diff --git a/lib/engine.io.ts b/lib/engine.io.ts
index f6d4cb1d..3ba9e67f 100644
--- a/lib/engine.io.ts
+++ b/lib/engine.io.ts
@@ -17,7 +17,6 @@ export const protocol = parser.protocol;
  * @param {Function} callback
  * @param {Object} options
  * @return {Server} websocket.io server
- * @api public
  */
 
 function listen(port, options: AttachOptions & ServerOptions, fn) {
@@ -46,7 +45,6 @@ function listen(port, options: AttachOptions & ServerOptions, fn) {
  * @param {http.Server} server
  * @param {Object} options
  * @return {Server} engine server
- * @api public
  */
 
 function attach(server, options: AttachOptions & ServerOptions) {
diff --git a/lib/server.ts b/lib/server.ts
index ba096f2e..e3c2c5fc 100644
--- a/lib/server.ts
+++ b/lib/server.ts
@@ -17,12 +17,13 @@ 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";
 
 const debug = debugModule("engine");
 
 const kResponseHeaders = Symbol("responseHeaders");
 
-type Transport = "polling" | "websocket";
+type Transport = "polling" | "websocket" | "webtransport";
 
 export interface AttachOptions {
   /**
@@ -169,7 +170,6 @@ export abstract class BaseServer extends EventEmitter {
    * Server constructor.
    *
    * @param {Object} opts - options
-   * @api public
    */
   constructor(opts: ServerOptions = {}) {
     super();
@@ -246,9 +246,8 @@ export abstract class BaseServer extends EventEmitter {
    * Returns a list of available transports for upgrade given a certain transport.
    *
    * @return {Array}
-   * @api public
    */
-  public upgrades(transport) {
+  public upgrades(transport: string) {
     if (!this.opts.allowUpgrades) return [];
     return transports[transport].upgradesTo || [];
   }
@@ -256,11 +255,16 @@ export abstract class BaseServer extends EventEmitter {
   /**
    * Verifies a request.
    *
-   * @param {http.IncomingMessage}
-   * @return {Boolean} whether the request is valid
-   * @api private
+   * @param {EngineRequest} req
+   * @param upgrade - whether it's an upgrade request
+   * @param fn
+   * @protected
    */
-  protected verify(req, upgrade, fn) {
+  protected verify(
+    req: any,
+    upgrade: boolean,
+    fn: (errorCode?: number, errorContext?: any) => void
+  ) {
     // transport check
     const transport = req._query.transport;
     // WebTransport does not go through the verify() method, see the onWebTransportSession() method
@@ -384,8 +388,6 @@ export abstract class BaseServer extends EventEmitter {
 
   /**
    * Closes all clients.
-   *
-   * @api public
    */
   public close() {
     debug("closing all open clients");
@@ -404,23 +406,26 @@ export abstract class BaseServer extends EventEmitter {
    * generate a socket id.
    * Overwrite this method to generate your custom socket id
    *
-   * @param {Object} request object
-   * @api public
+   * @param {IncomingMessage} req - the request object
    */
-  public generateId(req) {
+  public generateId(req: IncomingMessage) {
     return base64id.generateId();
   }
 
   /**
    * Handshakes a new client.
    *
-   * @param {String} transport name
-   * @param {Object} request object
+   * @param {String} transportName
+   * @param {Object} req - the request object
    * @param {Function} closeConnection
    *
-   * @api protected
+   * @protected
    */
-  protected async handshake(transportName, req, closeConnection) {
+  protected async handshake(
+    transportName: string,
+    req: any,
+    closeConnection: (errorCode?: number, errorContext?: any) => void
+  ) {
     const protocol = req._query.EIO === "4" ? 4 : 3; // 3rd revision by default
     if (protocol === 3 && !this.opts.allowEIO3) {
       debug("unsupported protocol version");
@@ -661,7 +666,7 @@ export class Server extends BaseServer {
   /**
    * Initialize websocket server
    *
-   * @api protected
+   * @protected
    */
   protected init() {
     if (!~this.opts.transports.indexOf("websocket")) return;
@@ -708,30 +713,30 @@ export class Server extends BaseServer {
   /**
    * Prepares a request by processing the query string.
    *
-   * @api private
+   * @private
    */
-  private prepare(req) {
+  private prepare(req: EngineRequest) {
     // try to leverage pre-existing `req._query` (e.g: from connect)
     if (!req._query) {
-      req._query = ~req.url.indexOf("?") ? qs.parse(parse(req.url).query) : {};
+      req._query = (
+        ~req.url.indexOf("?") ? qs.parse(parse(req.url).query) : {}
+      ) as Record<string, string>;
     }
   }
 
