diff --git a/packages/engine.io-parser/lib/commons.ts b/packages/engine.io-parser/lib/commons.ts
index 2562382f72..2930da8089 100644
--- a/packages/engine.io-parser/lib/commons.ts
+++ b/packages/engine.io-parser/lib/commons.ts
@@ -32,7 +32,7 @@ export type RawData = any;
 
 export interface Packet {
   type: PacketType;
-  options?: { compress: boolean };
+  options?: { compress: boolean, wsPreEncoded?: string, wsPreEncodedFrame?: boolean };
   data?: RawData;
 }
 
diff --git a/packages/engine.io/lib/engine.io.ts b/packages/engine.io/lib/engine.io.ts
index 3ba9e67f8f..83f87711a0 100644
--- a/packages/engine.io/lib/engine.io.ts
+++ b/packages/engine.io/lib/engine.io.ts
@@ -1,4 +1,4 @@
-import { createServer } from "http";
+import { createServer, Server as HttpServer } from "http";
 import { Server, AttachOptions, ServerOptions } from "./server";
 import transports from "./transports/index";
 import * as parser from "engine.io-parser";
@@ -11,17 +11,21 @@ export { Transport } from "./transport";
 export const protocol = parser.protocol;
 
 /**
- * Creates an http.Server exclusively used for WS upgrades.
+ * Creates an http.Server exclusively used for WS upgrades, and start listening.
  *
- * @param {Number} port
- * @param {Function} callback
- * @param {Object} options
- * @return {Server} websocket.io server
+ * @param port
+ * @param options
+ * @param listenCallback callback for http.Server.listen()
+ * @return engine.io server
  */
 
-function listen(port, options: AttachOptions & ServerOptions, fn) {
+function listen(
+  port: number,
+  options?: AttachOptions & ServerOptions,
+  listenCallback?: () => void,
+): Server {
   if ("function" === typeof options) {
-    fn = options;
+    listenCallback = options;
     options = {};
   }
 
@@ -34,7 +38,7 @@ function listen(port, options: AttachOptions & ServerOptions, fn) {
   const engine = attach(server, options);
   engine.httpServer = server;
 
-  server.listen(port, fn);
+  server.listen(port, listenCallback);
 
   return engine;
 }
@@ -42,12 +46,12 @@ function listen(port, options: AttachOptions & ServerOptions, fn) {
 /**
  * Captures upgrade requests for a http.Server.
  *
- * @param {http.Server} server
- * @param {Object} options
- * @return {Server} engine server
+ * @param server
+ * @param options
+ * @return engine.io server
  */
 
-function attach(server, options: AttachOptions & ServerOptions) {
+function attach(server: HttpServer, options: AttachOptions & ServerOptions): Server {
   const engine = new Server(options);
   engine.attach(server, options);
   return engine;
diff --git a/packages/engine.io/lib/parser-v3/index.ts b/packages/engine.io/lib/parser-v3/index.ts
index 367ae13767..8cf34dc2f2 100644
--- a/packages/engine.io/lib/parser-v3/index.ts
+++ b/packages/engine.io/lib/parser-v3/index.ts
@@ -59,8 +59,7 @@ const EMPTY_BUFFER = Buffer.concat([]);
  *
  * @api private
  */
-
-export function encodePacket (packet, supportsBinary, utf8encode, callback) {
+export function encodePacket (packet: any, supportsBinary?: any, utf8encode?: any, callback?: any) {
   if (typeof supportsBinary === 'function') {
     callback = supportsBinary;
     supportsBinary = null;
@@ -86,7 +85,7 @@ export function encodePacket (packet, supportsBinary, utf8encode, callback) {
   }
 
   return callback('' + encoded);
-};
+}
 
 /**
  * Encode Buffer data
@@ -120,16 +119,16 @@ export function encodeBase64Packet (packet, callback){
 /**
  * Decodes a packet. Data also available as an ArrayBuffer if requested.
  *
- * @return {Object} with `type` and `data` (if any)
+ * @return {import('engine.io-parser').Packet} with `type` and `data` (if any)
  * @api private
  */
 
-export function decodePacket (data, binaryType, utf8decode) {
+export function decodePacket (data: any, binaryType?: any, utf8decode?: any): any {
   if (data === undefined) {
     return err;
   }
 
