diff --git a/lib/socket.ts b/lib/socket.ts
index 0365c72e..761ab71e 100644
--- a/lib/socket.ts
+++ b/lib/socket.ts
@@ -313,19 +313,19 @@ export class SocketWithoutUpgrade extends Emitter<
   protected upgrading: boolean;
   protected setTimeoutFn: typeof setTimeout;
 
-  private prevBufferLen: number;
-  private pingInterval: number;
-  private pingTimeout: number;
-  private pingTimeoutTimer: NodeJS.Timer;
+  private _prevBufferLen: number = 0;
+  private _pingInterval: number = -1;
+  private _pingTimeout: number = -1;
+  private _maxPayload?: number = -1;
+  private _pingTimeoutTimer: NodeJS.Timer;
   private clearTimeoutFn: typeof clearTimeout;
-  private readonly beforeunloadEventListener: () => void;
-  private readonly offlineEventListener: () => void;
-  private maxPayload?: number;
+  private readonly _beforeunloadEventListener: () => void;
+  private readonly _offlineEventListener: () => void;
 
   private readonly secure: boolean;
   private readonly hostname: string;
   private readonly port: string | number;
-  private readonly transportsByName: Record<string, TransportCtor>;
+  private readonly _transportsByName: Record<string, TransportCtor>;
   /**
    * The cookie jar will store the cookies sent by the server (Node. js only).
    */
@@ -383,16 +383,13 @@ export class SocketWithoutUpgrade extends Emitter<
         : "80");
 
     this.transports = [];
-    this.transportsByName = {};
+    this._transportsByName = {};
     opts.transports.forEach((t) => {
       const transportName = t.prototype.name;
       this.transports.push(transportName);
-      this.transportsByName[transportName] = t;
+      this._transportsByName[transportName] = t;
     });
 
-    this.writeBuffer = [];
-    this.prevBufferLen = 0;
-
     this.opts = Object.assign(
       {
         path: "/engine.io",
@@ -420,35 +417,31 @@ export class SocketWithoutUpgrade extends Emitter<
       this.opts.query = decode(this.opts.query);
     }
 
-    // set on handshake
-    this.id = null;
-    this.pingInterval = null;
-    this.pingTimeout = null;
-
-    // set on heartbeat
-    this.pingTimeoutTimer = null;
-
     if (typeof addEventListener === "function") {
       if (this.opts.closeOnBeforeunload) {
         // Firefox closes the connection when the "beforeunload" event is emitted but not Chrome. This event listener
         // ensures every browser behaves the same (no "disconnect" event at the Socket.IO level when the page is
         // closed/reloaded)
-        this.beforeunloadEventListener = () => {
+        this._beforeunloadEventListener = () => {
           if (this.transport) {
             // silently close the transport
             this.transport.removeAllListeners();
             this.transport.close();
           }
         };
-        addEventListener("beforeunload", this.beforeunloadEventListener, false);
+        addEventListener(
+          "beforeunload",
+          this._beforeunloadEventListener,
+          false
+        );
       }
       if (this.hostname !== "localhost") {
-        this.offlineEventListener = () => {
-          this.onClose("transport close", {
+        this._offlineEventListener = () => {
+          this._onClose("transport close", {
             description: "network connection lost",
           });
         };
-        addEventListener("offline", this.offlineEventListener, false);
+        addEventListener("offline", this._offlineEventListener, false);
       }
     }
 
@@ -456,7 +449,7 @@ export class SocketWithoutUpgrade extends Emitter<
       this._cookieJar = createCookieJar();
     }
 
-    this.open();
+    this._open();
   }
 
   /**
@@ -494,7 +487,7 @@ export class SocketWithoutUpgrade extends Emitter<
 
     debug("options: %j", opts);
 
-    return new this.transportsByName[name](opts);
+    return new this._transportsByName[name](opts);
   }
 
   /**
@@ -502,7 +495,7 @@ export class SocketWithoutUpgrade extends Emitter<
    *
    * @private
    */
-  private open() {
+  private _open() {
     if (this.transports.length === 0) {
       // Emit error on next tick so it can be listened to
       this.setTimeoutFn(() => {
@@ -542,10 +535,10 @@ export class SocketWithoutUpgrade extends Emitter<
 
