socketio/engine.io/releases/tag/5.0.0	2021-03-10 11:14:33 +01:00
Maxime Kjaer	0107510ba8	feat: add support for typed events (#3822 ) Syntax: ```ts interface ClientToServerEvents { "my-event": (a: number, b: string, c: number[]) => void; } interface ServerToClientEvents { hello: (message: string) => void; } const io = new Server<ClientToServerEvents, ServerToClientEvents>(httpServer); io.emit("hello", "world"); io.on("connection", (socket) => { socket.on("my-event", (a, b, c) => { // ... }); socket.emit("hello", "again"); }); ``` The events are not typed by default (inferred as any), so this change is backward compatible. Note: we could also have reused the method here ([1]) to add types to the EventEmitter, instead of creating a StrictEventEmitter class. Related: https://github.com/socketio/socket.io/issues/3742 [1]: https://github.com/binier/tiny-typed-emitter	2021-03-10 00:18:13 +01:00
Damien Arrachequesne	b25495c069	feat: add some utility methods This commit adds the following methods: - fetchSockets: returns the matching socket instances Syntax: ```js // return all Socket instances const sockets = await io.fetchSockets(); // return all Socket instances of the "admin" namespace in the "room1" room const sockets = await io.of("/admin").in("room1").fetchSockets(); ``` - socketsJoin: makes the matching socket instances join the specified rooms Syntax: ```js // make all Socket instances join the "room1" room io.socketsJoin("room1"); // make all Socket instances of the "admin" namespace in the "room1" room join the "room2" room io.of("/admin").in("room1").socketsJoin("room2"); ``` - socketsLeave: makes the matching socket instances leave the specified rooms Syntax: ```js // make all Socket instances leave the "room1" room io.socketsLeave("room1"); // make all Socket instances of the "admin" namespace in the "room1" room leave the "room2" room io.of("/admin").in("room1").socketsLeave("room2"); ``` - disconnectSockets: makes the matching socket instances disconnect Syntax: ```js // make all Socket instances disconnect io.disconnectSockets(); // make all Socket instances of the "admin" namespace in the "room1" room disconnect io.of("/admin").in("room1").disconnectSockets(); ``` Those methods share the same semantics as broadcasting. They will also work with multiple Socket.IO servers when using the Redis adapter. In that case, the fetchSockets() method will return a list of RemoteSocket instances, which expose a subset of the methods and attributes of the Socket class (the "request" attribute cannot be mocked, for example). Related: - https://github.com/socketio/socket.io/issues/3042 - https://github.com/socketio/socket.io/issues/3418 - https://github.com/socketio/socket.io/issues/3570 - https://github.com/socketio/socket.io-redis/issues/283	2021-03-02 11:17:21 +01:00
Damien Arrachequesne	085d1de9df	feat: allow to pass an array to io.to(...) In some cases it is necessary to pass an array of rooms instead of a single room. New syntax: ``` io.to(["room1", "room2"]).except(["room3"]).emit(...); socket.to(["room1", "room2"]).except(["room3"]).emit(...); ``` Related: https://github.com/socketio/socket.io/issues/3048	2021-03-01 23:20:46 +01:00
Damien Arrachequesne	ac9e8ca6c7	fix: make io.to(...) immutable Previously, broadcasting to a given room (by calling `io.to()`) would mutate the io instance, which could lead to surprising behaviors, like: ```js io.to("room1"); io.to("room2").emit(...); // also sent to room1 // or with async/await io.to("room3").emit("details", await fetchDetails()); // random behavior: maybe in room3, maybe to all clients ``` Calling `io.to()` (or any other broadcast modifier) will now return an immutable instance. Related: - https://github.com/socketio/socket.io/issues/3431 - https://github.com/socketio/socket.io/issues/3444	2021-03-01 23:17:08 +01:00
Sebastiaan Marynissen	7de2e87e88	feat: allow to exclude specific rooms when broadcasting (#3789 ) New syntax: ``` io.except("room1").emit(...); io.to("room1").except("room2").emit(...); socket.broadcast.except("room1").emit(...); socket.to("room1").except("room2").emit(...); ``` Related: - https://github.com/socketio/socket.io/issues/3629 - https://github.com/socketio/socket.io/issues/3657	2021-03-01 09:30:58 +01:00
Damien Arrachequesne	225ade062a	chore(release): 3.1.2 Diff: https://github.com/socketio/socket.io/compare/3.1.1...3.1.2	2021-02-26 01:18:42 +01:00
Damien Arrachequesne	494c64e44f	fix: ignore packet received after disconnection Related: https://github.com/socketio/socket.io/issues/3095	2021-02-26 00:58:30 +01:00
Damien Arrachequesne	67a61e39e6	chore: loosen the version requirement of @types/node Related: https://github.com/socketio/socket.io/issues/3793	2021-02-26 00:58:01 +01:00
