From d1bfe40dbbf416dfed2c668df56d4dcbbbe2de6b Mon Sep 17 00:00:00 2001
From: David Fong <42986768+david-fong@users.noreply.github.com>
Date: Fri, 11 Dec 2020 03:19:20 -0800
Subject: [PATCH] refactor: add more typing info and upgrade prettier (#3725)

This upgrades prettier to 2.2.0 to gain support for TypeScript's new
type-only-imports feature.
---
 lib/client.ts           |  57 +++--
 lib/index.ts            | 171 +++++++------
 lib/namespace.ts        |  70 +++---
 lib/parent-namespace.ts |  28 ++-
 lib/socket.ts           |  66 ++---
 package-lock.json       |   6 +-
 package.json            |  11 +-
 test/socket.io.ts       | 530 +++++++++++++++++++---------------------
 test/support/util.ts    |   8 +-
 tsconfig.json           |  12 +-
 10 files changed, 483 insertions(+), 476 deletions(-)

diff --git a/lib/client.ts b/lib/client.ts
index a730425e..a4c34b70 100644
--- a/lib/client.ts
+++ b/lib/client.ts
@@ -1,10 +1,9 @@
 import { Decoder, Encoder, Packet, PacketType } from "socket.io-parser";
 import debugModule = require("debug");
-import { IncomingMessage } from "http";
-import { Server } from "./index";
-import { Socket } from "./socket";
-import { SocketId } from "socket.io-adapter";
-import { ParentNamespace } from "./parent-namespace";
+import type { IncomingMessage } from "http";
+import type { Namespace, Server } from "./index";
+import type { Socket } from "./socket";
+import type { SocketId } from "socket.io-adapter";
 
 const debug = debugModule("socket.io:client");
 
@@ -17,16 +16,16 @@ export class Client {
   private readonly decoder: Decoder;
   private sockets: Map<SocketId, Socket> = new Map();
   private nsps: Map<string, Socket> = new Map();
-  private connectTimeout: NodeJS.Timeout;
+  private connectTimeout?: NodeJS.Timeout;
 
   /**
    * Client constructor.
    *
-   * @param {Server} server instance
-   * @param {Socket} conn
+   * @param server instance
+   * @param conn
    * @package
    */
-  constructor(server: Server, conn) {
+  constructor(server: Server, conn: Socket) {
     this.server = server;
     this.conn = conn;
     this.encoder = server.encoder;
@@ -84,27 +83,31 @@ export class Client {
       return this.doConnect(name, auth);
     }
 
-    this.server._checkNamespace(name, auth, (dynamicNsp: ParentNamespace) => {
-      if (dynamicNsp) {
-        debug("dynamic namespace %s was created", dynamicNsp.name);
-        this.doConnect(name, auth);
-      } else {
-        debug("creation of namespace %s was denied", name);
-        this._packet({
-          type: PacketType.CONNECT_ERROR,
-          nsp: name,
-          data: {
-            message: "Invalid namespace"
-          }
-        });
+    this.server._checkNamespace(
+      name,
+      auth,
+      (dynamicNspName: Namespace | false) => {
+        if (dynamicNspName) {
+          debug("dynamic namespace %s was created", dynamicNspName);
+          this.doConnect(name, auth);
+        } else {
+          debug("creation of namespace %s was denied", name);
+          this._packet({
+            type: PacketType.CONNECT_ERROR,
+            nsp: name,
+            data: {
+              message: "Invalid namespace",
+            },
+          });
+        }
       }
-    });
+    );
   }
 
   /**
    * Connects a client to a namespace.
    *
-   * @param {String} name - the namespace
+   * @param name - the namespace
    * @param {Object} auth - the auth parameters
    *
    * @private
@@ -112,7 +115,7 @@ export class Client {
   private doConnect(name: string, auth: object) {
     if (this.connectTimeout) {
       clearTimeout(this.connectTimeout);
-      this.connectTimeout = null;
+      this.connectTimeout = undefined;
     }
     const nsp = this.server.of(name);
 
@@ -142,7 +145,7 @@ export class Client {
    */
   _remove(socket: Socket) {
     if (this.sockets.has(socket.id)) {
-      const nsp = this.sockets.get(socket.id).nsp.name;
+      const nsp = this.sockets.get(socket.id)!.nsp.name;
       this.sockets.delete(socket.id);
       this.nsps.delete(nsp);
     } else {
@@ -221,7 +224,7 @@ export class Client {
     } else {
       const socket = this.nsps.get(packet.nsp);
       if (socket) {
-        process.nextTick(function() {
+        process.nextTick(function () {
           socket._onpacket(packet);
         });
       } else {
diff --git a/lib/index.ts b/lib/index.ts
index 063e1a07..27f20380 100644
--- a/lib/index.ts
+++ b/lib/index.ts
@@ -11,11 +11,11 @@ import { ExtendedError, Namespace } from "./namespace";
 import { ParentNamespace } from "./parent-namespace";
 import { Adapter, Room, SocketId } from "socket.io-adapter";
 import * as parser from "socket.io-parser";
-import { Encoder } from "socket.io-parser";
+import type { Encoder } from "socket.io-parser";
 import debugModule from "debug";
 import { Socket } from "./socket";
-import { CookieSerializeOptions } from "cookie";
-import { CorsOptions } from "cors";
+import type { CookieSerializeOptions } from "cookie";
+import type { CorsOptions } from "cors";
 
 const debug = debugModule("socket.io:server");
 
@@ -23,6 +23,11 @@ const clientVersion = require("../package.json").version;
 const dotMapRegex = /\.map/;
 
 type Transport = "polling" | "websocket";
+type ParentNspNameMatchFn = (
+  name: string,
+  query: object,
+  fn: (err: Error | null, success: boolean) => void
+) => void;
 
 interface EngineOptions {
   /**
@@ -149,7 +154,7 @@ export class Server extends EventEmitter {
   public readonly sockets: Namespace;
 
   /** @private */
-  readonly _parser;
+  readonly _parser: typeof parser;
   /** @private */
   readonly encoder: Encoder;
 
@@ -157,17 +162,8 @@ export class Server extends EventEmitter {
    * @private
    */
   _nsps: Map<string, Namespace> = new Map();
-  private parentNsps: Map<
-    | string
-    | RegExp
-    | ((
-        name: string,
-        query: object,
-        fn: (err: Error, success: boolean) => void
-      ) => void),
-    ParentNamespace
-  > = new Map();
-  private _adapter: any;
+  private parentNsps: Map<ParentNspNameMatchFn, ParentNamespace> = new Map();
+  private _adapter?: typeof Adapter;
   private _serveClient: boolean;
   private opts: Partial<EngineOptions>;
   private eio;
@@ -184,18 +180,24 @@ export class Server extends EventEmitter {
   /**
    * Server constructor.
    *
-   * @param {http.Server|Number|Object} srv http server, port or options
-   * @param {Object} [opts]
+   * @param srv http server, port, or options
+   * @param [opts]
    * @public
    */
   constructor(opts?: Partial<ServerOptions>);
-  constructor(srv: http.Server, opts?: Partial<ServerOptions>);
-  constructor(srv: number, opts?: Partial<ServerOptions>);
-  constructor(srv?: any, opts: Partial<ServerOptions> = {}) {
+  constructor(srv?: http.Server | number, opts?: Partial<ServerOptions>);
+  constructor(
+    srv: undefined | Partial<ServerOptions> | http.Server | number,
+    opts: Partial<ServerOptions> = {}
+  ) {
     super();
-    if ("object" == typeof srv && srv instanceof Object && !srv.listen) {
-      opts = srv;
-      srv = null;
+    if (
+      "object" === typeof srv &&
+      srv instanceof Object &&
+      !(srv as Partial<http.Server>).listen
+    ) {
+      opts = srv as Partial<ServerOptions>;
+      srv = undefined;
     }
     this.path(opts.path || "/socket.io");
     this.connectTimeout(opts.connectTimeout || 45000);
@@ -205,44 +207,44 @@ export class Server extends EventEmitter {
     this.adapter(opts.adapter || Adapter);
     this.sockets = this.of("/");
     this.opts = opts;
-    if (srv) this.attach(srv);
+    if (srv) this.attach(srv as http.Server);
   }
 
   /**
    * Sets/gets whether client code is being served.
    *
-   * @param {Boolean} v - whether to serve client code
-   * @return {Server|Boolean} self when setting or value when getting
+   * @param v - whether to serve client code
+   * @return self when setting or value when getting
    * @public
    */
   public serveClient(v: boolean): Server;
   public serveClient(): boolean;
   public serveClient(v?: boolean): Server | boolean {
     if (!arguments.length) return this._serveClient;
-    this._serveClient = v;
+    this._serveClient = v!;
     return this;
   }
 
   /**
    * Executes the middleware for an incoming namespace not already created on the server.
    *
-   * @param {String} name - name of incoming namespace
+   * @param name - name of incoming namespace
    * @param {Object} auth - the auth parameters
-   * @param {Function} fn - callback
+   * @param fn - callback
    *
    * @private
    */
   _checkNamespace(
     name: string,
     auth: object,
-    fn: (nsp: Namespace | boolean) => void
+    fn: (nsp: Namespace | false) => void
   ) {
     if (this.parentNsps.size === 0) return fn(false);
 
     const keysIterator = this.parentNsps.keys();
 
     const run = () => {
-      let nextFn = keysIterator.next();
+      const nextFn = keysIterator.next();
       if (nextFn.done) {
         return fn(false);
       }
@@ -250,7 +252,7 @@ export class Server extends EventEmitter {
         if (err || !allow) {
           run();
         } else {
-          fn(this.parentNsps.get(nextFn.value).createChild(name));
+          fn(this.parentNsps.get(nextFn.value)!.createChild(name));
         }
       });
     };
@@ -270,7 +272,7 @@ export class Server extends EventEmitter {
   public path(v?: string): Server | string {
     if (!arguments.length) return this._path;
 
-    this._path = v.replace(/\/$/, "");
+    this._path = v!.replace(/\/$/, "");
 
     const escapedPath = this._path.replace(/[-\/\\^$*+?.()|[\]{}]/g, "\\$&");
     this.clientPathRegex = new RegExp(
@@ -297,13 +299,13 @@ export class Server extends EventEmitter {
   /**
    * Sets the adapter for rooms.
    *
-   * @param {Adapter} v pathname
-   * @return {Server|Adapter} self when setting or value when getting
+   * @param v pathname
+   * @return self when setting or value when getting
    * @public
    */
-  public adapter(): any;
-  public adapter(v: any);
-  public adapter(v?): Server | any {
+  public adapter(): typeof Adapter | undefined;
+  public adapter(v: typeof Adapter): Server;
+  public adapter(v?: typeof Adapter): typeof Adapter | undefined | Server {
     if (!arguments.length) return this._adapter;
     this._adapter = v;
     for (const nsp of this._nsps.values()) {
@@ -315,28 +317,30 @@ export class Server extends EventEmitter {
   /**
    * Attaches socket.io to a server or port.
    *
-   * @param {http.Server|Number} srv - server or port
-   * @param {Object} opts - options passed to engine.io
-   * @return {Server} self
+   * @param srv - server or port
+   * @param opts - options passed to engine.io
+   * @return self
    * @public
    */
-  public listen(srv: http.Server, opts?: Partial<ServerOptions>): Server;
-  public listen(srv: number, opts?: Partial<ServerOptions>): Server;
-  public listen(srv: any, opts: Partial<ServerOptions> = {}): Server {
+  public listen(
+    srv: http.Server | number,
+    opts: Partial<ServerOptions> = {}
+  ): Server {
     return this.attach(srv, opts);
   }
 