     // set up transport listeners
     transport
-      .on("drain", this.onDrain.bind(this))
-      .on("packet", this.onPacket.bind(this))
-      .on("error", this.onError.bind(this))
-      .on("close", (reason) => this.onClose("transport close", reason));
+      .on("drain", this._onDrain.bind(this))
+      .on("packet", this._onPacket.bind(this))
+      .on("error", this._onError.bind(this))
+      .on("close", (reason) => this._onClose("transport close", reason));
   }
 
   /**
@@ -567,7 +560,7 @@ export class SocketWithoutUpgrade extends Emitter<
    *
    * @private
    */
-  private onPacket(packet: Packet) {
+  private _onPacket(packet: Packet) {
     if (
       "opening" === this.readyState ||
       "open" === this.readyState ||
@@ -579,7 +572,7 @@ export class SocketWithoutUpgrade extends Emitter<
 
       // Socket is live - any packet counts
       this.emitReserved("heartbeat");
-      this.resetPingTimeout();
+      this._resetPingTimeout();
 
       switch (packet.type) {
         case "open":
@@ -587,7 +580,7 @@ export class SocketWithoutUpgrade extends Emitter<
           break;
 
         case "ping":
-          this.sendPacket("pong");
+          this._sendPacket("pong");
           this.emitReserved("ping");
           this.emitReserved("pong");
           break;
@@ -596,7 +589,7 @@ export class SocketWithoutUpgrade extends Emitter<
           const err = new Error("server error");
           // @ts-ignore
           err.code = packet.data;
-          this.onError(err);
+          this._onError(err);
           break;
 
         case "message":
@@ -619,13 +612,13 @@ export class SocketWithoutUpgrade extends Emitter<
     this.emitReserved("handshake", data);
     this.id = data.sid;
     this.transport.query.sid = data.sid;
-    this.pingInterval = data.pingInterval;
-    this.pingTimeout = data.pingTimeout;
-    this.maxPayload = data.maxPayload;
+    this._pingInterval = data.pingInterval;
+    this._pingTimeout = data.pingTimeout;
+    this._maxPayload = data.maxPayload;
     this.onOpen();
     // In case open handler closes socket
     if ("closed" === this.readyState) return;
-    this.resetPingTimeout();
+    this._resetPingTimeout();
   }
 
   /**
@@ -633,13 +626,13 @@ export class SocketWithoutUpgrade extends Emitter<
    *
    * @private
    */
-  private resetPingTimeout() {
-    this.clearTimeoutFn(this.pingTimeoutTimer);
-    this.pingTimeoutTimer = this.setTimeoutFn(() => {
-      this.onClose("ping timeout");
-    }, this.pingInterval + this.pingTimeout);
+  private _resetPingTimeout() {
+    this.clearTimeoutFn(this._pingTimeoutTimer);
+    this._pingTimeoutTimer = this.setTimeoutFn(() => {
+      this._onClose("ping timeout");
+    }, this._pingInterval + this._pingTimeout);
     if (this.opts.autoUnref) {
-      this.pingTimeoutTimer.unref();
+      this._pingTimeoutTimer.unref();
     }
   }
 
@@ -648,13 +641,13 @@ export class SocketWithoutUpgrade extends Emitter<
    *
    * @private
    */
-  private onDrain() {
-    this.writeBuffer.splice(0, this.prevBufferLen);
+  private _onDrain() {
+    this.writeBuffer.splice(0, this._prevBufferLen);
 