-  var type;
+  let type: string | number;
 
   // String data
   if (typeof data === 'string') {
@@ -147,6 +146,7 @@ export function decodePacket (data, binaryType, utf8decode) {
       }
     }
 
+    // @ts-expect-error
     if (Number(type) != type || !packetslist[type]) {
       return err;
     }
@@ -274,7 +274,7 @@ function map(ary, each, done) {
  * @api public
  */
 
-export function decodePayload (data, binaryType, callback) {
+export function decodePayload (data: any, binaryType?: any, callback?: any) {
   if (typeof data !== 'string') {
     return decodePayloadAsBinary(data, binaryType, callback);
   }
diff --git a/packages/engine.io/lib/server.ts b/packages/engine.io/lib/server.ts
index bf53f994ac..30cfa50dc2 100644
--- a/packages/engine.io/lib/server.ts
+++ b/packages/engine.io/lib/server.ts
@@ -6,7 +6,12 @@ import { EventEmitter } from "events";
 import { Socket } from "./socket";
 import debugModule from "debug";
 import { serialize } from "cookie";
-import { Server as DEFAULT_WS_ENGINE } from "ws";
+import {
+  Server as DEFAULT_WS_ENGINE,
+  type Server as WsServer,
+  type PerMessageDeflateOptions,
+  type WebSocket as WsWebSocket,
+} from "ws";
 import type {
   IncomingMessage,
   Server as HttpServer,
@@ -16,7 +21,7 @@ import type { CorsOptions, CorsOptionsDelegate } from "cors";
 import type { Duplex } from "stream";
 import { WebTransport } from "./transports/webtransport";
 import { createPacketDecoderStream } from "engine.io-parser";
-import type { EngineRequest } from "./transport";
+import type { EngineRequest, Transport as TransportImpl } from "./transport";
 import type { CookieSerializeOptions } from "./contrib/types.cookie";
 
 const debug = debugModule("engine");
@@ -25,6 +30,11 @@ const kResponseHeaders = Symbol("responseHeaders");
 
 type Transport = "polling" | "websocket" | "webtransport";
 
+export type ErrorCallback = (
+  errorCode?: (typeof Server.errors)[keyof typeof Server.errors],
+  errorContext?: Record<string, unknown> & { name?: string; message?: string },
+) => void;
+
 export interface AttachOptions {
   /**
    * name of the path to capture
@@ -100,7 +110,7 @@ export interface ServerOptions {
    * parameters of the WebSocket permessage-deflate extension (see ws module api docs). Set to false to disable.
    * @default false
    */
-  perMessageDeflate?: boolean | object;
+  perMessageDeflate?: boolean | PerMessageDeflateOptions;
   /**
    * parameters of the http compression for the polling transports (see zlib api docs). Set to false to disable.
    * @default true
@@ -113,7 +123,7 @@ export interface ServerOptions {
    *
    * @default `require("ws").Server`
    */
-  wsEngine?: any;
+  wsEngine?: typeof WsServer;
   /**
    * an optional packet which will be concatenated to the handshake packet emitted by Engine.IO.
    */
@@ -149,7 +159,7 @@ type Middleware = (
   next: (err?: any) => void,
 ) => void;
 
-function parseSessionId(data: string) {
+function parseSessionId(data: string): string | undefined {
   try {
     const parsed = JSON.parse(data);
     if (typeof parsed.sid === "string") {
@@ -224,7 +234,7 @@ export abstract class BaseServer extends EventEmitter {
     this.init();
   }
 
-  protected abstract init();
+  protected abstract init(): void;
 
   /**
    * Compute the pathname of the requests that are handled by the server
@@ -244,10 +254,8 @@ export abstract class BaseServer extends EventEmitter {
 