   /**
    * Attaches socket.io to a server or port.
    *
-   * @param {http.Server|Number} srv - server or port
-   * @param {Object} opts - options passed to engine.io
-   * @return {Server} self
+   * @param srv - server or port
+   * @param opts - options passed to engine.io
+   * @return self
    * @public
    */
-  public attach(srv: http.Server, opts?: Partial<ServerOptions>): Server;
-  public attach(port: number, opts?: Partial<ServerOptions>): Server;
-  public attach(srv: any, opts: Partial<ServerOptions> = {}): Server {
+  public attach(
+    srv: http.Server | number,
+    opts: Partial<ServerOptions> = {}
+  ): Server {
     if ("function" == typeof srv) {
       const msg =
         "You are trying to attach socket.io to an express " +
@@ -394,10 +398,10 @@ export class Server extends EventEmitter {
   /**
    * Attaches the static file serving.
    *
-   * @param {Function|http.Server} srv http server
+   * @param srv http server
    * @private
    */
-  private attachServe(srv) {
+  private attachServe(srv: http.Server) {
     debug("attaching client serving req handler");
 
     const evs = srv.listeners("request").slice(0);
@@ -416,12 +420,12 @@ export class Server extends EventEmitter {
   /**
    * Handles a request serving of client source and map
    *
-   * @param {http.IncomingMessage} req
-   * @param {http.ServerResponse} res
+   * @param req
+   * @param res
    * @private
    */
   private serve(req: http.IncomingMessage, res: http.ServerResponse) {
-    const filename = req.url.replace(this._path, "");
+    const filename = req.url!.replace(this._path, "");
     const isMap = dotMapRegex.test(filename);
     const type = isMap ? "map" : "source";
 
@@ -470,7 +474,7 @@ export class Server extends EventEmitter {
     );
     const encoding = accepts(req).encodings(["br", "gzip", "deflate"]);
 
-    const onError = err => {
+    const onError = (err: NodeJS.ErrnoException | null) => {
       if (err) {
         res.end();
       }
@@ -500,7 +504,7 @@ export class Server extends EventEmitter {
    * Binds socket.io to an engine.io instance.
    *
    * @param {engine.Server} engine engine.io (or compatible) server
-   * @return {Server} self
+   * @return self
    * @public
    */
   public bind(engine): Server {
@@ -513,7 +517,7 @@ export class Server extends EventEmitter {
    * Called with each incoming transport connection.
    *
    * @param {engine.Socket} conn
-   * @return {Server} self
+   * @return self
    * @private
    */
   private onconnection(conn): Server {
@@ -526,18 +530,11 @@ export class Server extends EventEmitter {
    * Looks up a namespace.
    *
    * @param {String|RegExp|Function} name nsp name
-   * @param {Function} [fn] optional, nsp `connection` ev handler
+   * @param [fn] optional, nsp `connection` ev handler
    * @public
    */
   public of(
-    name:
-      | string
-      | RegExp
-      | ((
-          name: string,
-          query: object,
-          fn: (err: Error, success: boolean) => void
-        ) => void),
+    name: string | RegExp | ParentNspNameMatchFn,
     fn?: (socket: Socket) => void
   ) {
     if (typeof name === "function" || name instanceof RegExp) {
@@ -573,7 +570,7 @@ export class Server extends EventEmitter {
   /**
    * Closes server connection
    *
-   * @param {Function} [fn] optional, called as `fn([err])` on error OR all conns closed
+   * @param [fn] optional, called as `fn([err])` on error OR all conns closed
    * @public
    */
   public close(fn?: (err?: Error) => void): void {
@@ -593,7 +590,7 @@ export class Server extends EventEmitter {
   /**
    * Sets up namespace middleware.
    *
-   * @return {Server} self
+   * @return self
    * @public
    */
   public use(
@@ -606,8 +603,8 @@ export class Server extends EventEmitter {
   /**
    * Targets a room when emitting.
    *
-   * @param {String} name
-   * @return {Server} self
+   * @param name
+   * @return self
    * @public
    */
   public to(name: Room): Server {
@@ -618,8 +615,8 @@ export class Server extends EventEmitter {
   /**
    * Targets a room when emitting.
    *
-   * @param {String} name
-   * @return {Server} self
+   * @param name
+   * @return self
    * @public
    */
   public in(name: Room): Server {
@@ -630,24 +627,22 @@ export class Server extends EventEmitter {
   /**
    * Sends a `message` event to all clients.
    *
-   * @return {Server} self
+   * @return self
    * @public
    */
-  public send(...args): Server {
-    args.unshift("message");
-    this.sockets.emit.apply(this.sockets, args);
+  public send(...args: readonly any[]): Server {
+    this.sockets.emit("message", ...args);
     return this;
   }
 
   /**
    * Sends a `message` event to all clients.
    *
-   * @return {Server} self
+   * @return self
    * @public
    */
-  public write(...args): Server {
-    args.unshift("message");
-    this.sockets.emit.apply(this.sockets, args);
+  public write(...args: readonly any[]): Server {
+    this.sockets.emit("message", ...args);
     return this;
   }
 
@@ -663,8 +658,8 @@ export class Server extends EventEmitter {
   /**
    * Sets the compress flag.
    *
-   * @param {Boolean} compress - if `true`, compresses the sending data
-   * @return {Server} self
+   * @param compress - if `true`, compresses the sending data
+   * @return self
    * @public
    */
   public compress(compress: boolean): Server {
@@ -677,7 +672,7 @@ export class Server extends EventEmitter {
    * 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 {Server} self
+   * @return self
    * @public
    */
   public get volatile(): Server {
@@ -688,7 +683,7 @@ export class Server extends EventEmitter {
   /**
    * Sets a modifier for a subsequent event emission that the event data will only be broadcast to the current node.
    *
-   * @return {Server} self
+   * @return self
    * @public
    */
   public get local(): Server {
@@ -701,14 +696,14 @@ export class Server extends EventEmitter {
  * Expose main namespace (/).
  */
 
-const emitterMethods = Object.keys(EventEmitter.prototype).filter(function(
+const emitterMethods = Object.keys(EventEmitter.prototype).filter(function (
   key
 ) {
   return typeof EventEmitter.prototype[key] === "function";
 });
 
-emitterMethods.forEach(function(fn) {
-  Server.prototype[fn] = function() {
+emitterMethods.forEach(function (fn) {
+  Server.prototype[fn] = function () {
     return this.sockets[fn].apply(this.sockets, arguments);
   };
 });
diff --git a/lib/namespace.ts b/lib/namespace.ts
index f445cff7..9c00ea8f 100644
--- a/lib/namespace.ts
+++ b/lib/namespace.ts
@@ -1,10 +1,10 @@
 import { Socket, RESERVED_EVENTS } from "./socket";
-import { Server } from "./index";
-import { Client } from "./client";
+import type { Server } from "./index";
+import type { Client } from "./client";
 import { EventEmitter } from "events";
 import { PacketType } from "socket.io-parser";
 import debugModule from "debug";
-import { Adapter, Room, SocketId } from "socket.io-adapter";
+import type { Adapter, Room, SocketId } from "socket.io-adapter";
 
 const debug = debugModule("socket.io:namespace");
 
@@ -23,7 +23,7 @@ export class Namespace extends EventEmitter {
 
   /** @private */
   _fns: Array<
-    (socket: Socket, next: (err: ExtendedError) => void) => void
+    (socket: Socket, next: (err?: ExtendedError) => void) => void
   > = [];
 
   /** @private */
@@ -38,8 +38,8 @@ export class Namespace extends EventEmitter {
   /**
    * Namespace constructor.
    *
-   * @param {Server} server instance
-   * @param {string} name
+   * @param server instance
+   * @param name
    */
   constructor(server: Server, name: string) {
     super();
@@ -56,13 +56,13 @@ export class Namespace extends EventEmitter {
    * @private
    */
   _initAdapter(): void {
-    this.adapter = new (this.server.adapter())(this);
+    this.adapter = new (this.server.adapter()!)(this);
   }
 
   /**
    * Sets up namespace middleware.
    *
-   * @return {Namespace} self
+   * @return self
    * @public
    */
   public use(
@@ -75,16 +75,16 @@ export class Namespace extends EventEmitter {
   /**
    * Executes the middleware for an incoming client.
    *
-   * @param {Socket} socket - the socket that will get added
-   * @param {Function} fn - last fn call in the middleware
+   * @param socket - the socket that will get added
+   * @param fn - last fn call in the middleware
    * @private
    */
-  private run(socket: Socket, fn: (err: ExtendedError) => void) {
+  private run(socket: Socket, fn: (err: ExtendedError | null) => void) {
     const fns = this._fns.slice(0);
     if (!fns.length) return fn(null);
 
-    function run(i) {
-      fns[i](socket, function(err) {
+    function run(i: number) {
+      fns[i](socket, function (err) {
         // upon error, short-circuit
         if (err) return fn(err);
 
@@ -102,8 +102,8 @@ export class Namespace extends EventEmitter {
   /**
    * Targets a room when emitting.
    *
-   * @param {String} name
-   * @return {Namespace} self
+   * @param name
+   * @return self
    * @public
    */
   public to(name: Room): Namespace {
@@ -114,8 +114,8 @@ export class Namespace extends EventEmitter {
   /**
    * Targets a room when emitting.
    *
-   * @param {String} name
-   * @return {Namespace} self
+   * @param name
+   * @return self
    * @public
    */
   public in(name: Room): Namespace {
@@ -132,13 +132,13 @@ export class Namespace extends EventEmitter {
   _add(client: Client, query, fn?: () => void): Socket {
     debug("adding socket to nsp %s", this.name);
     const socket = new Socket(this, client, query);
-    this.run(socket, err => {
+    this.run(socket, (err) => {
       process.nextTick(() => {
         if ("open" == client.conn.readyState) {
           if (err)
             return socket._error({
               message: err.message,
-              data: err.data
+              data: err.data,
             });
 