     // setting prevBufferLen = 0 is very important
     // for example, when upgrading, upgrade packet is sent over,
     // and a nonzero prevBufferLen could cause problems on `drain`
-    this.prevBufferLen = 0;
+    this._prevBufferLen = 0;
 
     if (0 === this.writeBuffer.length) {
       this.emitReserved("drain");
@@ -675,12 +668,12 @@ export class SocketWithoutUpgrade extends Emitter<
       !this.upgrading &&
       this.writeBuffer.length
     ) {
-      const packets = this.getWritablePackets();
+      const packets = this._getWritablePackets();
       debug("flushing %d packets in socket", packets.length);
       this.transport.send(packets);
       // keep track of current length of writeBuffer
       // splice writeBuffer and callbackBuffer on `drain`
-      this.prevBufferLen = packets.length;
+      this._prevBufferLen = packets.length;
       this.emitReserved("flush");
     }
   }
@@ -691,9 +684,9 @@ export class SocketWithoutUpgrade extends Emitter<
    *
    * @private
    */
-  private getWritablePackets() {
+  private _getWritablePackets() {
     const shouldCheckPayloadSize =
-      this.maxPayload &&
+      this._maxPayload &&
       this.transport.name === "polling" &&
       this.writeBuffer.length > 1;
     if (!shouldCheckPayloadSize) {
@@ -705,13 +698,13 @@ export class SocketWithoutUpgrade extends Emitter<
       if (data) {
         payloadSize += byteLength(data);
       }
-      if (i > 0 && payloadSize > this.maxPayload) {
+      if (i > 0 && payloadSize > this._maxPayload) {
         debug("only send %d out of %d packets", i, this.writeBuffer.length);
         return this.writeBuffer.slice(0, i);
       }
       payloadSize += 2; // separator + packet type
     }
-    debug("payload size is %d (max: %d)", payloadSize, this.maxPayload);
+    debug("payload size is %d (max: %d)", payloadSize, this._maxPayload);
     return this.writeBuffer;
   }
 
@@ -724,7 +717,7 @@ export class SocketWithoutUpgrade extends Emitter<
    * @return {Socket} for chaining.
    */
   public write(msg: RawData, options?: WriteOptions, fn?: () => void) {
-    this.sendPacket("message", msg, options, fn);
+    this._sendPacket("message", msg, options, fn);
     return this;
   }
 
@@ -737,7 +730,7 @@ export class SocketWithoutUpgrade extends Emitter<
    * @return {Socket} for chaining.
    */
   public send(msg: RawData, options?: WriteOptions, fn?: () => void) {
-    this.sendPacket("message", msg, options, fn);
+    this._sendPacket("message", msg, options, fn);
     return this;
   }
 
@@ -750,7 +743,7 @@ export class SocketWithoutUpgrade extends Emitter<
    * @param {Function} fn - callback function.
    * @private
    */
-  private sendPacket(
+  private _sendPacket(
     type: PacketType,
     data?: RawData,
     options?: WriteOptions,
@@ -789,7 +782,7 @@ export class SocketWithoutUpgrade extends Emitter<
    */
   public close() {
     const close = () => {
-      this.onClose("forced close");
+      this._onClose("forced close");
       debug("socket closing - telling transport to close");
       this.transport.close();
     };
@@ -832,7 +825,7 @@ export class SocketWithoutUpgrade extends Emitter<
    *
    * @private
    */
-  private onError(err: Error) {
+  private _onError(err: Error) {
     debug("socket error %j", err);
     SocketWithoutUpgrade.priorWebsocketSuccess = false;
 
@@ -843,11 +836,11 @@ export class SocketWithoutUpgrade extends Emitter<
     ) {
       debug("trying next transport");
       this.transports.shift();
-      return this.open();
+      return this._open();
     }
 
     this.emitReserved("error", err);
-    this.onClose("transport error", err);
+    this._onClose("transport error", err);
   }
 
   /**
@@ -855,7 +848,7 @@ export class SocketWithoutUpgrade extends Emitter<
    *
    * @private
    */
-  private onClose(reason: string, description?: CloseDetails | Error) {
+  private _onClose(reason: string, description?: CloseDetails | Error) {
     if (
       "opening" === this.readyState ||
       "open" === this.readyState ||
@@ -864,7 +857,7 @@ export class SocketWithoutUpgrade extends Emitter<
       debug('socket close with reason: "%s"', reason);
 
       // clear timers
-      this.clearTimeoutFn(this.pingTimeoutTimer);
+      this.clearTimeoutFn(this._pingTimeoutTimer);
 