   /**
    * Returns a list of available transports for upgrade given a certain transport.
-   *
-   * @return {Array}
    */
-  public upgrades(transport: string) {
+  public upgrades(transport: Transport): string[] {
     if (!this.opts.allowUpgrades) return [];
     return transports[transport].upgradesTo || [];
   }
@@ -259,17 +267,18 @@ export abstract class BaseServer extends EventEmitter {
    * @param upgrade - whether it's an upgrade request
    * @param fn
    * @protected
+   * @return whether the request is valid
    */
   protected verify(
-    req: any,
+    req: EngineRequest,
     upgrade: boolean,
-    fn: (errorCode?: number, errorContext?: any) => void,
-  ) {
+    fn: ErrorCallback,
+  ): void | boolean {
     // transport check
     const transport = req._query.transport;
     // WebTransport does not go through the verify() method, see the onWebTransportSession() method
     if (
-      !~this.opts.transports.indexOf(transport) ||
+      !~this.opts.transports.indexOf(transport as Transport) ||
       transport === "webtransport"
     ) {
       debug('unknown transport "%s"', transport);
@@ -408,7 +417,7 @@ export abstract class BaseServer extends EventEmitter {
    *
    * @param {IncomingMessage} req - the request object
    */
-  public generateId(req: IncomingMessage) {
+  public generateId(req: IncomingMessage): string | PromiseLike<string> {
     return base64id.generateId();
   }
 
@@ -423,8 +432,8 @@ export abstract class BaseServer extends EventEmitter {
    */
   protected async handshake(
     transportName: string,
-    req: any,
-    closeConnection: (errorCode?: number, errorContext?: any) => void,
+    req: EngineRequest,
+    closeConnection: ErrorCallback,
   ) {
     const protocol = req._query.EIO === "4" ? 4 : 3; // 3rd revision by default
     if (protocol === 3 && !this.opts.allowEIO3) {
@@ -600,7 +609,7 @@ export abstract class BaseServer extends EventEmitter {
     }
   }
 
-  protected abstract createTransport(transportName, req);
+  protected abstract createTransport(transportName: Transport, req: EngineRequest);
 
   /**
    * Protocol errors mappings.
@@ -613,7 +622,7 @@ export abstract class BaseServer extends EventEmitter {
     BAD_REQUEST: 3,
     FORBIDDEN: 4,
     UNSUPPORTED_PROTOCOL_VERSION: 5,
-  };
+  } as const;
 
   static errorMessages = {
     0: "Transport unknown",
@@ -622,7 +631,7 @@ export abstract class BaseServer extends EventEmitter {
     3: "Bad request",
     4: "Forbidden",
     5: "Unsupported protocol version",
-  };
+  } as const;
 }
 
 /**
@@ -667,7 +676,7 @@ class WebSocketResponse {
  */
 export class Server extends BaseServer {
   public httpServer?: HttpServer;
-  private ws: any;
+  private ws: WsServer;
 
   /**
    * Initialize websocket server
@@ -687,7 +696,7 @@ export class Server extends BaseServer {
     });
 
     if (typeof this.ws.on === "function") {
-      this.ws.on("headers", (headersArray, req) => {
+      this.ws.on("headers", (headersArray, req: EngineRequest) => {
         // note: 'ws' uses an array of headers, while Engine.IO uses an object (response.writeHead() accepts both formats)
         // we could also try to parse the array and then sync the values, but that will be error-prone
         const additionalHeaders = req[kResponseHeaders] || {};
@@ -730,7 +739,8 @@ export class Server extends BaseServer {
     }
   }
 
-  protected createTransport(transportName: string, req: IncomingMessage) {
+  protected createTransport(transportName: Transport, req: IncomingMessage) {
+    // @ts-expect-error 'polling' is a plain function used as constructor
     return new transports[transportName](req);
   }
 
@@ -745,7 +755,7 @@ export class Server extends BaseServer {
     this.prepare(req);
     req.res = res;
 
-    const callback = (errorCode, errorContext) => {
+    const callback: ErrorCallback = (errorCode, errorContext) => {
       if (errorCode !== undefined) {
         this.emit("connection_error", {
           req,
@@ -787,7 +797,7 @@ export class Server extends BaseServer {
     this.prepare(req);
 
     const res = new WebSocketResponse(req, socket);
-    const callback = (errorCode, errorContext) => {
+    const callback: ErrorCallback = (errorCode, errorContext) => {
       if (errorCode !== undefined) {
         this.emit("connection_error", {
           req,
@@ -823,11 +833,16 @@ export class Server extends BaseServer {
 