           // track socket
@@ -178,10 +178,10 @@ export class Namespace extends EventEmitter {
   /**
    * Emits to all clients.
    *
-   * @return {Boolean} Always true
+   * @return Always true
    * @public
    */
-  public emit(ev: string, ...args: any[]): boolean {
+  public emit(ev: string | Symbol, ...args: any[]): true {
     if (RESERVED_EVENTS.has(ev)) {
       throw new Error(`"${ev}" is a reserved event name`);
     }
@@ -189,7 +189,7 @@ export class Namespace extends EventEmitter {
     args.unshift(ev);
     const packet = {
       type: PacketType.EVENT,
-      data: args
+      data: args,
     };
 
     if ("function" == typeof args[args.length - 1]) {
@@ -205,7 +205,7 @@ export class Namespace extends EventEmitter {
 
     this.adapter.broadcast(packet, {
       rooms: rooms,
-      flags: flags
+      flags: flags,
     });
 
     return true;
@@ -214,31 +214,29 @@ export class Namespace extends EventEmitter {
   /**
    * Sends a `message` event to all clients.
    *
-   * @return {Namespace} self
+   * @return self
    * @public
    */
-  public send(...args): Namespace {
-    args.unshift("message");
-    this.emit.apply(this, args);
+  public send(...args: readonly any[]): Namespace {
+    this.emit("message", ...args);
     return this;
   }
 
   /**
    * Sends a `message` event to all clients.
    *
-   * @return {Namespace} self
+   * @return self
    * @public
    */
-  public write(...args): Namespace {
-    args.unshift("message");
-    this.emit.apply(this, args);
+  public write(...args: readonly any[]): Namespace {
+    this.emit("message", ...args);
     return this;
   }
 
   /**
    * Gets a list of clients.
    *
-   * @return {Namespace} self
+   * @return self
    * @public
    */
   public allSockets(): Promise<Set<SocketId>> {
@@ -255,8 +253,8 @@ export class Namespace extends EventEmitter {
   /**
    * Sets the compress flag.
    *
-   * @param {Boolean} compress - if `true`, compresses the sending data
-   * @return {Namespace} self
+   * @param compress - if `true`, compresses the sending data
+   * @return self
    * @public
    */
   public compress(compress: boolean): Namespace {
@@ -269,7 +267,7 @@ export class Namespace extends EventEmitter {
    * 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 {Namespace} self
+   * @return self
    * @public
    */
   public get volatile(): Namespace {
@@ -280,7 +278,7 @@ export class Namespace extends EventEmitter {
   /**
    * Sets a modifier for a subsequent event emission that the event data will only be broadcast to the current node.
    *
-   * @return {Namespace} self
+   * @return self
    * @public
    */
   public get local(): Namespace {
diff --git a/lib/parent-namespace.ts b/lib/parent-namespace.ts
index 88d07df8..9e1e2757 100644
--- a/lib/parent-namespace.ts
+++ b/lib/parent-namespace.ts
@@ -1,20 +1,26 @@
 import { Namespace } from "./namespace";
+import type { Server } from "./index";
 
 export class ParentNamespace extends Namespace {
   private static count: number = 0;
   private children: Set<Namespace> = new Set();
 
-  constructor(server) {
+  constructor(server: Server) {
     super(server, "/_" + ParentNamespace.count++);
   }
 
-  _initAdapter() {}
+  /**
+   * @private
+   */
+  _initAdapter(): void {
+    /* no-op */
+  }
 
-  public emit(...args: any[]): boolean {
-    this.children.forEach(nsp => {
+  public emit(ev: string | Symbol, ...args: [...any]): true {
+    this.children.forEach((nsp) => {
       nsp._rooms = this._rooms;
       nsp._flags = this._flags;
-      nsp.emit.apply(nsp, args);
+      nsp.emit(ev, ...args);
     });
     this._rooms.clear();
     this._flags = {};
@@ -22,16 +28,14 @@ export class ParentNamespace extends Namespace {
     return true;
   }
 
-  createChild(name) {
+  createChild(name: string): Namespace {
     const namespace = new Namespace(this.server, name);
     namespace._fns = this._fns.slice(0);
-    this.listeners("connect").forEach(listener =>
-      // @ts-ignore
-      namespace.on("connect", listener)
+    this.listeners("connect").forEach((listener) =>
+      namespace.on("connect", listener as (...args: any[]) => void)
     );
-    this.listeners("connection").forEach(listener =>
-      // @ts-ignore
-      namespace.on("connection", listener)
+    this.listeners("connection").forEach((listener) =>
+      namespace.on("connection", listener as (...args: any[]) => void)
     );
     this.children.add(namespace);
     this.server._nsps.set(name, namespace);
diff --git a/lib/socket.ts b/lib/socket.ts
index 57ef1488..f7c14cff 100644
--- a/lib/socket.ts
+++ b/lib/socket.ts
@@ -2,24 +2,29 @@ import { EventEmitter } from "events";
 import { PacketType } from "socket.io-parser";
 import url = require("url");
 import debugModule from "debug";
-import { Server } from "./index";
-import { Client } from "./client";
-import { Namespace } from "./namespace";
-import { IncomingMessage } from "http";
-import { Adapter, BroadcastFlags, Room, SocketId } from "socket.io-adapter";
+import type { Server } from "./index";
+import type { Client } from "./client";
+import type { Namespace } from "./namespace";
+import type { IncomingMessage } from "http";
+import type {
+  Adapter,
+  BroadcastFlags,
+  Room,
+  SocketId,
+} from "socket.io-adapter";
 import base64id from "base64id";
 
 const debug = debugModule("socket.io:socket");
 
-export const RESERVED_EVENTS = new Set([
+export const RESERVED_EVENTS = new Set(<const>[
   "connect",
   "connect_error",
   "disconnect",
   "disconnecting",
   // EventEmitter reserved events: https://nodejs.org/api/events.html#events_event_newlistener
   "newListener",
-  "removeListener"
-]);
+  "removeListener",
+]) as ReadonlySet<string | Symbol>;
 
 /**
  * The handshake details
@@ -86,7 +91,7 @@ export class Socket extends EventEmitter {
   > = [];
   private flags: BroadcastFlags = {};
   private _rooms: Set<Room> = new Set();
-  private _anyListeners: Array<(...args: any[]) => void>;
+  private _anyListeners?: Array<(...args: any[]) => void>;
 
   /**
    * Interface to a `Client` for a given `Namespace`.
@@ -120,16 +125,16 @@ export class Socket extends EventEmitter {
       // @ts-ignore
       secure: !!this.request.connection.encrypted,
       issued: +new Date(),
-      url: this.request.url,
-      query: url.parse(this.request.url, true).query,
-      auth
+      url: this.request.url!,
+      query: url.parse(this.request.url!, true).query,
+      auth,
     };
   }
 
   /**
    * Emits to this client.
    *
-   * @return {Boolean} Always true
+   * @return Always returns `true`.
    * @public
    */
   public emit(ev: string, ...args: any[]): boolean {
@@ -139,7 +144,7 @@ export class Socket extends EventEmitter {
     args.unshift(ev);
     const packet: any = {
       type: PacketType.EVENT,
-      data: args
+      data: args,
     };
 
     // access last argument to see if it's an ACK callback
@@ -164,7 +169,7 @@ export class Socket extends EventEmitter {
       this.adapter.broadcast(packet, {
         except: new Set([this.id]),
         rooms: rooms,
-        flags: flags
+        flags: flags,
       });
     } else {
       // dispatch packet
@@ -176,11 +181,11 @@ export class Socket extends EventEmitter {
   /**
    * Targets a room when broadcasting.
    *
-   * @param {String} name
-   * @return {Socket} self
+   * @param name
+   * @return self
    * @public
    */
-  public to(name: Room) {
+  public to(name: Room): Socket {
     this._rooms.add(name);
     return this;
   }
@@ -188,8 +193,8 @@ export class Socket extends EventEmitter {
   /**
    * Targets a room when broadcasting.
    *
-   * @param {String} name
-   * @return {Socket} self
+   * @param name
+   * @return self
    * @public
    */
   public in(name: Room): Socket {
@@ -200,24 +205,22 @@ export class Socket extends EventEmitter {
   /**
    * Sends a `message` event.
    *
-   * @return {Socket} self
+   * @return self
    * @public
    */
-  public send(...args): Socket {
-    args.unshift("message");
-    this.emit.apply(this, args);
+  public send(...args: readonly any[]): Socket {
+    this.emit("message", ...args);
     return this;
   }
 
   /**
    * Sends a `message` event.
    *
-   * @return {Socket} self
+   * @return self
    * @public
    */
-  public write(...args): Socket {
-    args.unshift("message");
-    this.emit.apply(this, args);
+  public write(...args: readonly any[]): Socket {
+    this.emit("message", ...args);
     return this;
   }
 
@@ -353,7 +356,7 @@ export class Socket extends EventEmitter {
   private ack(id: number) {
     const self = this;
     let sent = false;
-    return function() {
+    return function () {
       // prevent double callbacks
       if (sent) return;
       const args = Array.prototype.slice.call(arguments);
@@ -362,7 +365,7 @@ export class Socket extends EventEmitter {
       self.packet({
         id: id,
         type: PacketType.ACK,
-        data: args
+        data: args,
       });
 
       sent = true;
@@ -417,7 +420,7 @@ export class Socket extends EventEmitter {
    *
    * @private
    */
-  _onclose(reason: string) {
+  _onclose(reason: string): Socket | undefined {
     if (!this.connected) return this;
     debug("closing socket - reason %s", reason);
     super.emit("disconnecting", reason);
@@ -427,6 +430,7 @@ export class Socket extends EventEmitter {
     this.connected = false;
     this.disconnected = true;
     super.emit("disconnect", reason);
+    return;
   }
 
   /**
diff --git a/package-lock.json b/package-lock.json
index 6793e1bc..5045ab71 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -2083,9 +2083,9 @@
       "dev": true
     },
     "prettier": {
-      "version": "1.19.1",
-      "resolved": "https://registry.npmjs.org/prettier/-/prettier-1.19.1.tgz",
-      "integrity": "sha512-s7PoyDv/II1ObgQunCbB9PdLmUcBZcnWOcxDh7O0N/UwDEsHyqkW+Qh28jW+mVuCdx7gLB0BotYI1Y6uI9iyew==",
+      "version": "2.2.1",
+      "resolved": "https://registry.npmjs.org/prettier/-/prettier-2.2.1.tgz",
+      "integrity": "sha512-PqyhM2yCjg/oKkFPtTGUojv7gnZAoG80ttl45O6x2Ug/rMJw4wcc9k6aaf2hibP7BGVCCM33gZoGjyvt9mm16Q==",
       "dev": true
     },
     "process-nextick-args": {
diff --git a/package.json b/package.json
index 87ed5975..aa4c5314 100644
--- a/package.json
+++ b/package.json
@@ -14,8 +14,15 @@
   "files": [
     "dist/",
     "client-dist/",
-    "wrapper.mjs"
+    "wrapper.mjs",
+    "!**/*.tsbuildinfo"
   ],
+  "directories": {
+    "doc": "docs/",
+    "example": "example/",
+    "lib": "lib/",
+    "test": "test/"
+  },
   "type": "commonjs",
   "main": "./dist/index.js",
   "exports": {
@@ -54,7 +61,7 @@
     "expect.js": "0.3.1",
     "mocha": "^3.5.3",
     "nyc": "^15.1.0",
-    "prettier": "^1.19.1",
+    "prettier": "^2.2.0",
     "rimraf": "^3.0.2",
     "socket.io-client": "3.0.4",
     "superagent": "^3.8.2",
diff --git a/test/socket.io.ts b/test/socket.io.ts
index cc9fc072..6c938b46 100644
--- a/test/socket.io.ts
+++ b/test/socket.io.ts
@@ -7,7 +7,7 @@ import { join } from "path";
 import { exec } from "child_process";
 import request from "supertest";
 import expect from "expect.js";
-import { AddressInfo } from "net";
+import type { AddressInfo } from "net";
 
 const ioc = require("socket.io-client");
 