       // stop event from firing again for transport
       this.transport.removeAllListeners("close");
@@ -878,10 +871,10 @@ export class SocketWithoutUpgrade extends Emitter<
       if (typeof removeEventListener === "function") {
         removeEventListener(
           "beforeunload",
-          this.beforeunloadEventListener,
+          this._beforeunloadEventListener,
           false
         );
-        removeEventListener("offline", this.offlineEventListener, false);
+        removeEventListener("offline", this._offlineEventListener, false);
       }
 
       // set ready state
@@ -896,7 +889,7 @@ export class SocketWithoutUpgrade extends Emitter<
       // clean buffers after, so users can still
       // grab the buffers on `close` event
       this.writeBuffer = [];
-      this.prevBufferLen = 0;
+      this._prevBufferLen = 0;
     }
   }
 }
@@ -925,17 +918,15 @@ export class SocketWithoutUpgrade extends Emitter<
  * @see Socket
  */
 export class SocketWithUpgrade extends SocketWithoutUpgrade {
-  private upgrades: string[] = [];
+  private _upgrades: string[] = [];
 
   override onOpen() {
     super.onOpen();
 
     if ("open" === this.readyState && this.opts.upgrade) {
       debug("starting upgrade probes");
-      let i = 0;
-      const l = this.upgrades.length;
-      for (; i < l; i++) {
-        this.probe(this.upgrades[i]);
+      for (let i = 0; i < this._upgrades.length; i++) {
+        this._probe(this._upgrades[i]);
       }
     }
   }
@@ -946,7 +937,7 @@ export class SocketWithUpgrade extends SocketWithoutUpgrade {
    * @param {String} name - transport name
    * @private
    */
-  private probe(name: string) {
+  private _probe(name: string) {
     debug('probing transport "%s"', name);
     let transport = this.createTransport(name);
     let failed = false;
@@ -1052,7 +1043,7 @@ export class SocketWithUpgrade extends SocketWithoutUpgrade {
     this.once("upgrading", onupgrade);
 
     if (
-      this.upgrades.indexOf("webtransport") !== -1 &&
+      this._upgrades.indexOf("webtransport") !== -1 &&
       name !== "webtransport"
     ) {
       // favor WebTransport
@@ -1067,7 +1058,7 @@ export class SocketWithUpgrade extends SocketWithoutUpgrade {
   }
 
   override onHandshake(data: HandshakeData) {
-    this.upgrades = this.filterUpgrades(data.upgrades);
+    this._upgrades = this._filterUpgrades(data.upgrades);
     super.onHandshake(data);
   }
 
@@ -1077,7 +1068,7 @@ export class SocketWithUpgrade extends SocketWithoutUpgrade {
    * @param {Array} upgrades - server upgrades
    * @private
    */
-  private filterUpgrades(upgrades: string[]) {
+  private _filterUpgrades(upgrades: string[]) {
     const filteredUpgrades = [];
     for (let i = 0; i < upgrades.length; i++) {
       if (~this.transports.indexOf(upgrades[i]))
diff --git a/lib/transports/polling-xhr.ts b/lib/transports/polling-xhr.ts
index ac213546..d24ef75e 100644
--- a/lib/transports/polling-xhr.ts
+++ b/lib/transports/polling-xhr.ts
@@ -2,10 +2,7 @@ import { Polling } from "./polling.js";
 import { Emitter } from "@socket.io/component-emitter";
 import type { SocketOptions } from "../socket.js";
 import { installTimerFunctions, pick } from "../util.js";
-import {
-  globalThisShim as globalThis,
-  createCookieJar,
-} from "../globals.node.js";
+import { globalThisShim as globalThis } from "../globals.node.js";
 import type { CookieJar } from "../globals.node.js";
 import type { RawData } from "engine.io-parser";
 import { hasCORS } from "../contrib/has-cors.js";
@@ -104,14 +101,14 @@ export class Request extends Emitter<
   Record<never, never>,
   RequestReservedEvents
 > {
-  private readonly opts: RequestOptions;
-  private readonly method: string;
-  private readonly uri: string;
-  private readonly data: string | ArrayBuffer;
+  private readonly _opts: RequestOptions;
+  private readonly _method: string;
+  private readonly _uri: string;
+  private readonly _data: string | ArrayBuffer;
 