   /**
    * Called upon a ws.io connection.
-   *
-   * @param {ws.Socket} websocket
+   * @param req
+   * @param socket
+   * @param websocket
    * @private
    */
-  private onWebSocket(req, socket, websocket) {
+  private onWebSocket(
+    req: EngineRequest,
+    socket: Duplex,
+    websocket: WsWebSocket,
+  ) {
     websocket.on("error", onUpgradeError);
 
     if (
@@ -862,7 +877,11 @@ export class Server extends BaseServer {
         // transport error handling takes over
         websocket.removeListener("error", onUpgradeError);
 
-        const transport = this.createTransport(req._query.transport, req);
+        const transport: TransportImpl = this.createTransport(
+          req._query.transport,
+          req
+        );
+        // @ts-expect-error this option is only for WebSocket impl
         transport.perMessageDeflate = this.opts.perMessageDeflate;
         client._maybeUpgrade(transport);
       }
@@ -947,7 +966,11 @@ export class Server extends BaseServer {
  * @private
  */
 
-function abortRequest(res, errorCode, errorContext) {
+function abortRequest(
+  res: ServerResponse,
+  errorCode: number,
+  errorContext?: { message?: string },
+) {
   const statusCode = errorCode === Server.errors.FORBIDDEN ? 403 : 400;
   const message =
     errorContext && errorContext.message
@@ -1030,7 +1053,7 @@ const validHdrChars = [
   1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1  // ... 255
 ]
 
-function checkInvalidHeaderChar(val) {
+function checkInvalidHeaderChar(val?: string) {
   val += "";
   if (val.length < 1) return false;
   if (!validHdrChars[val.charCodeAt(0)]) {
diff --git a/packages/engine.io/lib/socket.ts b/packages/engine.io/lib/socket.ts
index bfe6d3ba1d..0dbcccf9cf 100644
--- a/packages/engine.io/lib/socket.ts
+++ b/packages/engine.io/lib/socket.ts
@@ -5,6 +5,7 @@ import type { EngineRequest, Transport } from "./transport";
 import type { BaseServer } from "./server";
 import { setTimeout, clearTimeout } from "timers";
 import type { Packet, PacketType, RawData } from "engine.io-parser";
+import type transports from "./transports";
 
 const debug = debugModule("engine:socket");
 
@@ -537,9 +538,9 @@ export class Socket extends EventEmitter {
    */
   private getAvailableUpgrades() {
     const availableUpgrades = [];
-    const allUpgrades = this.server.upgrades(this.transport.name);
+    const allUpgrades = this.server.upgrades(this.transport.name as keyof typeof transports);
     for (let i = 0; i < allUpgrades.length; ++i) {
-      const upg = allUpgrades[i];
+      const upg = allUpgrades[i] as keyof typeof transports;
       if (this.server.opts.transports.indexOf(upg) !== -1) {
         availableUpgrades.push(upg);
       }
diff --git a/packages/engine.io/lib/transport.ts b/packages/engine.io/lib/transport.ts
index 1121e4e348..a23e6de70b 100644
--- a/packages/engine.io/lib/transport.ts
+++ b/packages/engine.io/lib/transport.ts
@@ -4,6 +4,7 @@ import * as parser_v3 from "./parser-v3/index";
 import debugModule from "debug";
 import type { IncomingMessage, ServerResponse } from "http";
 import { Packet, RawData } from "engine.io-parser";
+import type { WebSocket } from "ws";
 
 const debug = debugModule("engine:transport");
 
@@ -15,7 +16,11 @@ export type EngineRequest = IncomingMessage & {
   _query: Record<string, string>;
   res?: ServerResponse;
   cleanup?: Function;
-  websocket?: any;
+  websocket?: WebSocket & {
+    _socket?: {
+      remoteAddress: string;
+    };
+  };
 };
 
 export abstract class Transport extends EventEmitter {
@@ -37,7 +42,7 @@ export abstract class Transport extends EventEmitter {
    *
    * @see https://github.com/socketio/engine.io-protocol
    */
-  public protocol: number;
+  public protocol: 3 | 4;
 