@@ -17,7 +17,7 @@ import "./support/util";
 function client(srv, nsp?: string | object, opts?: object) {
   if ("object" == typeof nsp) {
     opts = nsp;
-    nsp = null;
+    nsp = undefined;
   }
   let addr = srv.address();
   if (!addr) addr = srv.listen().address();
@@ -35,7 +35,7 @@ describe("socket.io", () => {
     describe("http.Server", () => {
       const clientVersion = require("socket.io-client/package.json").version;
 
-      const testSource = filename => done => {
+      const testSource = (filename) => (done) => {
         const srv = createServer();
         new Server(srv);
         request(srv)
@@ -52,7 +52,7 @@ describe("socket.io", () => {
           });
       };
 
-      const testSourceMap = filename => done => {
+      const testSourceMap = (filename) => (done) => {
         const srv = createServer();
         new Server(srv);
         request(srv)
@@ -77,7 +77,7 @@ describe("socket.io", () => {
         testSourceMap("socket.io.min.js.map")
       );
 
-      it("should serve client (gzip)", done => {
+      it("should serve client (gzip)", (done) => {
         const srv = createServer();
         new Server(srv);
         request(srv)
@@ -102,7 +102,7 @@ describe("socket.io", () => {
         testSourceMap("socket.io.msgpack.min.js.map")
       );
 
-      it("should handle 304", done => {
+      it("should handle 304", (done) => {
         const srv = createServer();
         new Server(srv);
         request(srv)
@@ -115,15 +115,13 @@ describe("socket.io", () => {
           });
       });
 
-      it("should not serve static files", done => {
+      it("should not serve static files", (done) => {
         const srv = createServer();
         new Server(srv, { serveClient: false });
-        request(srv)
-          .get("/socket.io/socket.io.js")
-          .expect(400, done);
+        request(srv).get("/socket.io/socket.io.js").expect(400, done);
       });
 
-      it("should work with #attach", done => {
+      it("should work with #attach", (done) => {
         const srv = createServer((req, res) => {
           res.writeHead(404);
           res.end();
@@ -145,10 +143,10 @@ describe("socket.io", () => {
           res.end();
         });
         const server = new Server({
-          pingTimeout: 6000
+          pingTimeout: 6000,
         });
         server.attach(srv, {
-          pingInterval: 24000
+          pingInterval: 24000,
         });
         // @ts-ignore
         expect(server.eio.opts.pingTimeout).to.eql(6000);
@@ -159,28 +157,28 @@ describe("socket.io", () => {
     });
 
     describe("port", () => {
-      it("should be bound", done => {
+      it("should be bound", (done) => {
         const sockets = new Server(54010);
         request("http://localhost:54010")
           .get("/socket.io/socket.io.js")
           .expect(200, done);
       });
 
-      it("should be bound as a string", done => {
+      it("should be bound as a string", (done) => {
         const sockets = new Server(54020);
         request("http://localhost:54020")
           .get("/socket.io/socket.io.js")
           .expect(200, done);
       });
 
-      it("with listen", done => {
+      it("with listen", (done) => {
         const sockets = new Server().listen(54011);
         request("http://localhost:54011")
           .get("/socket.io/socket.io.js")
           .expect(200, done);
       });
 
-      it("as a string", done => {
+      it("as a string", (done) => {
         const sockets = new Server().listen(54012);
         request("http://localhost:54012")
           .get("/socket.io/socket.io.js")
@@ -192,14 +190,14 @@ describe("socket.io", () => {
   describe("handshake", () => {
     const request = require("superagent");
 
-    it("should send the Access-Control-Allow-xxx headers on OPTIONS request", done => {
+    it("should send the Access-Control-Allow-xxx headers on OPTIONS request", (done) => {
       const sockets = new Server(54013, {
         cors: {
           origin: "http://localhost:54023",
           methods: ["GET", "POST"],
           allowedHeaders: ["content-type"],
-          credentials: true
-        }
+          credentials: true,
+        },
       });
       request
         .options("http://localhost:54013/socket.io/default/")
@@ -220,14 +218,14 @@ describe("socket.io", () => {
         });
     });
 
-    it("should send the Access-Control-Allow-xxx headers on GET request", done => {
+    it("should send the Access-Control-Allow-xxx headers on GET request", (done) => {
       const sockets = new Server(54014, {
         cors: {
           origin: "http://localhost:54024",
           methods: ["GET", "POST"],
           allowedHeaders: ["content-type"],
-          credentials: true
-        }
+          credentials: true,
+        },
       });
       request
         .get("http://localhost:54014/socket.io/default/")
@@ -244,9 +242,9 @@ describe("socket.io", () => {
         });
     });
 
-    it("should allow request if custom function in opts.allowRequest returns true", done => {
+    it("should allow request if custom function in opts.allowRequest returns true", (done) => {
       const sockets = new Server(createServer().listen(54022), {
-        allowRequest: (req, callback) => callback(null, true)
+        allowRequest: (req, callback) => callback(null, true),
       });
 
       request
@@ -258,9 +256,9 @@ describe("socket.io", () => {
         });
     });
 
-    it("should disallow request if custom function in opts.allowRequest returns false", done => {
+    it("should disallow request if custom function in opts.allowRequest returns false", (done) => {
       const sockets = new Server(createServer().listen(54023), {
-        allowRequest: (req, callback) => callback(null, false)
+        allowRequest: (req, callback) => callback(null, false),
       });
       request
         .get("http://localhost:54023/socket.io/default/")
@@ -295,7 +293,7 @@ describe("socket.io", () => {
 
       server.once("listening", () => {
         // PORT should be free
-        server.close(error => {
+        server.close((error) => {
           expect(error).to.be(undefined);
         });
       });
@@ -321,7 +319,7 @@ describe("socket.io", () => {
 
       server.once("listening", () => {
         // PORT should be free
-        server.close(error => {
+        server.close((error) => {
           expect(error).to.be(undefined);
         });
       });
@@ -338,7 +336,7 @@ describe("socket.io", () => {
         );
       }
 
-      it("should stop socket and timers", done => {
+      it("should stop socket and timers", (done) => {
         exec(fixture("server-close.ts"), done);
       });
     });
@@ -369,7 +367,7 @@ describe("socket.io", () => {
       delete sio.sockets._flags;
     });
 
-    it("should automatically connect", done => {
+    it("should automatically connect", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       srv.listen(() => {
@@ -380,7 +378,7 @@ describe("socket.io", () => {
       });
     });
 
-    it("should fire a `connection` event", done => {
+    it("should fire a `connection` event", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       srv.listen(() => {
@@ -392,19 +390,19 @@ describe("socket.io", () => {
       });
     });
 
-    it("should fire a `connect` event", done => {
+    it("should fire a `connect` event", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       srv.listen(() => {
         const socket = client(srv);
-        sio.on("connect", socket => {
+        sio.on("connect", (socket) => {
           expect(socket).to.be.a(Socket);
           done();
         });
       });
     });
 
-    it("should work with many sockets", done => {
+    it("should work with many sockets", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       srv.listen(() => {
@@ -422,7 +420,7 @@ describe("socket.io", () => {
       });
     });
 
-    it('should be able to equivalently start with "" or "/" on server', done => {
+    it('should be able to equivalently start with "" or "/" on server', (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       let total = 2;
@@ -436,7 +434,7 @@ describe("socket.io", () => {
       const c2 = client(srv, "/abc");
     });
 
-    it('should be equivalent for "" and "/" on client', done => {
+    it('should be equivalent for "" and "/" on client', (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       sio.of("/").on("connection", () => {
@@ -445,43 +443,43 @@ describe("socket.io", () => {
       const c1 = client(srv, "");
     });
 
-    it("should work with `of` and many sockets", done => {
+    it("should work with `of` and many sockets", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       srv.listen(() => {
         const chat = client(srv, "/chat");
         const news = client(srv, "/news");
         let total = 2;
-        sio.of("/news").on("connection", socket => {
+        sio.of("/news").on("connection", (socket) => {
           expect(socket).to.be.a(Socket);
           --total || done();
         });
-        sio.of("/news").on("connection", socket => {
+        sio.of("/news").on("connection", (socket) => {
           expect(socket).to.be.a(Socket);
           --total || done();
         });
       });
     });
 
-    it("should work with `of` second param", done => {
+    it("should work with `of` second param", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       srv.listen(() => {
         const chat = client(srv, "/chat");
         const news = client(srv, "/news");
         let total = 2;
-        sio.of("/news", socket => {
+        sio.of("/news", (socket) => {
           expect(socket).to.be.a(Socket);
           --total || done();
         });
-        sio.of("/news", socket => {
+        sio.of("/news", (socket) => {
           expect(socket).to.be.a(Socket);
           --total || done();
         });
       });
     });
 
-    it("should disconnect upon transport disconnection", done => {
+    it("should disconnect upon transport disconnection", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       srv.listen(() => {
@@ -490,15 +488,15 @@ describe("socket.io", () => {
         let total = 2;
         let totald = 2;
         let s;
-        sio.of("/news", socket => {
-          socket.on("disconnect", reason => {
+        sio.of("/news", (socket) => {
+          socket.on("disconnect", (reason) => {
             --totald || done();
           });
           --total || close();
         });
-        sio.of("/chat", socket => {
+        sio.of("/chat", (socket) => {
           s = socket;
-          socket.on("disconnect", reason => {
+          socket.on("disconnect", (reason) => {
             --totald || done();
           });
           --total || close();
@@ -509,24 +507,24 @@ describe("socket.io", () => {
       });
     });
 
-    it("should fire a `disconnecting` event just before leaving all rooms", done => {
+    it("should fire a `disconnecting` event just before leaving all rooms", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       srv.listen(() => {
         const socket = client(srv);
 
-        sio.on("connection", s => {
+        sio.on("connection", (s) => {
           s.join("a");
           // FIXME not sure why process.nextTick() is needed here
           process.nextTick(() => s.disconnect());
 
           let total = 2;
-          s.on("disconnecting", reason => {
+          s.on("disconnecting", (reason) => {
             expect(s.rooms).to.contain(s.id, "a");
             total--;
           });
 
-          s.on("disconnect", reason => {
+          s.on("disconnect", (reason) => {
             expect(s.rooms.size).to.eql(0);
             --total || done();
           });
@@ -534,19 +532,19 @@ describe("socket.io", () => {
       });
     });
 
-    it("should return error connecting to non-existent namespace", done => {
+    it("should return error connecting to non-existent namespace", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       srv.listen(() => {
         const socket = client(srv, "/doesnotexist");
-        socket.on("connect_error", err => {
+        socket.on("connect_error", (err) => {
           expect(err.message).to.be("Invalid namespace");
           done();
         });
       });
     });
 