-  protected createTransport(transportName, req) {
+  protected createTransport(transportName: string, req: IncomingMessage) {
     return new transports[transportName](req);
   }
 
   /**
    * Handles an Engine.IO HTTP request.
    *
-   * @param {IncomingMessage} req
+   * @param {EngineRequest} req
    * @param {ServerResponse} res
-   * @api public
    */
-  public handleRequest(req: IncomingMessage, res: ServerResponse) {
+  public handleRequest(req: EngineRequest, res: ServerResponse) {
     debug('handling "%s" http request "%s"', req.method, req.url);
     this.prepare(req);
-    // @ts-ignore
     req.res = res;
 
     const callback = (errorCode, errorContext) => {
@@ -746,7 +751,6 @@ export class Server extends BaseServer {
         return;
       }
 
-      // @ts-ignore
       if (req._query.sid) {
         debug("setting new request for existing client");
         // @ts-ignore
@@ -754,7 +758,6 @@ export class Server extends BaseServer {
       } else {
         const closeConnection = (errorCode, errorContext) =>
           abortRequest(res, errorCode, errorContext);
-        // @ts-ignore
         this.handshake(req._query.transport, req, closeConnection);
       }
     };
@@ -770,11 +773,9 @@ export class Server extends BaseServer {
 
   /**
    * Handles an Engine.IO HTTP Upgrade.
-   *
-   * @api public
    */
   public handleUpgrade(
-    req: IncomingMessage,
+    req: EngineRequest,
     socket: Duplex,
     upgradeHead: Buffer
   ) {
@@ -819,7 +820,7 @@ export class Server extends BaseServer {
    * Called upon a ws.io connection.
    *
    * @param {ws.Socket} websocket
-   * @api private
+   * @private
    */
   private onWebSocket(req, socket, websocket) {
     websocket.on("error", onUpgradeError);
@@ -877,7 +878,6 @@ export class Server extends BaseServer {
    *
    * @param {http.Server} server
    * @param {Object} options
-   * @api public
    */
   public attach(server: HttpServer, options: AttachOptions = {}) {
     const path = this._computePath(options);
@@ -898,7 +898,7 @@ export class Server extends BaseServer {
     server.on("request", (req, res) => {
       if (check(req)) {
         debug('intercepting request for path "%s"', path);
-        this.handleRequest(req, res);
+        this.handleRequest(req as EngineRequest, res);
       } else {
         let i = 0;
         const l = listeners.length;
@@ -911,7 +911,7 @@ export class Server extends BaseServer {
     if (~this.opts.transports.indexOf("websocket")) {
       server.on("upgrade", (req, socket, head) => {
         if (check(req)) {
-          this.handleUpgrade(req, socket, head);
+          this.handleUpgrade(req as EngineRequest, socket, head);
         } else if (false !== options.destroyUpgrade) {
           // default node behavior is to disconnect when no handlers
           // but by adding a handler, we prevent that
@@ -939,7 +939,7 @@ export class Server extends BaseServer {
  * @param errorCode - the error code
  * @param errorContext - additional error context
  *
- * @api private
+ * @private
  */
 
 function abortRequest(res, errorCode, errorContext) {
@@ -964,8 +964,6 @@ function abortRequest(res, errorCode, errorContext) {
  * @param {net.Socket} socket
  * @param {string} errorCode - the error code
  * @param {object} errorContext - additional error context
- *
- * @api private
  */
 
 function abortUpgrade(
diff --git a/lib/socket.ts b/lib/socket.ts
index 1dfa460b..d9f58a68 100644
--- a/lib/socket.ts
+++ b/lib/socket.ts
@@ -1,10 +1,10 @@
 import { EventEmitter } from "events";
 import debugModule from "debug";
-import { IncomingMessage } from "http";
-import { Transport } from "./transport";
-import { Server } from "./server";
+import type { IncomingMessage } from "http";
+import type { EngineRequest, Transport } from "./transport";
+import type { BaseServer } from "./server";
 import { setTimeout, clearTimeout } from "timers";
-import { Packet, PacketType, RawData } from "engine.io-parser";
+import type { Packet, PacketType, RawData } from "engine.io-parser";
 
 const debug = debugModule("engine:socket");
 