-  private xhr: any;
+  private _xhr: any;
   private setTimeoutFn: typeof setTimeout;
-  private index: number;
+  private _index: number;
 
   static requestsCount = 0;
   static requests = {};
@@ -129,13 +126,13 @@ export class Request extends Emitter<
   ) {
     super();
     installTimerFunctions(this, opts);
-    this.opts = opts;
+    this._opts = opts;
 
-    this.method = opts.method || "GET";
-    this.uri = uri;
-    this.data = undefined !== opts.data ? opts.data : null;
+    this._method = opts.method || "GET";
+    this._uri = uri;
+    this._data = undefined !== opts.data ? opts.data : null;
 
-    this.create();
+    this._create();
   }
 
   /**
@@ -143,9 +140,9 @@ export class Request extends Emitter<
    *
    * @private
    */
-  private create() {
+  private _create() {
     const opts = pick(
-      this.opts,
+      this._opts,
       "agent",
       "pfx",
       "key",
@@ -156,26 +153,26 @@ export class Request extends Emitter<
       "rejectUnauthorized",
       "autoUnref"
     );
-    opts.xdomain = !!this.opts.xd;
+    opts.xdomain = !!this._opts.xd;
 
-    const xhr = (this.xhr = this.createRequest(opts));
+    const xhr = (this._xhr = this.createRequest(opts));
 
     try {
-      debug("xhr open %s: %s", this.method, this.uri);
-      xhr.open(this.method, this.uri, true);
+      debug("xhr open %s: %s", this._method, this._uri);
+      xhr.open(this._method, this._uri, true);
       try {
-        if (this.opts.extraHeaders) {
+        if (this._opts.extraHeaders) {
           // @ts-ignore
           xhr.setDisableHeaderCheck && xhr.setDisableHeaderCheck(true);
-          for (let i in this.opts.extraHeaders) {
-            if (this.opts.extraHeaders.hasOwnProperty(i)) {
-              xhr.setRequestHeader(i, this.opts.extraHeaders[i]);
+          for (let i in this._opts.extraHeaders) {
+            if (this._opts.extraHeaders.hasOwnProperty(i)) {
+              xhr.setRequestHeader(i, this._opts.extraHeaders[i]);
             }
           }
         }
       } catch (e) {}
 
-      if ("POST" === this.method) {
+      if ("POST" === this._method) {
         try {
           xhr.setRequestHeader("Content-type", "text/plain;charset=UTF-8");
         } catch (e) {}
@@ -185,20 +182,20 @@ export class Request extends Emitter<
         xhr.setRequestHeader("Accept", "*/*");
       } catch (e) {}
 
-      this.opts.cookieJar?.addCookies(xhr);
+      this._opts.cookieJar?.addCookies(xhr);
 
       // ie6 check
       if ("withCredentials" in xhr) {
-        xhr.withCredentials = this.opts.withCredentials;
+        xhr.withCredentials = this._opts.withCredentials;
       }
 
-      if (this.opts.requestTimeout) {
-        xhr.timeout = this.opts.requestTimeout;
+      if (this._opts.requestTimeout) {
+        xhr.timeout = this._opts.requestTimeout;
       }
 
       xhr.onreadystatechange = () => {
         if (xhr.readyState === 3) {
-          this.opts.cookieJar?.parseCookies(
+          this._opts.cookieJar?.parseCookies(
             // @ts-ignore
             xhr.getResponseHeader("set-cookie")
           );
@@ -206,31 +203,31 @@ export class Request extends Emitter<
 
         if (4 !== xhr.readyState) return;
         if (200 === xhr.status || 1223 === xhr.status) {
-          this.onLoad();
+          this._onLoad();
         } else {
           // make sure the `error` event handler that's user-set
           // does not throw in the same tick and gets caught here
           this.setTimeoutFn(() => {
-            this.onError(typeof xhr.status === "number" ? xhr.status : 0);
+            this._onError(typeof xhr.status === "number" ? xhr.status : 0);
           }, 0);
         }
       };
 