-    it("should not reuse same-namespace connections", done => {
+    it("should not reuse same-namespace connections", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       let connections = 0;
@@ -563,21 +561,21 @@ describe("socket.io", () => {
       });
     });
 
-    it("should find all clients in a namespace", done => {
+    it("should find all clients in a namespace", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
-      const chatSids = [];
+      const chatSids: string[] = [];
       let otherSid = null;
       srv.listen(() => {
         const c1 = client(srv, "/chat");
         const c2 = client(srv, "/chat", { forceNew: true });
         const c3 = client(srv, "/other", { forceNew: true });
         let total = 3;
-        sio.of("/chat").on("connection", socket => {
+        sio.of("/chat").on("connection", (socket) => {
           chatSids.push(socket.id);
           --total || getSockets();
         });
-        sio.of("/other").on("connection", socket => {
+        sio.of("/other").on("connection", (socket) => {
           otherSid = socket.id;
           --total || getSockets();
         });
@@ -591,7 +589,7 @@ describe("socket.io", () => {
       }
     });
 
-    it("should find all clients in a namespace room", done => {
+    it("should find all clients in a namespace room", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       let chatFooSid = null;
@@ -603,7 +601,7 @@ describe("socket.io", () => {
         const c3 = client(srv, "/other", { forceNew: true });
         let chatIndex = 0;
         let total = 3;
-        sio.of("/chat").on("connection", socket => {
+        sio.of("/chat").on("connection", (socket) => {
           if (chatIndex++) {
             socket.join("foo");
             chatFooSid = socket.id;
@@ -614,17 +612,14 @@ describe("socket.io", () => {
             --total || getSockets();
           }
         });
-        sio.of("/other").on("connection", socket => {
+        sio.of("/other").on("connection", (socket) => {
           socket.join("foo");
           otherSid = socket.id;
           --total || getSockets();
         });
       });
       async function getSockets() {
-        const sids = await sio
-          .of("/chat")
-          .in("foo")
-          .allSockets();
+        const sids = await sio.of("/chat").in("foo").allSockets();
 
         expect(sids).to.contain(chatFooSid);
         expect(sids).to.not.contain(chatBarSid);
@@ -633,7 +628,7 @@ describe("socket.io", () => {
       }
     });
 
-    it("should find all clients across namespace rooms", done => {
+    it("should find all clients across namespace rooms", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       let chatFooSid = null;
@@ -645,7 +640,7 @@ describe("socket.io", () => {
         const c3 = client(srv, "/other", { forceNew: true });
         let chatIndex = 0;
         let total = 3;
-        sio.of("/chat").on("connection", socket => {
+        sio.of("/chat").on("connection", (socket) => {
           if (chatIndex++) {
             socket.join("foo");
             chatFooSid = socket.id;
@@ -656,7 +651,7 @@ describe("socket.io", () => {
             --total || getSockets();
           }
         });
-        sio.of("/other").on("connection", socket => {
+        sio.of("/other").on("connection", (socket) => {
           socket.join("foo");
           otherSid = socket.id;
           --total || getSockets();
@@ -670,13 +665,13 @@ describe("socket.io", () => {
       }
     });
 
-    it("should not emit volatile event after regular event", done => {
+    it("should not emit volatile event after regular event", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
 
       let counter = 0;
       srv.listen(() => {
-        sio.of("/chat").on("connection", s => {
+        sio.of("/chat").on("connection", (s) => {
           // Wait to make sure there are no packets being sent for opening the connection
           setTimeout(() => {
             sio.of("/chat").emit("ev", "data");
@@ -696,13 +691,13 @@ describe("socket.io", () => {
       }, 500);
     });
 
-    it("should emit volatile event", done => {
+    it("should emit volatile event", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
 
       let counter = 0;
       srv.listen(() => {
-        sio.of("/chat").on("connection", s => {
+        sio.of("/chat").on("connection", (s) => {
           // Wait to make sure there are no packets being sent for opening the connection
           setTimeout(() => {
             sio.of("/chat").volatile.emit("ev", "data");
@@ -721,13 +716,13 @@ describe("socket.io", () => {
       }, 500);
     });
 
-    it("should enable compression by default", done => {
+    it("should enable compression by default", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       srv.listen(() => {
         const socket = client(srv, "/chat");
-        sio.of("/chat").on("connection", s => {
-          s.conn.once("packetCreate", packet => {
+        sio.of("/chat").on("connection", (s) => {
+          s.conn.once("packetCreate", (packet) => {
             expect(packet.options.compress).to.be(true);
             done();
           });
@@ -736,20 +731,17 @@ describe("socket.io", () => {
       });
     });
 
-    it("should disable compression", done => {
+    it("should disable compression", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       srv.listen(() => {
         const socket = client(srv, "/chat");
-        sio.of("/chat").on("connection", s => {
-          s.conn.once("packetCreate", packet => {
+        sio.of("/chat").on("connection", (s) => {
+          s.conn.once("packetCreate", (packet) => {
             expect(packet.options.compress).to.be(false);
             done();
           });
-          sio
-            .of("/chat")
-            .compress(false)
-            .emit("woot", "hi");
+          sio.of("/chat").compress(false).emit("woot", "hi");
         });
       });
     });
@@ -762,10 +754,10 @@ describe("socket.io", () => {
       );
     });
 
-    it("should close a client without namespace", done => {
+    it("should close a client without namespace", (done) => {
       const srv = createServer();
       const sio = new Server(srv, {
-        connectTimeout: 10
+        connectTimeout: 10,
       });
 
       srv.listen(() => {
@@ -782,7 +774,7 @@ describe("socket.io", () => {
     });
 
     describe("dynamic namespaces", () => {
-      it("should allow connections to dynamic namespaces with a regex", done => {
+      it("should allow connections to dynamic namespaces with a regex", (done) => {
         const srv = createServer();
         const sio = new Server(srv);
         let count = 0;
@@ -790,7 +782,7 @@ describe("socket.io", () => {
           const socket = client(srv, "/dynamic-101");
           let dynamicNsp = sio
             .of(/^\/dynamic-\d+$/)
-            .on("connect", socket => {
+            .on("connect", (socket) => {
               expect(socket.nsp.name).to.be("/dynamic-101");
               dynamicNsp.emit("hello", 1, "2", { 3: "4" });
               if (++count === 4) done();
@@ -799,7 +791,7 @@ describe("socket.io", () => {
               next();
               if (++count === 4) done();
             });
-          socket.on("connect_error", err => {
+          socket.on("connect_error", (err) => {
             expect().fail();
           });
           socket.on("connect", () => {
@@ -814,7 +806,7 @@ describe("socket.io", () => {
         });
       });
 
-      it("should allow connections to dynamic namespaces with a function", done => {
+      it("should allow connections to dynamic namespaces with a function", (done) => {
         const srv = createServer();
         const sio = new Server(srv);
         srv.listen(() => {
@@ -824,14 +816,14 @@ describe("socket.io", () => {
         });
       });
 
-      it("should disallow connections when no dynamic namespace matches", done => {
+      it("should disallow connections when no dynamic namespace matches", (done) => {
         const srv = createServer();
         const sio = new Server(srv);
         srv.listen(() => {
           const socket = client(srv, "/abc");
           sio.of(/^\/dynamic-\d+$/);
           sio.of((name, query, next) => next(null, "/dynamic-101" === name));
-          socket.on("connect_error", err => {
+          socket.on("connect_error", (err) => {
             expect(err.message).to.be("Invalid namespace");
             done();
           });
@@ -841,7 +833,7 @@ describe("socket.io", () => {
   });
 
   describe("socket", () => {
-    it("should not fire events more than once after manually reconnecting", done => {
+    it("should not fire events more than once after manually reconnecting", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       srv.listen(() => {
@@ -858,13 +850,13 @@ describe("socket.io", () => {
       });
     });
 
-    it("should not fire reconnect_failed event more than once when server closed", done => {
+    it("should not fire reconnect_failed event more than once when server closed", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       srv.listen(() => {
         const clientSocket = client(srv, {
           reconnectionAttempts: 3,
-          reconnectionDelay: 10
+          reconnectionDelay: 10,
         });
         clientSocket.on("connect", () => {
           srv.close();
@@ -876,12 +868,12 @@ describe("socket.io", () => {
       });
     });
 
-    it("should receive events", done => {
+    it("should receive events", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       srv.listen(() => {
         const socket = client(srv);
-        sio.on("connection", s => {
+        sio.on("connection", (s) => {
           s.on("random", (a, b, c) => {
             expect(a).to.be(1);
             expect(b).to.be("2");
@@ -893,13 +885,13 @@ describe("socket.io", () => {
       });
     });
 
-    it("should receive message events through `send`", done => {
+    it("should receive message events through `send`", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       srv.listen(() => {
         const socket = client(srv);
-        sio.on("connection", s => {
-          s.on("message", a => {
+        sio.on("connection", (s) => {
+          s.on("message", (a) => {
             expect(a).to.be(1337);
             done();
           });
@@ -908,13 +900,13 @@ describe("socket.io", () => {
       });
     });
 
-    it("should error with null messages", done => {
+    it("should error with null messages", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       srv.listen(() => {
         const socket = client(srv);
-        sio.on("connection", s => {
-          s.on("message", a => {
+        sio.on("connection", (s) => {
+          s.on("message", (a) => {
             expect(a).to.be(null);
             done();
           });
@@ -923,15 +915,15 @@ describe("socket.io", () => {
       });
     });
 
-    it("should handle transport null messages", done => {
+    it("should handle transport null messages", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       srv.listen(() => {
         const socket = client(srv, { reconnection: false });
-        sio.on("connection", s => {
-          s.on("error", err => {
+        sio.on("connection", (s) => {
+          s.on("error", (err) => {
             expect(err).to.be.an(Error);
-            s.on("disconnect", reason => {
+            s.on("disconnect", (reason) => {
               expect(reason).to.be("forced close");
               done();
             });
@@ -941,28 +933,28 @@ describe("socket.io", () => {
       });
     });
 
-    it("should emit events", done => {
+    it("should emit events", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       srv.listen(() => {
         const socket = client(srv);
-        socket.on("woot", a => {
+        socket.on("woot", (a) => {
           expect(a).to.be("tobi");
           done();
         });
-        sio.on("connection", s => {
+        sio.on("connection", (s) => {
           s.emit("woot", "tobi");
         });
       });
     });
 
-    it("should emit events with utf8 multibyte character", done => {
+    it("should emit events with utf8 multibyte character", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       srv.listen(() => {
         const socket = client(srv);
         let i = 0;
-        socket.on("hoot", a => {
+        socket.on("hoot", (a) => {
           expect(a).to.be("utf8 — string");
           i++;
 
@@ -970,7 +962,7 @@ describe("socket.io", () => {
             done();
           }
         });
-        sio.on("connection", s => {
+        sio.on("connection", (s) => {
           s.emit("hoot", "utf8 — string");
           s.emit("hoot", "utf8 — string");
           s.emit("hoot", "utf8 — string");
@@ -978,20 +970,20 @@ describe("socket.io", () => {
       });
     });
 