@@ -17,15 +17,38 @@ type ReadyState = "opening" | "open" | "closing" | "closed";
 type SendCallback = (transport: Transport) => void;
 
 export class Socket extends EventEmitter {
+  /**
+   * The revision of the protocol:
+   *
+   * - 3rd is used in Engine.IO v3 / Socket.IO v2
+   * - 4th is used in Engine.IO v4 and above / Socket.IO v3 and above
+   *
+   * It is found in the `EIO` query parameters of the HTTP requests.
+   *
+   * @see https://github.com/socketio/engine.io-protocol
+   */
   public readonly protocol: number;
-  // TODO for the next major release: do not keep the reference to the first HTTP request, as it stays in memory
+  /**
+   * A reference to the first HTTP request of the session
+   *
+   * TODO for the next major release: remove it
+   */
   public request: IncomingMessage;
+  /**
+   * The IP address of the client.
+   */
   public readonly remoteAddress: string;
 
+  /**
+   * The current state of the socket.
+   */
   public _readyState: ReadyState = "opening";
+  /**
+   * The current low-level transport.
+   */
   public transport: Transport;
 
-  private server: Server;
+  private server: BaseServer;
   /* private */ upgrading = false;
   /* private */ upgraded = false;
   private writeBuffer: Packet[] = [];
@@ -52,12 +75,13 @@ export class Socket extends EventEmitter {
     this._readyState = state;
   }
 
-  /**
-   * Client class (abstract).
-   *
-   * @api private
-   */
-  constructor(id, server, transport, req, protocol) {
+  constructor(
+    id: string,
+    server: BaseServer,
+    transport: Transport,
+    req: EngineRequest,
+    protocol: number
+  ) {
     super();
     this.id = id;
     this.server = server;
@@ -86,7 +110,7 @@ export class Socket extends EventEmitter {
   /**
    * Called upon transport considered open.
    *
-   * @api private
+   * @private
    */
   private onOpen() {
     this.readyState = "open";
@@ -123,7 +147,7 @@ export class Socket extends EventEmitter {
    * Called upon transport packet.
    *
    * @param {Object} packet
-   * @api private
+   * @private
    */
   private onPacket(packet: Packet) {
     if ("open" !== this.readyState) {
@@ -136,7 +160,7 @@ export class Socket extends EventEmitter {
     switch (packet.type) {
       case "ping":
         if (this.transport.protocol !== 3) {
-          this.onError("invalid heartbeat direction");
+          this.onError(new Error("invalid heartbeat direction"));
           return;
         }
         debug("got ping");
@@ -147,7 +171,7 @@ export class Socket extends EventEmitter {
 