   /**
    * The current state of the transport.
@@ -53,7 +58,7 @@ export abstract class Transport extends EventEmitter {
    * The parser to use (depends on the revision of the {@link Transport#protocol}.
    * @protected
    */
-  protected parser: any;
+  protected parser: typeof parser_v4 | typeof parser_v3;
   /**
    * Whether the transport supports binary payloads (else it will be base64-encoded)
    * @protected
@@ -148,7 +153,7 @@ export abstract class Transport extends EventEmitter {
   /**
    * Called with the encoded packet data.
    *
-   * @param {String} data
+   * @param data
    * @protected
    */
   protected onData(data: RawData) {
diff --git a/packages/engine.io/lib/transports-uws/polling.ts b/packages/engine.io/lib/transports-uws/polling.ts
index 090270ec2f..342d25b4e3 100644
--- a/packages/engine.io/lib/transports-uws/polling.ts
+++ b/packages/engine.io/lib/transports-uws/polling.ts
@@ -3,6 +3,8 @@ import { createGzip, createDeflate } from "zlib";
 import * as accepts from "accepts";
 import debugModule from "debug";
 import { HttpRequest, HttpResponse } from "uWebSockets.js";
+import type * as parser_v4 from "engine.io-parser";
+import type * as parser_v3 from "../parser-v3/index";
 
 const debug = debugModule("engine:polling");
 
@@ -228,9 +230,9 @@ export class Polling extends Transport {
     };
 
     if (this.protocol === 3) {
-      this.parser.decodePayload(data, callback);
+      (this.parser as typeof parser_v3).decodePayload(data, callback);
     } else {
-      this.parser.decodePayload(data).forEach(callback);
+      (this.parser as typeof parser_v4).decodePayload(data).forEach(callback);
     }
   }
 
@@ -263,7 +265,7 @@ export class Polling extends Transport {
       this.shouldClose = null;
     }
 
-    const doWrite = (data) => {
+    const doWrite = (data: string) => {
       const compress = packets.some((packet) => {
         return packet.options && packet.options.compress;
       });
@@ -271,9 +273,9 @@ export class Polling extends Transport {
     };
 
     if (this.protocol === 3) {
-      this.parser.encodePayload(packets, this.supportsBinary, doWrite);
+      (this.parser as typeof parser_v3).encodePayload(packets, this.supportsBinary, doWrite);
     } else {
-      this.parser.encodePayload(packets, doWrite);
+      (this.parser as typeof parser_v4).encodePayload(packets, doWrite);
     }
   }
 
diff --git a/packages/engine.io/lib/transports/index.ts b/packages/engine.io/lib/transports/index.ts
index e585112bb1..6f41324801 100644
--- a/packages/engine.io/lib/transports/index.ts
+++ b/packages/engine.io/lib/transports/index.ts
@@ -2,9 +2,10 @@ import { Polling as XHR } from "./polling";
 import { JSONP } from "./polling-jsonp";
 import { WebSocket } from "./websocket";
 import { WebTransport } from "./webtransport";
+import { EngineRequest } from "../transport";
 
 export default {
-  polling: polling,
+  polling,
   websocket: WebSocket,
   webtransport: WebTransport,
 };
@@ -12,8 +13,7 @@ export default {
 /**
  * Polling polymorphic constructor.
  */
-
-function polling(req) {
+function polling(req: EngineRequest) {
   if ("string" === typeof req._query.j) {
     return new JSONP(req);
   } else {
diff --git a/packages/engine.io/lib/transports/polling.ts b/packages/engine.io/lib/transports/polling.ts
index 1f46335934..618f36d43a 100644
--- a/packages/engine.io/lib/transports/polling.ts
+++ b/packages/engine.io/lib/transports/polling.ts
@@ -4,6 +4,8 @@ import * as accepts from "accepts";
 import debugModule from "debug";
 import type { IncomingMessage, ServerResponse } from "http";
 import type { Packet, RawData } from "engine.io-parser";
+import type * as parser_v4 from "engine.io-parser";
+import type * as parser_v3 from "../parser-v3/index";
 
 const debug = debugModule("engine:polling");
 
@@ -196,9 +198,9 @@ export class Polling extends Transport {
     };
 
     if (this.protocol === 3) {
-      this.parser.decodePayload(data, callback);
+      (this.parser as typeof parser_v3).decodePayload(data, callback);
     } else {
-      this.parser.decodePayload(data).forEach(callback);
+      (this.parser as typeof parser_v4).decodePayload(data).forEach(callback);
     }
   }
 