-    it("should emit events with binary data", done => {
+    it("should emit events with binary data", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       srv.listen(() => {
         const socket = client(srv);
         let imageData;
-        socket.on("doge", a => {
+        socket.on("doge", (a) => {
           expect(Buffer.isBuffer(a)).to.be(true);
           expect(imageData.length).to.equal(a.length);
           expect(imageData[0]).to.equal(a[0]);
           expect(imageData[imageData.length - 1]).to.equal(a[a.length - 1]);
           done();
         });
-        sio.on("connection", s => {
+        sio.on("connection", (s) => {
           fs.readFile(join(__dirname, "support", "doge.jpg"), (err, data) => {
             if (err) return done(err);
             imageData = data;
@@ -1001,7 +993,7 @@ describe("socket.io", () => {
       });
     });
 
-    it("should emit events with several types of data (including binary)", done => {
+    it("should emit events with several types of data (including binary)", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       srv.listen(() => {
@@ -1017,7 +1009,7 @@ describe("socket.io", () => {
           expect(Buffer.isBuffer(f[2])).to.be(true);
           done();
         });
-        sio.on("connection", s => {
+        sio.on("connection", (s) => {
           fs.readFile(join(__dirname, "support", "doge.jpg"), (err, data) => {
             if (err) return done(err);
             const buf = Buffer.from("asdfasdf", "utf8");
@@ -1027,13 +1019,13 @@ describe("socket.io", () => {
       });
     });
 
-    it("should receive events with binary data", done => {
+    it("should receive events with binary data", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       srv.listen(() => {
         const socket = client(srv);
-        sio.on("connection", s => {
-          s.on("buff", a => {
+        sio.on("connection", (s) => {
+          s.on("buff", (a) => {
             expect(Buffer.isBuffer(a)).to.be(true);
             done();
           });
@@ -1043,12 +1035,12 @@ describe("socket.io", () => {
       });
     });
 
-    it("should receive events with several types of data (including binary)", done => {
+    it("should receive events with several types of data (including binary)", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       srv.listen(() => {
         const socket = client(srv);
-        sio.on("connection", s => {
+        sio.on("connection", (s) => {
           s.on("multiple", (a, b, c, d, e, f) => {
             expect(a).to.be(1);
             expect(Buffer.isBuffer(b)).to.be(true);
@@ -1066,20 +1058,20 @@ describe("socket.io", () => {
             socket.emit("multiple", 1, data, "3", [4], buf, [
               data,
               "swag",
-              buf
+              buf,
             ]);
           });
         });
       });
     });
 
-    it("should not emit volatile event after regular event (polling)", done => {
+    it("should not emit volatile event after regular event (polling)", (done) => {
       const srv = createServer();
       const sio = new Server(srv, { transports: ["polling"] });
 
       let counter = 0;
       srv.listen(() => {
-        sio.on("connection", s => {
+        sio.on("connection", (s) => {
           s.emit("ev", "data");
           s.volatile.emit("ev", "data");
         });
@@ -1096,13 +1088,13 @@ describe("socket.io", () => {
       }, 200);
     });
 
-    it("should not emit volatile event after regular event (ws)", done => {
+    it("should not emit volatile event after regular event (ws)", (done) => {
       const srv = createServer();
       const sio = new Server(srv, { transports: ["websocket"] });
 
       let counter = 0;
       srv.listen(() => {
-        sio.on("connection", s => {
+        sio.on("connection", (s) => {
           s.emit("ev", "data");
           s.volatile.emit("ev", "data");
         });
@@ -1119,13 +1111,13 @@ describe("socket.io", () => {
       }, 200);
     });
 
-    it("should emit volatile event (polling)", done => {
+    it("should emit volatile event (polling)", (done) => {
       const srv = createServer();
       const sio = new Server(srv, { transports: ["polling"] });
 
       let counter = 0;
       srv.listen(() => {
-        sio.on("connection", s => {
+        sio.on("connection", (s) => {
           // Wait to make sure there are no packets being sent for opening the connection
           setTimeout(() => {
             s.volatile.emit("ev", "data");
@@ -1144,13 +1136,13 @@ describe("socket.io", () => {
       }, 500);
     });
 
-    it("should emit volatile event (ws)", done => {
+    it("should emit volatile event (ws)", (done) => {
       const srv = createServer();
       const sio = new Server(srv, { transports: ["websocket"] });
 
       let counter = 0;
       srv.listen(() => {
-        sio.on("connection", s => {
+        sio.on("connection", (s) => {
           // Wait to make sure there are no packets being sent for opening the connection
           setTimeout(() => {
             s.volatile.emit("ev", "data");
@@ -1169,13 +1161,13 @@ describe("socket.io", () => {
       }, 200);
     });
 
-    it("should emit only one consecutive volatile event (polling)", done => {
+    it("should emit only one consecutive volatile event (polling)", (done) => {
       const srv = createServer();
       const sio = new Server(srv, { transports: ["polling"] });
 
       let counter = 0;
       srv.listen(() => {
-        sio.on("connection", s => {
+        sio.on("connection", (s) => {
           // Wait to make sure there are no packets being sent for opening the connection
           setTimeout(() => {
             s.volatile.emit("ev", "data");
@@ -1195,13 +1187,13 @@ describe("socket.io", () => {
       }, 500);
     });
 
-    it("should emit only one consecutive volatile event (ws)", done => {
+    it("should emit only one consecutive volatile event (ws)", (done) => {
       const srv = createServer();
       const sio = new Server(srv, { transports: ["websocket"] });
 
       let counter = 0;
       srv.listen(() => {
-        sio.on("connection", s => {
+        sio.on("connection", (s) => {
           // Wait to make sure there are no packets being sent for opening the connection
           setTimeout(() => {
             s.volatile.emit("ev", "data");
@@ -1221,13 +1213,13 @@ describe("socket.io", () => {
       }, 200);
     });
 
-    it("should emit regular events after trying a failed volatile event (polling)", done => {
+    it("should emit regular events after trying a failed volatile event (polling)", (done) => {
       const srv = createServer();
       const sio = new Server(srv, { transports: ["polling"] });
 
       let counter = 0;
       srv.listen(() => {
-        sio.on("connection", s => {
+        sio.on("connection", (s) => {
           // Wait to make sure there are no packets being sent for opening the connection
           setTimeout(() => {
             s.emit("ev", "data");
@@ -1248,13 +1240,13 @@ describe("socket.io", () => {
       }, 200);
     });
 
-    it("should emit regular events after trying a failed volatile event (ws)", done => {
+    it("should emit regular events after trying a failed volatile event (ws)", (done) => {
       const srv = createServer();
       const sio = new Server(srv, { transports: ["websocket"] });
 
       let counter = 0;
       srv.listen(() => {
-        sio.on("connection", s => {
+        sio.on("connection", (s) => {
           // Wait to make sure there are no packets being sent for opening the connection
           setTimeout(() => {
             s.emit("ev", "data");
@@ -1275,28 +1267,28 @@ describe("socket.io", () => {
       }, 200);
     });
 
-    it("should emit message events through `send`", done => {
+    it("should emit message events through `send`", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       srv.listen(() => {
         const socket = client(srv);
-        socket.on("message", a => {
+        socket.on("message", (a) => {
           expect(a).to.be("a");
           done();
         });
-        sio.on("connection", s => {
+        sio.on("connection", (s) => {
           s.send("a");
         });
       });
     });
 
-    it("should receive event with callbacks", done => {
+    it("should receive event with callbacks", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       srv.listen(() => {
         const socket = client(srv);
-        sio.on("connection", s => {
-          s.on("woot", fn => {
+        sio.on("connection", (s) => {
+          s.on("woot", (fn) => {
             fn(1, 2);
           });
           socket.emit("woot", (a, b) => {
@@ -1308,13 +1300,13 @@ describe("socket.io", () => {
       });
     });
 
-    it("should receive all events emitted from namespaced client immediately and in order", done => {
+    it("should receive all events emitted from namespaced client immediately and in order", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       let total = 0;
       srv.listen(() => {
-        sio.of("/chat", s => {
-          s.on("hi", letter => {
+        sio.of("/chat", (s) => {
+          s.on("hi", (letter) => {
             total++;
             if (total == 2 && letter == "b") {
               done();
@@ -1332,13 +1324,13 @@ describe("socket.io", () => {
       });
     });
 
-    it("should emit events with callbacks", done => {
+    it("should emit events with callbacks", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       srv.listen(() => {
         const socket = client(srv);
-        sio.on("connection", s => {
-          socket.on("hi", fn => {
+        sio.on("connection", (s) => {
+          socket.on("hi", (fn) => {
             fn();
           });
           s.emit("hi", () => {
@@ -1348,12 +1340,12 @@ describe("socket.io", () => {
       });
     });
 
-    it("should receive events with args and callback", done => {
+    it("should receive events with args and callback", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       srv.listen(() => {
         const socket = client(srv);
-        sio.on("connection", s => {
+        sio.on("connection", (s) => {
           s.on("woot", (a, b, fn) => {
             expect(a).to.be(1);
             expect(b).to.be(2);
@@ -1366,12 +1358,12 @@ describe("socket.io", () => {
       });
     });
 
-    it("should emit events with args and callback", done => {
+    it("should emit events with args and callback", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       srv.listen(() => {
         const socket = client(srv);
-        sio.on("connection", s => {
+        sio.on("connection", (s) => {
           socket.on("hi", (a, b, fn) => {
             expect(a).to.be(1);
             expect(b).to.be(2);
@@ -1384,12 +1376,12 @@ describe("socket.io", () => {
       });
     });
 
-    it("should receive events with binary args and callbacks", done => {
+    it("should receive events with binary args and callbacks", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       srv.listen(() => {
         const socket = client(srv);
-        sio.on("connection", s => {
+        sio.on("connection", (s) => {
           s.on("woot", (buf, fn) => {
             expect(Buffer.isBuffer(buf)).to.be(true);
             fn(1, 2);
@@ -1403,12 +1395,12 @@ describe("socket.io", () => {
       });
     });
 
-    it("should emit events with binary args and callback", done => {
+    it("should emit events with binary args and callback", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       srv.listen(() => {
         const socket = client(srv);
-        sio.on("connection", s => {
+        sio.on("connection", (s) => {
           socket.on("hi", (a, fn) => {
             expect(Buffer.isBuffer(a)).to.be(true);
             fn();
@@ -1420,16 +1412,16 @@ describe("socket.io", () => {
       });
     });
 
-    it("should emit events and receive binary data in a callback", done => {
+    it("should emit events and receive binary data in a callback", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       srv.listen(() => {
         const socket = client(srv);
-        sio.on("connection", s => {
-          socket.on("hi", fn => {
+        sio.on("connection", (s) => {
+          socket.on("hi", (fn) => {
             fn(Buffer.alloc(1));
           });
-          s.emit("hi", a => {
+          s.emit("hi", (a) => {
             expect(Buffer.isBuffer(a)).to.be(true);
             done();
           });
@@ -1437,16 +1429,16 @@ describe("socket.io", () => {
       });
     });
 