       case "pong":
         if (this.transport.protocol === 3) {
-          this.onError("invalid heartbeat direction");
+          this.onError(new Error("invalid heartbeat direction"));
           return;
         }
         debug("got pong");
@@ -171,9 +195,9 @@ export class Socket extends EventEmitter {
    * Called upon transport error.
    *
    * @param {Error} err - error object
-   * @api private
+   * @private
    */
-  private onError(err) {
+  private onError(err: Error) {
     debug("transport error");
     this.onClose("transport error", err);
   }
@@ -182,7 +206,7 @@ export class Socket extends EventEmitter {
    * Pings client every `this.pingInterval` and expects response
    * within `this.pingTimeout` or closes connection.
    *
-   * @api private
+   * @private
    */
   private schedulePing() {
     this.pingIntervalTimer = setTimeout(() => {
@@ -198,7 +222,7 @@ export class Socket extends EventEmitter {
   /**
    * Resets ping timeout.
    *
-   * @api private
+   * @private
    */
   private resetPingTimeout() {
     clearTimeout(this.pingTimeoutTimer);
@@ -217,9 +241,9 @@ export class Socket extends EventEmitter {
    * Attaches handlers for the given transport.
    *
    * @param {Transport} transport
-   * @api private
+   * @private
    */
-  private setTransport(transport) {
+  private setTransport(transport: Transport) {
     const onError = this.onError.bind(this);
     const onReady = () => this.flush();
     const onPacket = this.onPacket.bind(this);
@@ -263,7 +287,7 @@ export class Socket extends EventEmitter {
    * Upgrades socket to the given transport
    *
    * @param {Transport} transport
-   * @api private
+   * @private
    */
   /* private */ _maybeUpgrade(transport: Transport) {
     debug(
@@ -357,7 +381,7 @@ export class Socket extends EventEmitter {
   /**
    * Clears listeners and timers associated with current transport.
    *
-   * @api private
+   * @private
    */
   private clearTransport() {
     let cleanup;
@@ -412,7 +436,6 @@ export class Socket extends EventEmitter {
    * @param {Object} options
    * @param {Function} callback
    * @return {Socket} for chaining
-   * @api public
    */
   public send(data: RawData, options?: SendOptions, callback?: SendCallback) {
     this.sendPacket("message", data, options, callback);
@@ -439,7 +462,7 @@ export class Socket extends EventEmitter {
    * @param {Object} options
    * @param {Function} callback
    *
-   * @api private
+   * @private
    */
   private sendPacket(
     type: PacketType,
@@ -480,7 +503,7 @@ export class Socket extends EventEmitter {
   /**
    * Attempts to flush the packets buffer.
    *
-   * @api private
+   * @private
    */
   private flush() {
     if (
@@ -510,14 +533,12 @@ export class Socket extends EventEmitter {
   /**
    * Get available upgrades for this socket.
    *
-   * @api private
+   * @private
    */
   private getAvailableUpgrades() {
     const availableUpgrades = [];
     const allUpgrades = this.server.upgrades(this.transport.name);
-    let i = 0;
-    const l = allUpgrades.length;
-    for (; i < l; ++i) {
+    for (let i = 0; i < allUpgrades.length; ++i) {
       const upg = allUpgrades[i];
       if (this.server.opts.transports.indexOf(upg) !== -1) {
         availableUpgrades.push(upg);
@@ -531,7 +552,6 @@ export class Socket extends EventEmitter {
    *
    * @param {Boolean} discard - optional, discard the transport
    * @return {Socket} for chaining
-   * @api public
    */
   public close(discard?: boolean) {
     if ("open" !== this.readyState) return;
@@ -558,9 +578,9 @@ export class Socket extends EventEmitter {
    * Closes the underlying transport.
    *
    * @param {Boolean} discard
-   * @api private
+   * @private
    */
-  private closeTransport(discard) {
+  private closeTransport(discard: boolean) {
     debug("closing the transport (discard? %s)", discard);
     if (discard) this.transport.discard();
     this.transport.close(this.onClose.bind(this, "forced close"));
diff --git a/lib/transport.ts b/lib/transport.ts
index 6318fb01..8d72c3f3 100644
--- a/lib/transport.ts
+++ b/lib/transport.ts
@@ -2,30 +2,63 @@ import { EventEmitter } from "events";
 import * as parser_v4 from "engine.io-parser";
 import * as parser_v3 from "./parser-v3/index";
 import debugModule from "debug";
-import { IncomingMessage } from "http";
-import { Packet } from "engine.io-parser";
+import type { IncomingMessage, ServerResponse } from "http";
+import { Packet, RawData } from "engine.io-parser";
 
 const debug = debugModule("engine:transport");
 
-/**
- * Noop function.
- *
- * @api private
- */
-
 function noop() {}
 
 type ReadyState = "open" | "closing" | "closed";
 
+export type EngineRequest = IncomingMessage & {
+  _query: Record<string, string>;
+  res?: ServerResponse;
+  cleanup?: Function;
+  websocket?: any;
+};
+
 export abstract class Transport extends EventEmitter {
+  /**
+   * The session ID.
+   */
   public sid: string;
+  /**
+   * Whether the transport is currently ready to send packets.
+   */
   public writable = false;
+  /**
+   * The revision of the protocol:
+   *
+   * - 3 is used in Engine.IO v3 / Socket.IO v2
+   * - 4 is used in Engine.IO v4 and above / Socket.IO v3 and above
+   *
+   * It is found in the `EIO` query parameters of the HTTP requests.
+   *
+   * @see https://github.com/socketio/engine.io-protocol
+   */
   public protocol: number;
 