@@ -225,7 +227,7 @@ export class Polling extends Transport {
       this.shouldClose = null;
     }
 
-    const doWrite = (data) => {
+    const doWrite = (data: string) => {
       const compress = packets.some((packet) => {
         return packet.options && packet.options.compress;
       });
@@ -233,9 +235,9 @@ export class Polling extends Transport {
     };
 
     if (this.protocol === 3) {
-      this.parser.encodePayload(packets, this.supportsBinary, doWrite);
+      (this.parser as typeof parser_v3).encodePayload(packets, this.supportsBinary, doWrite);
     } else {
-      this.parser.encodePayload(packets, doWrite);
+      (this.parser as typeof parser_v4).encodePayload(packets, doWrite);
     }
   }
 
diff --git a/packages/engine.io/lib/transports/websocket.ts b/packages/engine.io/lib/transports/websocket.ts
index a95f4b0f19..fe084674fe 100644
--- a/packages/engine.io/lib/transports/websocket.ts
+++ b/packages/engine.io/lib/transports/websocket.ts
@@ -1,12 +1,18 @@
 import { EngineRequest, Transport } from "../transport";
 import debugModule from "debug";
 import type { Packet, RawData } from "engine.io-parser";
+import type { PerMessageDeflateOptions, WebSocket as WsWebSocket } from 'ws';
 
 const debug = debugModule("engine:ws");
 
+/**
+ * WebSocket object in 'ws' package
+ * names started with _ are not in web-WebSocket standard
+ */
 export class WebSocket extends Transport {
-  protected perMessageDeflate: any;
-  private socket: any;
+  perMessageDeflate?: boolean | PerMessageDeflateOptions;
+  private socket: WsWebSocket;
+  static upgradesTo?: undefined;
 
   /**
    * WebSocket transport
@@ -51,8 +57,8 @@ export class WebSocket extends Transport {
       if (this._canSendPreEncodedFrame(packet)) {
         // the WebSocket frame was computed with WebSocket.Sender.frame()
         // see https://github.com/websockets/ws/issues/617#issuecomment-283002469
+        // @ts-expect-error use of untyped member
         this.socket._sender.sendFrame(
-          // @ts-ignore
           packet.options.wsPreEncodedFrame,
           isLast ? this._onSentLast : this._onSent,
         );
@@ -74,8 +80,8 @@ export class WebSocket extends Transport {
   private _canSendPreEncodedFrame(packet: Packet) {
     return (
       !this.perMessageDeflate &&
+      // @ts-expect-error use of untyped member
       typeof this.socket?._sender?.sendFrame === "function" &&
-      // @ts-ignore
       packet.options?.wsPreEncodedFrame !== undefined
     );
   }
diff --git a/packages/engine.io/lib/transports/webtransport.ts b/packages/engine.io/lib/transports/webtransport.ts
index f675e7c37b..1cbe69295b 100644
--- a/packages/engine.io/lib/transports/webtransport.ts
+++ b/packages/engine.io/lib/transports/webtransport.ts
@@ -8,6 +8,7 @@ const debug = debugModule("engine:webtransport");
  * Reference: https://developer.mozilla.org/en-US/docs/Web/API/WebTransport_API
  */
 export class WebTransport extends Transport {
+  static upgradesTo?: undefined;
   private readonly writer;
 
   constructor(
diff --git a/packages/engine.io/lib/userver.ts b/packages/engine.io/lib/userver.ts
index e8fb47f580..9ef754b736 100644
--- a/packages/engine.io/lib/userver.ts
+++ b/packages/engine.io/lib/userver.ts
@@ -2,6 +2,7 @@ import debugModule from "debug";
 import { AttachOptions, BaseServer, Server } from "./server";
 import { HttpRequest, HttpResponse, TemplatedApp } from "uWebSockets.js";
 import transports from "./transports-uws";
+import { EngineRequest } from "./transport";
 
 const debug = debugModule("engine:uws");
 