Damien Arrachequesne	7467216e02	docs(examples): 4th and final part of the "private messaging" example See also: https://socket.io/get-started/private-messaging-part-4/	2021-02-17 00:24:23 +01:00
Damien Arrachequesne	7247b4051f	docs(examples): 3rd part of the "private messaging" example See also: https://socket.io/get-started/private-messaging-part-3/	2021-02-17 00:22:55 +01:00
Damien Arrachequesne	992c9380c3	docs(examples): 2nd part of the "private messaging" example See also: https://socket.io/get-started/private-messaging-part-2/	2021-02-15 01:09:12 +01:00
Damien Arrachequesne	8b404f424b	docs(examples): 1st part of the "private messaging" example	2021-02-10 00:33:39 +01:00
Damien Arrachequesne	12221f296d	chore(release): 3.1.1 Diff: https://github.com/socketio/socket.io/compare/3.1.0...3.1.1	2021-02-03 22:57:21 +01:00
Damien Arrachequesne	6f4bd7f8e7	fix: properly parse the CONNECT packet in v2 compatibility mode In Socket.IO v2, the Socket query option was appended to the namespace in the CONNECT packet: { type: 0, nsp: "/my-namespace?abc=123" } Note: the "query" option on the client-side (v2) will be found in the "auth" attribute on the server-side: ``` // client-side const socket = io("/nsp1", { query: { abc: 123 } }); socket.query = { abc: 456 }; // server-side const io = require("socket.io")(httpServer, { allowEIO3: true // enable compatibility mode }); io.of("/nsp1").on("connection", (socket) => { console.log(socket.handshake.auth); // { abc: 456 } (the Socket query) console.log(socket.handshake.query.abc); // 123 (the Manager query) }); More information here: https://socket.io/docs/v3/migrating-from-2-x-to-3-0/#Add-a-clear-distinction-between-the-Manager-query-option-and-the-Socket-query-option Related: https://github.com/socketio/socket.io/issues/3791	2021-02-03 22:54:07 +01:00
Damien Arrachequesne	4f2e9a716d	fix(typings): update the types of "query", "auth" and "headers" Related: https://github.com/socketio/socket.io/issues/3770	2021-02-03 22:53:38 +01:00
david-fong	9e8f288ca9	fix(typings): add return types and general-case overload signatures (#3776 ) See also: https://stackoverflow.com/questions/52760509/typescript-returntype-of-overloaded-function/52760599#52760599	2021-02-02 11:50:08 +01:00
Damien Arrachequesne	86eb4227b2	docs(examples): add example with traefik Reference: https://doc.traefik.io/traefik/v2.0/	2021-01-31 23:47:23 +01:00
Damien Arrachequesne	cf873fd831	docs(examples): update cluster examples to Socket.IO v3	2021-01-28 11:21:38 +01:00
Damien Arrachequesne	0d10e6131b	docs(examples): update the nginx cluster example Related: https://github.com/socketio/socket.io/discussions/3778 Reference: http://nginx.org/en/docs/http/ngx_http_upstream_module.html#hash	2021-01-28 10:52:26 +01:00
JPSO	10aafbbc16	ci: add Node.js 15 (#3765 )	2021-01-20 22:34:51 +01:00
Hamdi Bayhan	f34cfca26d	docs: fix broken link (#3759 )	2021-01-17 22:10:37 +01:00
PrashoonB	d412e876b8	docs: add installation with yarn (#3757 )	2021-01-15 22:20:04 +01:00
Damien Arrachequesne	f05a4a6f82	chore(release): 3.1.0 Diff: https://github.com/socketio/socket.io/compare/3.0.5...3.1.0	2021-01-15 02:21:54 +01:00
Damien Arrachequesne	2c883f5d4e	chore: bump socket.io-adapter version Diff: https://github.com/socketio/socket.io-adapter/compare/2.0.3...2.1.0 Includes: - add room events ([`155fa63`](`155fa6333a`)) - make rooms and sids public ([`313c5a9`](`313c5a9fb6`))	2021-01-15 01:41:37 +01:00
Jakob Ackermann	161091dd4c	feat: confirm a weak but matching ETag (#3485 ) When handling compression at the proxy server level, the client receives a weak ETag. Weak ETags are prefixed with `W/`, e.g. `W/"2.2.0"`. Upon cache validation we should take care of these too. Reference: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag	2021-01-15 01:04:55 +01:00
汪心禾 Wang, Xinhe	d52532b7be	docs: add other client implementations (#3593 ) Reference: https://socket.io/docs/#Other-client-implementations	2021-01-15 00:37:40 +01:00
Sergio Lenza	6b1d7901db	docs(examples): Improve the chat example with more ES6 features (#3240 )	2021-01-15 00:36:17 +01:00
EloniX-X	b55892ae80	docs: add run on repl.it badge to README (#3617 )	2021-01-15 00:30:17 +01:00
Pablo Tejada	233650c222	feat(esm): export the Namespace and Socket class (#3699 )	2021-01-15 00:28:20 +01:00
Damien Arrachequesne	9925746c8e	feat: add support for Socket.IO v2 clients In order to ease the migration to Socket.IO v3, the Socket.IO server can now communicate with v2 clients. ```js const io = require("socket.io")({ allowEIO3: true }); ``` This feature is disabled by default.	2021-01-14 23:38:24 +01:00
Rohan Chougule	de8dffd252	refactor: strict type check in if expressions (#3744 )	2021-01-08 14:58:37 +01:00