+  /**
+   * The current state of the transport.
+   * @protected
+   */
   protected _readyState: ReadyState = "open";
+  /**
+   * Whether the transport is discarded and can be safely closed (used during upgrade).
+   * @protected
+   */
   protected discarded = false;
+  /**
+   * The parser to use (depends on the revision of the {@link Transport#protocol}.
+   * @protected
+   */
   protected parser: any;
-  protected req: IncomingMessage & { cleanup: Function };
+  protected req: EngineRequest;
+  /**
+   * Whether the transport supports binary payloads (else it will be base64-encoded)
+   * @protected
+   */
   protected supportsBinary: boolean;
 
   get readyState() {
@@ -45,10 +78,9 @@ export abstract class Transport extends EventEmitter {
   /**
    * Transport constructor.
    *
-   * @param {http.IncomingMessage} req
-   * @api public
+   * @param {EngineRequest} req
    */
-  constructor(req) {
+  constructor(req: EngineRequest) {
     super();
     this.protocol = req._query.EIO === "4" ? 4 : 3; // 3rd revision by default
     this.parser = this.protocol === 4 ? parser_v4 : parser_v3;
@@ -58,7 +90,7 @@ export abstract class Transport extends EventEmitter {
   /**
    * Flags the transport as discarded.
    *
-   * @api private
+   * @package
    */
   discard() {
     this.discarded = true;
@@ -67,10 +99,10 @@ export abstract class Transport extends EventEmitter {
   /**
    * Called with an incoming HTTP request.
    *
-   * @param {http.IncomingMessage} req
-   * @api protected
+   * @param {EngineRequest} req
+   * @protected
    */
-  protected onRequest(req) {
+  protected onRequest(req: EngineRequest) {
     debug("setting request");
     this.req = req;
   }
@@ -78,9 +110,9 @@ export abstract class Transport extends EventEmitter {
   /**
    * Closes the transport.
    *
-   * @api private
+   * @package
    */
-  close(fn?) {
+  close(fn?: () => void) {
     if ("closed" === this.readyState || "closing" === this.readyState) return;
 
     this.readyState = "closing";
@@ -92,7 +124,7 @@ export abstract class Transport extends EventEmitter {
    *
    * @param {String} msg - message error
    * @param {Object} desc - error description
-   * @api protected
+   * @protected
    */
   protected onError(msg: string, desc?) {
     if (this.listeners("error").length) {
@@ -111,7 +143,7 @@ export abstract class Transport extends EventEmitter {
    * Called with parsed out a packets from the data stream.
    *
    * @param {Object} packet
-   * @api protected
+   * @protected
    */
   protected onPacket(packet: Packet) {
     this.emit("packet", packet);
@@ -121,16 +153,16 @@ export abstract class Transport extends EventEmitter {
    * Called with the encoded packet data.
    *
    * @param {String} data
-   * @api protected
+   * @protected
    */
-  protected onData(data) {
+  protected onData(data: RawData) {
     this.onPacket(this.parser.decodePacket(data));
   }
 
   /**
    * Called upon transport close.
    *
-   * @api protected
+   * @protected
    */
   protected onClose() {
     this.readyState = "closed";
@@ -153,5 +185,5 @@ export abstract class Transport extends EventEmitter {
   /**
    * Closes the transport.
    */
-  abstract doClose(fn?);
+  abstract doClose(fn?: () => void);
 }
diff --git a/lib/transports-uws/polling.ts b/lib/transports-uws/polling.ts
index 01a82f6e..1fc643f8 100644
--- a/lib/transports-uws/polling.ts
+++ b/lib/transports-uws/polling.ts
@@ -24,8 +24,6 @@ export class Polling extends Transport {
 
   /**
    * HTTP polling constructor.
-   *
-   * @api public.
    */
   constructor(req) {
     super(req);
@@ -35,8 +33,6 @@ export class Polling extends Transport {
 