-    it("should receive events and pass binary data in a callback", done => {
+    it("should receive events and pass binary data in a callback", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       srv.listen(() => {
         const socket = client(srv);
-        sio.on("connection", s => {
-          s.on("woot", fn => {
+        sio.on("connection", (s) => {
+          s.on("woot", (fn) => {
             fn(Buffer.alloc(2));
           });
-          socket.emit("woot", a => {
+          socket.emit("woot", (a) => {
             expect(Buffer.isBuffer(a)).to.be(true);
             done();
           });
@@ -1454,24 +1446,24 @@ describe("socket.io", () => {
       });
     });
 
-    it("should have access to the client", done => {
+    it("should have access to the client", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       srv.listen(() => {
         const socket = client(srv);
-        sio.on("connection", s => {
+        sio.on("connection", (s) => {
           expect(s.client).to.be.an("object");
           done();
         });
       });
     });
 
-    it("should have access to the connection", done => {
+    it("should have access to the connection", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       srv.listen(() => {
         const socket = client(srv);
-        sio.on("connection", s => {
+        sio.on("connection", (s) => {
           expect(s.client.conn).to.be.an("object");
           expect(s.conn).to.be.an("object");
           done();
@@ -1479,12 +1471,12 @@ describe("socket.io", () => {
       });
     });
 
-    it("should have access to the request", done => {
+    it("should have access to the request", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       srv.listen(() => {
         const socket = client(srv);
-        sio.on("connection", s => {
+        sio.on("connection", (s) => {
           expect(s.client.request.headers).to.be.an("object");
           expect(s.request.headers).to.be.an("object");
           done();
@@ -1492,12 +1484,12 @@ describe("socket.io", () => {
       });
     });
 
-    it("should see query parameters in the request", done => {
+    it("should see query parameters in the request", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       srv.listen(() => {
         const socket = client(srv, { query: { key1: 1, key2: 2 } });
-        sio.on("connection", s => {
+        sio.on("connection", (s) => {
           const parsed = require("url").parse(s.request.url);
           const query = require("querystring").parse(parsed.query);
           expect(query.key1).to.be("1");
@@ -1507,15 +1499,15 @@ describe("socket.io", () => {
       });
     });
 
-    it("should see query parameters sent from secondary namespace connections in handshake object", done => {
+    it("should see query parameters sent from secondary namespace connections in handshake object", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       const client1 = client(srv);
       const client2 = client(srv, "/connection2", {
-        auth: { key1: "aa", key2: "&=bb" }
+        auth: { key1: "aa", key2: "&=bb" },
       });
-      sio.on("connection", s => {});
-      sio.of("/connection2").on("connection", s => {
+      sio.on("connection", (s) => {});
+      sio.of("/connection2").on("connection", (s) => {
         expect(s.handshake.query.key1).to.be(undefined);
         expect(s.handshake.query.EIO).to.be("4");
         expect(s.handshake.auth.key1).to.be("aa");
@@ -1524,19 +1516,19 @@ describe("socket.io", () => {
       });
     });
 
-    it("should handle very large json", function(done) {
+    it("should handle very large json", function (done) {
       this.timeout(30000);
       const srv = createServer();
       const sio = new Server(srv, { perMessageDeflate: false });
       let received = 0;
       srv.listen(() => {
         const socket = client(srv);
-        socket.on("big", a => {
+        socket.on("big", (a) => {
           expect(Buffer.isBuffer(a.json)).to.be(false);
           if (++received == 3) done();
           else socket.emit("big", a);
         });
-        sio.on("connection", s => {
+        sio.on("connection", (s) => {
           fs.readFile(
             join(__dirname, "fixtures", "big.json"),
             (err, data: any) => {
@@ -1545,31 +1537,31 @@ describe("socket.io", () => {
               s.emit("big", { hello: "friend", json: data });
             }
           );
-          s.on("big", a => {
+          s.on("big", (a) => {
             s.emit("big", a);
           });
         });
       });
     });
 
-    it("should handle very large binary data", function(done) {
+    it("should handle very large binary data", function (done) {
       this.timeout(30000);
       const srv = createServer();
       const sio = new Server(srv, { perMessageDeflate: false });
       let received = 0;
       srv.listen(() => {
         const socket = client(srv);
-        socket.on("big", a => {
+        socket.on("big", (a) => {
           expect(Buffer.isBuffer(a.image)).to.be(true);
           if (++received == 3) done();
           else socket.emit("big", a);
         });
-        sio.on("connection", s => {
+        sio.on("connection", (s) => {
           fs.readFile(join(__dirname, "fixtures", "big.jpg"), (err, data) => {
             if (err) return done(err);
             s.emit("big", { hello: "friend", image: data });
           });
-          s.on("big", a => {
+          s.on("big", (a) => {
             expect(Buffer.isBuffer(a.image)).to.be(true);
             s.emit("big", a);
           });
@@ -1577,12 +1569,12 @@ describe("socket.io", () => {
       });
     });
 
-    it("should be able to emit after server close and restart", done => {
+    it("should be able to emit after server close and restart", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
 
-      sio.on("connection", socket => {
-        socket.on("ev", data => {
+      sio.on("connection", (socket) => {
+        socket.on("ev", (data) => {
           expect(data).to.be("payload");
           done();
         });
@@ -1592,7 +1584,7 @@ describe("socket.io", () => {
         const { port } = srv.address() as AddressInfo;
         const clientSocket = client(srv, {
           reconnectionAttempts: 10,
-          reconnectionDelay: 100
+          reconnectionDelay: 100,
         });
         clientSocket.once("connect", () => {
           srv.close(() => {
@@ -1605,13 +1597,13 @@ describe("socket.io", () => {
       });
     });
 
-    it("should enable compression by default", done => {
+    it("should enable compression by default", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       srv.listen(() => {
         const socket = client(srv, "/chat");
-        sio.of("/chat").on("connection", s => {
-          s.conn.once("packetCreate", packet => {
+        sio.of("/chat").on("connection", (s) => {
+          s.conn.once("packetCreate", (packet) => {
             expect(packet.options.compress).to.be(true);
             done();
           });
@@ -1620,30 +1612,27 @@ describe("socket.io", () => {
       });
     });
 
-    it("should disable compression", done => {
+    it("should disable compression", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       srv.listen(() => {
         const socket = client(srv, "/chat");
-        sio.of("/chat").on("connection", s => {
-          s.conn.once("packetCreate", packet => {
+        sio.of("/chat").on("connection", (s) => {
+          s.conn.once("packetCreate", (packet) => {
             expect(packet.options.compress).to.be(false);
             done();
           });
-          sio
-            .of("/chat")
-            .compress(false)
-            .emit("woot", "hi");
+          sio.of("/chat").compress(false).emit("woot", "hi");
         });
       });
     });
 
-    it("should error with raw binary and warn", done => {
+    it("should error with raw binary and warn", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       srv.listen(() => {
         const socket = client(srv, { reconnection: false });
-        sio.on("connection", s => {
+        sio.on("connection", (s) => {
           s.conn.on("upgrade", () => {
             console.log(
               "\u001b[96mNote: warning expected and normal in test.\u001b[39m"
@@ -1657,12 +1646,12 @@ describe("socket.io", () => {
       });
     });
 
-    it("should not crash when receiving an error packet without handler", done => {
+    it("should not crash when receiving an error packet without handler", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       srv.listen(() => {
         const socket = client(srv, { reconnection: false });
-        sio.on("connection", s => {
+        sio.on("connection", (s) => {
           s.conn.on("upgrade", () => {
             console.log(
               "\u001b[96mNote: warning expected and normal in test.\u001b[39m"
@@ -1676,13 +1665,13 @@ describe("socket.io", () => {
       });
     });
 
-    it("should not crash with raw binary", done => {
+    it("should not crash with raw binary", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       srv.listen(() => {
         const socket = client(srv, { reconnection: false });
-        sio.on("connection", s => {
-          s.once("error", err => {
+        sio.on("connection", (s) => {
+          s.once("error", (err) => {
             expect(err.message).to.match(/Illegal attachments/);
             done();
           });
@@ -1693,13 +1682,13 @@ describe("socket.io", () => {
       });
     });
 
-    it("should handle empty binary packet", done => {
+    it("should handle empty binary packet", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       srv.listen(() => {
         const socket = client(srv, { reconnection: false });
-        sio.on("connection", s => {
-          s.once("error", err => {
+        sio.on("connection", (s) => {
+          s.once("error", (err) => {
             expect(err.message).to.match(/Illegal attachments/);
             done();
           });
@@ -1710,7 +1699,7 @@ describe("socket.io", () => {
       });
     });
 
-    it("should not crash when messing with Object prototype (and other globals)", done => {
+    it("should not crash when messing with Object prototype (and other globals)", (done) => {
       // @ts-ignore
       Object.prototype.foo = "bar";
       // @ts-ignore
@@ -1722,7 +1711,7 @@ describe("socket.io", () => {
       srv.listen(() => {
         const socket = client(srv);
 
-        sio.on("connection", s => {
+        sio.on("connection", (s) => {
           s.disconnect(true);
           sio.close();
           setTimeout(() => {
@@ -1732,13 +1721,13 @@ describe("socket.io", () => {
       });
     });
 
-    it("should throw on reserved event", done => {
+    it("should throw on reserved event", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
 
       srv.listen(() => {
         const socket = client(srv);
-        sio.on("connection", s => {
+        sio.on("connection", (s) => {
           expect(() => s.emit("connect_error")).to.throwException(
             /"connect_error" is a reserved event name/
           );
@@ -1749,7 +1738,7 @@ describe("socket.io", () => {
     });
 
     describe("onAny", () => {
-      it("should call listener", done => {
+      it("should call listener", (done) => {
         const srv = createServer();
         const sio = new Server(srv);
 
@@ -1768,7 +1757,7 @@ describe("socket.io", () => {
         });
       });
 
-      it("should prepend listener", done => {
+      it("should prepend listener", (done) => {
         const srv = createServer();
         const sio = new Server(srv);
 
@@ -1796,7 +1785,7 @@ describe("socket.io", () => {
         });
       });
 
-      it("should remove listener", done => {
+      it("should remove listener", (done) => {
         const srv = createServer();
         const sio = new Server(srv);
 
@@ -1822,7 +1811,7 @@ describe("socket.io", () => {
   });
 
   describe("messaging many", () => {
-    it("emits to a namespace", done => {
+    it("emits to a namespace", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       let total = 2;
@@ -1831,11 +1820,11 @@ describe("socket.io", () => {
         const socket1 = client(srv, { multiplex: false });
         const socket2 = client(srv, { multiplex: false });
         const socket3 = client(srv, "/test");
-        socket1.on("a", a => {
+        socket1.on("a", (a) => {
           expect(a).to.be("b");
           --total || done();
         });
-        socket2.on("a", a => {
+        socket2.on("a", (a) => {
           expect(a).to.be("b");
           --total || done();
         });
@@ -1844,10 +1833,10 @@ describe("socket.io", () => {
         });
 
         let sockets = 3;
-        sio.on("connection", socket => {
+        sio.on("connection", (socket) => {
           --sockets || emit();
         });
-        sio.of("/test", socket => {
+        sio.of("/test", (socket) => {
           --sockets || emit();
         });
 