-      debug("xhr data %s", this.data);
-      xhr.send(this.data);
+      debug("xhr data %s", this._data);
+      xhr.send(this._data);
     } catch (e) {
       // Need to defer since .create() is called directly from the constructor
       // and thus the 'error' event can only be only bound *after* this exception
       // occurs.  Therefore, also, we cannot throw here at all.
       this.setTimeoutFn(() => {
-        this.onError(e);
+        this._onError(e);
       }, 0);
       return;
     }
 
     if (typeof document !== "undefined") {
-      this.index = Request.requestsCount++;
-      Request.requests[this.index] = this;
+      this._index = Request.requestsCount++;
+      Request.requests[this._index] = this;
     }
   }
 
@@ -239,9 +236,9 @@ export class Request extends Emitter<
    *
    * @private
    */
-  private onError(err: number | Error) {
-    this.emitReserved("error", err, this.xhr);
-    this.cleanup(true);
+  private _onError(err: number | Error) {
+    this.emitReserved("error", err, this._xhr);
+    this._cleanup(true);
   }
 
   /**
@@ -249,23 +246,23 @@ export class Request extends Emitter<
    *
    * @private
    */
-  private cleanup(fromError?) {
-    if ("undefined" === typeof this.xhr || null === this.xhr) {
+  private _cleanup(fromError?) {
+    if ("undefined" === typeof this._xhr || null === this._xhr) {
       return;
     }
-    this.xhr.onreadystatechange = empty;
+    this._xhr.onreadystatechange = empty;
 
     if (fromError) {
       try {
-        this.xhr.abort();
+        this._xhr.abort();
       } catch (e) {}
     }
 
     if (typeof document !== "undefined") {
-      delete Request.requests[this.index];
+      delete Request.requests[this._index];
     }
 
-    this.xhr = null;
+    this._xhr = null;
   }
 
   /**
@@ -273,12 +270,12 @@ export class Request extends Emitter<
    *
    * @private
    */
-  private onLoad() {
-    const data = this.xhr.responseText;
+  private _onLoad() {
+    const data = this._xhr.responseText;
     if (data !== null) {
       this.emitReserved("data", data);
       this.emitReserved("success");
-      this.cleanup();
+      this._cleanup();
     }
   }
 
@@ -288,7 +285,7 @@ export class Request extends Emitter<
    * @package
    */
   public abort() {
-    this.cleanup();
+    this._cleanup();
   }
 }
 
diff --git a/lib/transports/polling.ts b/lib/transports/polling.ts
index 0750c733..12e270ff 100644
--- a/lib/transports/polling.ts
+++ b/lib/transports/polling.ts
@@ -6,7 +6,7 @@ import debugModule from "debug"; // debug()
 const debug = debugModule("engine.io-client:polling"); // debug()
 
 export abstract class Polling extends Transport {
-  private polling: boolean = false;
+  private _polling: boolean = false;
 
   override get name() {
     return "polling";
@@ -19,7 +19,7 @@ export abstract class Polling extends Transport {
    * @protected
    */
   override doOpen() {
-    this.poll();
+    this._poll();
   }
 
   /**
@@ -37,10 +37,10 @@ export abstract class Polling extends Transport {
       onPause();
     };
 
-    if (this.polling || !this.writable) {
+    if (this._polling || !this.writable) {
       let total = 0;
 
-      if (this.polling) {
+      if (this._polling) {
         debug("we are currently polling - waiting to pause");
         total++;
         this.once("pollComplete", function () {
@@ -67,9 +67,9 @@ export abstract class Polling extends Transport {
    *
    * @private
    */
-  poll() {
+  private _poll() {
     debug("polling");
-    this.polling = true;
+    this._polling = true;
     this.doPoll();
     this.emitReserved("poll");
   }
@@ -103,11 +103,11 @@ export abstract class Polling extends Transport {
     // if an event did not trigger closing
     if ("closed" !== this.readyState) {
       // if we got data we're not polling
-      this.polling = false;
+      this._polling = false;
       this.emitReserved("pollComplete");
 