   /**
    * Transport name
-   *
-   * @api public
    */
   get name() {
     return "polling";
@@ -47,7 +43,7 @@ export class Polling extends Transport {
    *
    * @param req
    *
-   * @api private
+   * @private
    */
   onRequest(req) {
     const res = req.res;
@@ -67,7 +63,7 @@ export class Polling extends Transport {
   /**
    * The client sends a request awaiting for us to send data.
    *
-   * @api private
+   * @private
    */
   onPollRequest(req, res) {
     if (this.req) {
@@ -109,7 +105,7 @@ export class Polling extends Transport {
   /**
    * The client sends a request with data.
    *
-   * @api private
+   * @private
    */
   onDataRequest(req, res) {
     if (this.dataReq) {
@@ -206,7 +202,7 @@ export class Polling extends Transport {
   /**
    * Cleanup request.
    *
-   * @api private
+   * @private
    */
   private onDataRequestCleanup() {
     this.dataReq = this.dataRes = null;
@@ -216,7 +212,7 @@ export class Polling extends Transport {
    * Processes the incoming data payload.
    *
    * @param {String} encoded payload
-   * @api private
+   * @private
    */
   onData(data) {
     debug('received "%s"', data);
@@ -240,7 +236,7 @@ export class Polling extends Transport {
   /**
    * Overrides onClose.
    *
-   * @api private
+   * @private
    */
   onClose() {
     if (this.writable) {
@@ -254,7 +250,7 @@ export class Polling extends Transport {
    * Writes a packet payload.
    *
    * @param {Object} packet
-   * @api private
+   * @private
    */
   send(packets) {
     this.writable = false;
@@ -285,7 +281,7 @@ export class Polling extends Transport {
    *
    * @param {String} data
    * @param {Object} options
-   * @api private
+   * @private
    */
   write(data, options) {
     debug('writing "%s"', data);
@@ -298,7 +294,7 @@ export class Polling extends Transport {
   /**
    * Performs the write.
    *
-   * @api private
+   * @private
    */
   doWrite(data, options, callback) {
     // explicit UTF-8 is required for pages not served under utf
@@ -355,7 +351,7 @@ export class Polling extends Transport {
   /**
    * Compresses data.
    *
-   * @api private
+   * @private
    */
   compress(data, encoding, callback) {
     debug("compressing");
@@ -378,7 +374,7 @@ export class Polling extends Transport {
   /**
    * Closes the transport.
    *
-   * @api private
+   * @private
    */
   doClose(fn) {
     debug("closing");
@@ -410,7 +406,7 @@ export class Polling extends Transport {
    *
    * @param req - request
    * @param {Object} extra headers
-   * @api private
+   * @private
    */
   headers(req, headers) {
     headers = headers || {};
diff --git a/lib/transports-uws/websocket.ts b/lib/transports-uws/websocket.ts
index d9aaf2ae..aa5e1c93 100644
--- a/lib/transports-uws/websocket.ts
+++ b/lib/transports-uws/websocket.ts
@@ -11,7 +11,6 @@ export class WebSocket extends Transport {
    * WebSocket transport
    *
    * @param req
-   * @api public
    */
   constructor(req) {
     super(req);
@@ -21,8 +20,6 @@ export class WebSocket extends Transport {
 
   /**
    * Transport name
-   *
-   * @api public
    */
   get name() {
     return "websocket";
@@ -30,8 +27,6 @@ export class WebSocket extends Transport {
 
   /**
    * Advertise upgrade support.
-   *
-   * @api public
    */
   get handlesUpgrades() {
     return true;
@@ -41,7 +36,7 @@ export class WebSocket extends Transport {
    * Writes a packet payload.
    *
    * @param {Array} packets
-   * @api private
+   * @private
    */
   send(packets) {
     this.writable = false;
@@ -77,7 +72,7 @@ export class WebSocket extends Transport {
   /**
    * Closes the transport.
    *
-   * @api private
+   * @private
    */
   doClose(fn) {
     debug("closing");
diff --git a/lib/transports/index.ts b/lib/transports/index.ts
index 5c8449d4..e585112b 100644
--- a/lib/transports/index.ts
+++ b/lib/transports/index.ts
@@ -11,8 +11,6 @@ export default {
 
 /**
  * Polling polymorphic constructor.
- *
- * @api private
  */
 
 function polling(req) {
diff --git a/lib/transports/polling-jsonp.ts b/lib/transports/polling-jsonp.ts
index d94ba23a..52c4b0bd 100644
--- a/lib/transports/polling-jsonp.ts
+++ b/lib/transports/polling-jsonp.ts
@@ -1,5 +1,6 @@
 import { Polling } from "./polling";
 import * as qs from "querystring";
+import type { RawData } from "engine.io-parser";
 
 const rDoubleSlashes = /\\\\n/g;
 const rSlashes = /(\\)?\\n/g;
@@ -10,8 +11,6 @@ export class JSONP extends Polling {
 