@@ -1857,7 +1846,7 @@ describe("socket.io", () => {
       });
     });
 
-    it("emits binary data to a namespace", done => {
+    it("emits binary data to a namespace", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       let total = 2;
@@ -1866,11 +1855,11 @@ describe("socket.io", () => {
         const socket1 = client(srv, { multiplex: false });
         const socket2 = client(srv, { multiplex: false });
         const socket3 = client(srv, "/test");
-        socket1.on("bin", a => {
+        socket1.on("bin", (a) => {
           expect(Buffer.isBuffer(a)).to.be(true);
           --total || done();
         });
-        socket2.on("bin", a => {
+        socket2.on("bin", (a) => {
           expect(Buffer.isBuffer(a)).to.be(true);
           --total || done();
         });
@@ -1879,10 +1868,10 @@ describe("socket.io", () => {
         });
 
         let sockets = 3;
-        sio.on("connection", socket => {
+        sio.on("connection", (socket) => {
           --sockets || emit();
         });
-        sio.of("/test", socket => {
+        sio.of("/test", (socket) => {
           --sockets || emit();
         });
 
@@ -1892,7 +1881,7 @@ describe("socket.io", () => {
       });
     });
 
-    it("emits to the rest", done => {
+    it("emits to the rest", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       const total = 2;
@@ -1901,7 +1890,7 @@ describe("socket.io", () => {
         const socket1 = client(srv, { multiplex: false });
         const socket2 = client(srv, { multiplex: false });
         const socket3 = client(srv, "/test");
-        socket1.on("a", a => {
+        socket1.on("a", (a) => {
           expect(a).to.be("b");
           socket1.emit("finish");
         });
@@ -1914,7 +1903,7 @@ describe("socket.io", () => {
         });
 
         const sockets = 2;
-        sio.on("connection", socket => {
+        sio.on("connection", (socket) => {
           socket.on("broadcast", () => {
             socket.broadcast.emit("a", "b");
           });
@@ -1925,7 +1914,7 @@ describe("socket.io", () => {
       });
     });
 
-    it("emits to rooms", done => {
+    it("emits to rooms", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       const total = 2;
@@ -1943,20 +1932,20 @@ describe("socket.io", () => {
         socket1.emit("join", "woot");
         socket1.emit("emit", "woot");
 
-        sio.on("connection", socket => {
+        sio.on("connection", (socket) => {
           socket.on("join", (room, fn) => {
             socket.join(room);
             fn && fn();
           });
 
-          socket.on("emit", room => {
+          socket.on("emit", (room) => {
             sio.in(room).emit("a");
           });
         });
       });
     });
 
-    it("emits to rooms avoiding dupes", done => {
+    it("emits to rooms avoiding dupes", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       let total = 2;
@@ -1981,24 +1970,21 @@ describe("socket.io", () => {
           socket2.emit("emit");
         });
 
-        sio.on("connection", socket => {
+        sio.on("connection", (socket) => {
           socket.on("join", (room, fn) => {
             socket.join(room);
             fn && fn();
           });
 
-          socket.on("emit", room => {
-            sio
-              .in("woot")
-              .in("test")
-              .emit("a");
+          socket.on("emit", (room) => {
+            sio.in("woot").in("test").emit("a");
             sio.in("third").emit("b");
           });
         });
       });
     });
 
-    it("broadcasts to rooms", done => {
+    it("broadcasts to rooms", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       let total = 2;
@@ -2027,7 +2013,7 @@ describe("socket.io", () => {
           --total || done();
         });
 
-        sio.on("connection", socket => {
+        sio.on("connection", (socket) => {
           socket.on("join", (room, fn) => {
             socket.join(room);
             fn && fn();
@@ -2041,7 +2027,7 @@ describe("socket.io", () => {
       });
     });
 
-    it("broadcasts binary data to rooms", done => {
+    it("broadcasts binary data to rooms", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       let total = 2;
@@ -2057,25 +2043,25 @@ describe("socket.io", () => {
           socket3.emit("broadcast");
         });
 
-        socket1.on("bin", data => {
+        socket1.on("bin", (data) => {
           throw new Error("got bin in socket1");
         });
-        socket2.on("bin", data => {
+        socket2.on("bin", (data) => {
           expect(Buffer.isBuffer(data)).to.be(true);
           --total || done();
         });
-        socket2.on("bin2", data => {
+        socket2.on("bin2", (data) => {
           throw new Error("socket2 got bin2");
         });
-        socket3.on("bin", data => {
+        socket3.on("bin", (data) => {
           throw new Error("socket3 got bin");
         });
-        socket3.on("bin2", data => {
+        socket3.on("bin2", (data) => {
           expect(Buffer.isBuffer(data)).to.be(true);
           --total || done();
         });
 
-        sio.on("connection", socket => {
+        sio.on("connection", (socket) => {
           socket.on("join", (room, fn) => {
             socket.join(room);
             fn && fn();
@@ -2088,13 +2074,13 @@ describe("socket.io", () => {
       });
     });
 
-    it("keeps track of rooms", done => {
+    it("keeps track of rooms", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
 
       srv.listen(() => {
         const socket = client(srv);
-        sio.on("connection", s => {
+        sio.on("connection", (s) => {
           s.join("a");
           expect(s.rooms).to.contain(s.id, "a");
           s.join("b");
@@ -2110,13 +2096,13 @@ describe("socket.io", () => {
       });
     });
 
-    it("deletes empty rooms", done => {
+    it("deletes empty rooms", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
 
       srv.listen(() => {
         const socket = client(srv);
-        sio.on("connection", s => {
+        sio.on("connection", (s) => {
           s.join("a");
           expect(s.nsp.adapter.rooms).to.contain("a");
           s.leave("a");
@@ -2126,13 +2112,13 @@ describe("socket.io", () => {
       });
     });
 
-    it("should properly cleanup left rooms", done => {
+    it("should properly cleanup left rooms", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
 
       srv.listen(() => {
         const socket = client(srv);
-        sio.on("connection", s => {
+        sio.on("connection", (s) => {
           s.join("a");
           expect(s.rooms).to.contain(s.id, "a");
           s.join("b");
@@ -2146,13 +2132,13 @@ describe("socket.io", () => {
       });
     });
 
-    it("allows to join several rooms at once", done => {
+    it("allows to join several rooms at once", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
 
       srv.listen(() => {
         const socket = client(srv);
-        sio.on("connection", s => {
+        sio.on("connection", (s) => {
           s.join(["a", "b", "c"]);
           expect(s.rooms).to.contain(s.id, "a", "b", "c");
           done();
@@ -2164,7 +2150,7 @@ describe("socket.io", () => {
   describe("middleware", () => {
     const { Socket } = require("../dist/socket");
 
-    it("should call functions", done => {
+    it("should call functions", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       let run = 0;
@@ -2187,7 +2173,7 @@ describe("socket.io", () => {
       });
     });
 
-    it("should pass errors", done => {
+    it("should pass errors", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       const run = 0;
@@ -2202,14 +2188,14 @@ describe("socket.io", () => {
         socket.on("connect", () => {
           done(new Error("nope"));
         });
-        socket.on("connect_error", err => {
+        socket.on("connect_error", (err) => {
           expect(err.message).to.be("Authentication error");
           done();
         });
       });
     });
 
-    it("should pass an object", done => {
+    it("should pass an object", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       sio.use((socket, next) => {
@@ -2223,7 +2209,7 @@ describe("socket.io", () => {
         socket.on("connect", () => {
           done(new Error("nope"));
         });
-        socket.on("connect_error", err => {
+        socket.on("connect_error", (err) => {
           expect(err).to.be.an(Error);
           expect(err.message).to.eql("Authentication error");
           expect(err.data).to.eql({ a: "b", c: 3 });
@@ -2232,7 +2218,7 @@ describe("socket.io", () => {
       });
     });
 
-    it("should only call connection after fns", done => {
+    it("should only call connection after fns", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       sio.use((socket: any, next) => {
@@ -2241,14 +2227,14 @@ describe("socket.io", () => {
       });
       srv.listen(() => {
         const socket = client(srv);
-        sio.on("connection", socket => {
+        sio.on("connection", (socket) => {
           expect(socket.name).to.be("guillermo");
           done();
         });
       });
     });
 
-    it("should only call connection after (lengthy) fns", done => {
+    it("should only call connection after (lengthy) fns", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       let authenticated = false;
@@ -2268,7 +2254,7 @@ describe("socket.io", () => {
       });
     });
 
-    it("should be ignored if socket gets closed", done => {
+    it("should be ignored if socket gets closed", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
       let socket;
@@ -2283,16 +2269,16 @@ describe("socket.io", () => {
       });
       srv.listen(() => {
         socket = client(srv);
-        sio.on("connection", socket => {
+        sio.on("connection", (socket) => {
           done(new Error("should not fire"));
         });
       });
     });
 
-    it("should call functions in expected order", done => {
+    it("should call functions in expected order", (done) => {
       const srv = createServer();
       const sio = new Server(srv);
-      const result = [];
+      const result: number[] = [];
 
       sio.use(() => {
         done(new Error("should not fire"));
@@ -2319,7 +2305,7 @@ describe("socket.io", () => {
       });
     });
 
-    it("should disable the merge of handshake packets", done => {
+    it("should disable the merge of handshake packets", (done) => {
       const srv = createServer();
       const sio = new Server();
       sio.use((socket, next) => {
@@ -2332,7 +2318,7 @@ describe("socket.io", () => {
       });
     });
 
-    it("should work with a custom namespace", done => {
+    it("should work with a custom namespace", (done) => {
       const srv = createServer();
       const sio = new Server();
       sio.listen(srv);
diff --git a/test/support/util.ts b/test/support/util.ts
index b89a92e6..1cfb3c3e 100644
--- a/test/support/util.ts
+++ b/test/support/util.ts
@@ -3,15 +3,15 @@ const i = expect.stringify;
 
 // add support for Set/Map
 const contain = expect.Assertion.prototype.contain;
-expect.Assertion.prototype.contain = function(...args) {
+expect.Assertion.prototype.contain = function (...args) {
   if (typeof this.obj === "object") {
-    args.forEach(obj => {
+    args.forEach((obj) => {
       this.assert(
         this.obj.has(obj),
-        function() {
+        function () {
           return "expected " + i(this.obj) + " to contain " + i(obj);
         },
-        function() {
+        function () {
           return "expected " + i(this.obj) + " to not contain " + i(obj);
         }
       );
diff --git a/tsconfig.json b/tsconfig.json
index c2197027..eab8e447 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -4,7 +4,17 @@
     "target": "es2017",
     "module": "commonjs",
     "declaration": true,
-    "esModuleInterop": true
+    "esModuleInterop": true,
+
+    "incremental": true,
+    "skipLibCheck": true,
+
+    "strict": true,
+    "noImplicitAny": false,
+    "noImplicitThis": false,
+    "strictPropertyInitialization": false,
+    "noImplicitReturns": true,
+    "importsNotUsedAsValues": "error",
   },
   "include": [
     "./lib/**/*"