@@ -36,7 +37,7 @@ export class uServer extends BaseServer {
    *
    * @private
    */
-  private prepare(req, res: HttpResponse) {
+  private prepare(req: HttpRequest & EngineRequest, res: HttpResponse) {
     req.method = req.getMethod().toUpperCase();
     req.url = req.getUrl();
 
@@ -48,6 +49,7 @@ export class uServer extends BaseServer {
       req.headers[key] = value;
     });
 
+    // @ts-expect-error
     req.connection = {
       remoteAddress: Buffer.from(res.getRemoteAddressAsText()).toString(),
     };
@@ -57,7 +59,7 @@ export class uServer extends BaseServer {
     });
   }
 
-  protected createTransport(transportName, req) {
+  protected createTransport(transportName: string, req: EngineRequest) {
     return new transports[transportName](req);
   }
 
@@ -238,8 +240,8 @@ export class uServer extends BaseServer {
 
   private abortRequest(
     res: HttpResponse | ResponseWrapper,
-    errorCode,
-    errorContext,
+    errorCode: number,
+    errorContext?: {message?: string},
   ) {
     const statusCode =
       errorCode === Server.errors.FORBIDDEN
diff --git a/packages/socket.io-adapter/lib/in-memory-adapter.ts b/packages/socket.io-adapter/lib/in-memory-adapter.ts
index 2b78e8285e..84c338d8d9 100644
--- a/packages/socket.io-adapter/lib/in-memory-adapter.ts
+++ b/packages/socket.io-adapter/lib/in-memory-adapter.ts
@@ -1,6 +1,7 @@
 import { EventEmitter } from "events";
 import { yeast } from "./contrib/yeast";
-import WebSocket = require("ws");
+import type {Namespace, Socket} from 'socket.io';
+import * as WebSocket from 'ws';
 
 // @ts-expect-error
 const canPreComputeFrame = typeof WebSocket?.Sender?.frame === "function";
@@ -51,9 +52,9 @@ export class Adapter extends EventEmitter {
   /**
    * In-memory adapter constructor.
    *
-   * @param {Namespace} nsp
+   * @param nsp
    */
-  constructor(readonly nsp: any) {
+  constructor(readonly nsp: Namespace) {
     super();
     this.encoder = nsp.server.encoder;
   }
@@ -171,10 +172,13 @@ export class Adapter extends EventEmitter {
     const encodedPackets = this._encode(packet, packetOpts);
 
     this.apply(opts, (socket) => {
+      // @ts-expect-error use of private
       if (typeof socket.notifyOutgoingListeners === "function") {
+        // @ts-expect-error use of private
         socket.notifyOutgoingListeners(packet);
       }
 
+      // @ts-expect-error use of private
       socket.client.writeToEngine(encodedPackets, packetOpts);
     });
   }
@@ -219,12 +223,16 @@ export class Adapter extends EventEmitter {
       // track the total number of acknowledgements that are expected
       clientCount++;
       // call the ack callback for each client response
+      // @ts-expect-error use of private
       socket.acks.set(packet.id, ack);
 
+      // @ts-expect-error use of private
       if (typeof socket.notifyOutgoingListeners === "function") {
+        // @ts-expect-error use of private
         socket.notifyOutgoingListeners(packet);
       }
 
+      // @ts-expect-error use of private
       socket.client.writeToEngine(encodedPackets, packetOpts);
     });
 
@@ -330,7 +338,7 @@ export class Adapter extends EventEmitter {
     });
   }
 
-  private apply(opts: BroadcastOptions, callback: (socket) => void): void {
+  private apply(opts: BroadcastOptions, callback: (socket: Socket) => void): void {
     const rooms = opts.rooms;
     const except = this.computeExceptSids(opts.except);
 