   /**
    * JSON-P polling transport.
-   *
-   * @api public
    */
   constructor(req) {
     super(req);
@@ -24,12 +23,12 @@ export class JSONP extends Polling {
    * Handles incoming data.
    * Due to a bug in \n handling by browsers, we expect a escaped string.
    *
-   * @api private
+   * @protected
    */
-  onData(data) {
+  protected onData(data: RawData) {
     // we leverage the qs module so that we get built-in DoS protection
     // and the fast alternative to decodeURIComponent
-    data = qs.parse(data).d;
+    data = qs.parse(data).d as string;
     if ("string" === typeof data) {
       // client will send already escaped newlines as \\\\n and newlines as \\n
       // \\n must be replaced with \n and \\\\n with \\n
@@ -43,9 +42,9 @@ export class JSONP extends Polling {
   /**
    * Performs the write.
    *
-   * @api private
+   * @protected
    */
-  doWrite(data, options, callback) {
+  protected doWrite(data, options, callback) {
     // we must output valid javascript, not valid json
     // see: http://timelessrepo.com/json-isnt-a-javascript-subset
     const js = JSON.stringify(data)
diff --git a/lib/transports/polling.ts b/lib/transports/polling.ts
index 09af20fb..d1f65a0a 100644
--- a/lib/transports/polling.ts
+++ b/lib/transports/polling.ts
@@ -1,8 +1,9 @@
-import { Transport } from "../transport";
+import { EngineRequest, Transport } from "../transport";
 import { createGzip, createDeflate } from "zlib";
 import * as accepts from "accepts";
 import debugModule from "debug";
-import { IncomingMessage, ServerResponse } from "http";
+import type { IncomingMessage, ServerResponse } from "http";
+import type { Packet, RawData } from "engine.io-parser";
 
 const debug = debugModule("engine:polling");
 
@@ -18,14 +19,12 @@ export class Polling extends Transport {
   private res: ServerResponse;
   private dataReq: IncomingMessage;
   private dataRes: ServerResponse;
-  private shouldClose: Function;
+  private shouldClose: () => void;
 
   private readonly closeTimeout: number;
 
   /**
    * HTTP polling constructor.
-   *
-   * @api public.
    */
   constructor(req) {
     super(req);
@@ -35,8 +34,6 @@ export class Polling extends Transport {
 
   /**
    * Transport name
-   *
-   * @api public
    */
   get name() {
     return "polling";
@@ -45,10 +42,10 @@ export class Polling extends Transport {
   /**
    * Overrides onRequest.
    *
-   * @param {http.IncomingMessage}
-   * @api private
+   * @param {EngineRequest} req
+   * @private
    */
-  onRequest(req: IncomingMessage & { res: ServerResponse }) {
+  onRequest(req: EngineRequest) {
     const res = req.res;
     // remove the reference to the ServerResponse object (as the first request of the session is kept in memory by default)
     req.res = null;
@@ -66,7 +63,7 @@ export class Polling extends Transport {
   /**
    * The client sends a request awaiting for us to send data.
    *
-   * @api private
+   * @private
    */
   onPollRequest(req, res) {
     if (this.req) {
@@ -108,7 +105,7 @@ export class Polling extends Transport {
   /**
    * The client sends a request with data.
    *
-   * @api private
+   * @private
    */
   onDataRequest(req: IncomingMessage, res: ServerResponse) {
     if (this.dataReq) {
@@ -165,7 +162,7 @@ export class Polling extends Transport {
         // text/html is required instead of text/plain to avoid an
         // unwanted download dialog on certain user-agents (GH-43)
         "Content-Type": "text/html",
-        "Content-Length": 2,
+        "Content-Length": "2",
       };
 
       res.writeHead(200, this.headers(req, headers));
@@ -182,10 +179,10 @@ export class Polling extends Transport {
   /**
    * Processes the incoming data payload.
    *
-   * @param {String} encoded payload
-   * @api private
+   * @param data - encoded payload
+   * @protected
    */
-  onData(data) {
+  protected onData(data: RawData) {
     debug('received "%s"', data);
     const callback = (packet) => {
       if ("close" === packet.type) {
@@ -207,7 +204,7 @@ export class Polling extends Transport {
   /**
    * Overrides onClose.
    *
-   * @api private
+   * @private
    */
   onClose() {
     if (this.writable) {
@@ -217,13 +214,7 @@ export class Polling extends Transport {
     super.onClose();
   }
 
-  /**
-   * Writes a packet payload.
-   *
-   * @param {Object} packet
-   * @api private
-   */
-  send(packets) {
+  send(packets: Packet[]) {
     this.writable = false;
 
     if (this.shouldClose) {
@@ -252,9 +243,9 @@ export class Polling extends Transport {
    *
    * @param {String} data
    * @param {Object} options
-   * @api private
+   * @private
    */
-  write(data, options) {
+  private write(data, options) {
     debug('writing "%s"', data);
     this.doWrite(data, options, () => {
       this.req.cleanup();
@@ -265,9 +256,9 @@ export class Polling extends Transport {
   /**
    * Performs the write.
    *
-   * @api private
+   * @protected
    */
-  doWrite(data, options, callback) {
+  protected doWrite(data, options, callback) {
     // explicit UTF-8 is required for pages not served under utf
     const isString = typeof data === "string";
     const contentType = isString
@@ -319,7 +310,7 @@ export class Polling extends Transport {
   /**
    * Compresses data.
    *
-   * @api private
+   * @private
    */
   compress(data, encoding, callback) {
     debug("compressing");
@@ -342,9 +333,9 @@ export class Polling extends Transport {
   /**
    * Closes the transport.
    *
-   * @api private
+   * @private
    */
-  doClose(fn) {
+  doClose(fn: () => void) {
     debug("closing");
 
     let closeTimeoutTimer;
@@ -377,13 +368,11 @@ export class Polling extends Transport {
   /**
    * Returns headers for a response.
    *
-   * @param {http.IncomingMessage} request
-   * @param {Object} extra headers
-   * @api private
+   * @param {http.IncomingMessage} req
+   * @param {Object} headers - extra headers
+   * @private
    */
-  headers(req, headers) {
-    headers = headers || {};
-
+  private headers(req: IncomingMessage, headers: Record<string, string> = {}) {
     // prevent XSS warnings on IE
     // https://github.com/LearnBoost/socket.io/pull/1333
     const ua = req.headers["user-agent"];
diff --git a/lib/transports/websocket.ts b/lib/transports/websocket.ts
index 788094f4..71ac94a7 100644
--- a/lib/transports/websocket.ts
+++ b/lib/transports/websocket.ts
@@ -1,4 +1,4 @@
-import { Transport } from "../transport";
+import { EngineRequest, Transport } from "../transport";
 import debugModule from "debug";
 import type { Packet, RawData } from "engine.io-parser";
 
@@ -11,10 +11,9 @@ export class WebSocket extends Transport {
   /**
    * WebSocket transport
    *
-   * @param {http.IncomingMessage}
-   * @api public
+   * @param {EngineRequest} req
    */
-  constructor(req) {
+  constructor(req: EngineRequest) {
     super(req);
     this.socket = req.websocket;
     this.socket.on("message", (data, isBinary) => {
@@ -30,8 +29,6 @@ export class WebSocket extends Transport {
 
   /**
    * Transport name
-   *
-   * @api public
    */
   get name() {
     return "websocket";
@@ -39,8 +36,6 @@ export class WebSocket extends Transport {
 
   /**
    * Advertise upgrade support.
-   *
-   * @api public
    */
   get handlesUpgrades() {
     return true;
diff --git a/lib/transports/webtransport.ts b/lib/transports/webtransport.ts
index 07852282..534a5241 100644
--- a/lib/transports/webtransport.ts
+++ b/lib/transports/webtransport.ts
@@ -11,6 +11,7 @@ export class WebTransport extends Transport {
   private readonly writer;
 
   constructor(private readonly session, stream, reader) {
+    // @ts-expect-error
     super({ _query: { EIO: "4" } });
 
     const transformStream = createPacketEncoderStream();
diff --git a/lib/userver.ts b/lib/userver.ts
index c7c91c76..ce56575a 100644
--- a/lib/userver.ts
+++ b/lib/userver.ts
@@ -30,7 +30,7 @@ export class uServer extends BaseServer {
   /**
    * Prepares a request by processing the query string.
    *
-   * @api private
+   * @private
    */
   private prepare(req, res: HttpResponse) {
     req.method = req.getMethod().toUpperCase();