diff --git a/packages/socket.io/lib/client.ts b/packages/socket.io/lib/client.ts
index 6ca7b77818..d65f3cf075 100644
--- a/packages/socket.io/lib/client.ts
+++ b/packages/socket.io/lib/client.ts
@@ -1,6 +1,6 @@
 import { Decoder, Encoder, Packet, PacketType } from "socket.io-parser";
-import debugModule = require("debug");
-import url = require("url");
+import debugModule from "debug";
+import url from "url";
 import type { IncomingMessage } from "http";
 import type { Server } from "./index";
 import type { Namespace } from "./namespace";
@@ -61,12 +61,13 @@ export class Client<
    */
   constructor(
     server: Server<ListenEvents, EmitEvents, ServerSideEvents, SocketData>,
-    conn: any,
+    conn: RawSocket
   ) {
     this.server = server;
     this.conn = conn;
     this.encoder = server.encoder;
     this.decoder = new server._parser.Decoder();
+    // @ts-expect-error use of private
     this.id = conn.id;
     this.setup();
   }
@@ -216,13 +217,13 @@ export class Client<
    * @param {Object} opts
    * @private
    */
-  _packet(packet: Packet | any[], opts: WriteOptions = {}): void {
+  _packet(packet: Packet | Packet[], opts: WriteOptions = {}): void {
     if (this.conn.readyState !== "open") {
       debug("ignoring packet write %j", packet);
       return;
     }
     const encodedPackets = opts.preEncoded
-      ? (packet as any[]) // previous versions of the adapter incorrectly used socket.packet() instead of writeToEngine()
+      ? (packet as Packet[]) // previous versions of the adapter incorrectly used socket.packet() instead of writeToEngine()
       : this.encoder.encode(packet as Packet);
     this.writeToEngine(encodedPackets, opts);
   }
@@ -250,7 +251,7 @@ export class Client<
    *
    * @private
    */
-  private ondata(data): void {
+  private ondata(data: unknown): void {
     // try/catch is needed for protocol violations (GH-1880)
     try {
       this.decoder.add(data);
diff --git a/packages/socket.io/lib/index.ts b/packages/socket.io/lib/index.ts
index b77b8fd3fd..8ec5dcdb0d 100644
--- a/packages/socket.io/lib/index.ts
+++ b/packages/socket.io/lib/index.ts
@@ -1,4 +1,4 @@
-import http = require("http");
+import http from 'http';
 import type { Server as HTTPSServer } from "https";
 import type { Http2SecureServer, Http2Server } from "http2";
 import { createReadStream } from "fs";
@@ -7,7 +7,7 @@ import accepts = require("accepts");
 import { pipeline } from "stream";
 import path = require("path");
 import { attach, Server as Engine, uServer } from "engine.io";
-import type { ServerOptions as EngineOptions, AttachOptions } from "engine.io";
+import type { ServerOptions as EngineOptions, AttachOptions, Socket as RawSocket } from "engine.io";
 import { Client } from "./client";
 import { EventEmitter } from "events";
 import { ExtendedError, Namespace, ServerReservedEventsMap } from "./namespace";
@@ -506,6 +506,11 @@ export class Server<
     return this;
   }
 
+  /**
+   * attach uServer app
+   * @param app
+   * @param opts
+   */
   public attachApp(app /*: TemplatedApp */, opts: Partial<ServerOptions> = {}) {
     // merge the options passed to the Socket.IO server
     Object.assign(opts, this.opts);
@@ -582,7 +587,7 @@ export class Server<
   ): void {
     // initialize engine
     debug("creating engine.io instance with opts %j", opts);
-    this.eio = attach(srv, opts);
+    this.eio = attach(srv as http.Server, opts);
 
     // attach static file serving
     if (this._serveClient) this.attachServe(srv);
@@ -723,11 +728,12 @@ export class Server<
    * @return self
    * @private
    */
-  private onconnection(conn): this {
+  private onconnection(conn: RawSocket): this {
+    // @ts-expect-error use of private
     debug("incoming connection with id %s", conn.id);
     const client = new Client(this, conn);
     if (conn.protocol === 3) {
-      // @ts-ignore
+      // @ts-expect-error use of private
       client.connect("/");
     }
     return this;
diff --git a/packages/socket.io/lib/socket.ts b/packages/socket.io/lib/socket.ts
index 719b597e59..55131fcee7 100644
--- a/packages/socket.io/lib/socket.ts
+++ b/packages/socket.io/lib/socket.ts
@@ -163,7 +163,7 @@ export class Socket<
   ) {
     super();
     this.server = nsp.server;
-    this.adapter = this.nsp.adapter;
+    this.adapter = nsp.adapter;
     if (previousSession) {
       this.id = previousSession.sid;
       this.pid = previousSession.pid;