       if ("open" === this.readyState) {
-        this.poll();
+        this._poll();
       } else {
         debug('ignoring poll - transport state "%s"', this.readyState);
       }
diff --git a/lib/transports/webtransport.ts b/lib/transports/webtransport.ts
index 74238197..0062c3e5 100644
--- a/lib/transports/webtransport.ts
+++ b/lib/transports/webtransport.ts
@@ -18,8 +18,8 @@ const debug = debugModule("engine.io-client:webtransport"); // debug()
  * @see https://caniuse.com/webtransport
  */
 export class WT extends Transport {
-  private transport: any;
-  private writer: any;
+  private _transport: any;
+  private _writer: any;
 
   get name() {
     return "webtransport";
@@ -28,7 +28,7 @@ export class WT extends Transport {
   protected doOpen() {
     try {
       // @ts-ignore
-      this.transport = new WebTransport(
+      this._transport = new WebTransport(
         this.createUri("https"),
         this.opts.transportOptions[this.name]
       );
@@ -36,7 +36,7 @@ export class WT extends Transport {
       return this.emitReserved("error", err);
     }
 
-    this.transport.closed
+    this._transport.closed
       .then(() => {
         debug("transport closed gracefully");
         this.onClose();
@@ -47,8 +47,8 @@ export class WT extends Transport {
       });
 
     // note: we could have used async/await, but that would require some additional polyfills
-    this.transport.ready.then(() => {
-      this.transport.createBidirectionalStream().then((stream) => {
+    this._transport.ready.then(() => {
+      this._transport.createBidirectionalStream().then((stream) => {
         const decoderStream = createPacketDecoderStream(
           Number.MAX_SAFE_INTEGER,
           this.socket.binaryType
@@ -57,7 +57,7 @@ export class WT extends Transport {
 
         const encoderStream = createPacketEncoderStream();
         encoderStream.readable.pipeTo(stream.writable);
-        this.writer = encoderStream.writable.getWriter();
+        this._writer = encoderStream.writable.getWriter();
 
         const read = () => {
           reader
@@ -82,7 +82,7 @@ export class WT extends Transport {
         if (this.query.sid) {
           packet.data = `{"sid":"${this.query.sid}"}`;
         }
-        this.writer.write(packet).then(() => this.onOpen());
+        this._writer.write(packet).then(() => this.onOpen());
       });
     });
   }
@@ -94,7 +94,7 @@ export class WT extends Transport {
       const packet = packets[i];
       const lastPacket = i === packets.length - 1;
 
-      this.writer.write(packet).then(() => {
+      this._writer.write(packet).then(() => {
         if (lastPacket) {
           nextTick(() => {
             this.writable = true;
@@ -106,6 +106,6 @@ export class WT extends Transport {
   }
 
   protected doClose() {
-    this.transport?.close();
+    this._transport?.close();
   }
 }
diff --git a/support/rollup.config.esm.js b/support/rollup.config.esm.js
index 9f910341..44f7837b 100644
--- a/support/rollup.config.esm.js
+++ b/support/rollup.config.esm.js
@@ -14,7 +14,15 @@ module.exports = {
     file: "./dist/engine.io.esm.min.js",
     format: "esm",
     sourcemap: true,
-    plugins: [terser()],
+    plugins: [
+      terser({
+        mangle: {
+          properties: {
+            regex: /^_/,
+          },
+        },
+      }),
+    ],
     banner,
   },
   plugins: [
diff --git a/support/rollup.config.umd.js b/support/rollup.config.umd.js
index d87c25cc..2b8a5300 100644
--- a/support/rollup.config.umd.js
+++ b/support/rollup.config.umd.js
@@ -47,7 +47,15 @@ module.exports = [
       format: "umd",
       name: "eio",
       sourcemap: true,
-      plugins: [terser()],
+      plugins: [
+        terser({
+          mangle: {
+            properties: {
+              regex: /^_/,
+            },
+          },
+        }),
+      ],
       banner,
     },
     plugins: [
diff --git a/test/socket.js b/test/socket.js
index ea4de6c0..e1f999f1 100644
--- a/test/socket.js
+++ b/test/socket.js
@@ -16,7 +16,7 @@ describe("Socket", function () {
   describe("filterUpgrades", () => {
     it("should return only available transports", () => {
       const socket = new Socket({ transports: ["polling"] });
-      expect(socket.filterUpgrades(["polling", "websocket"])).to.eql([
+      expect(socket._filterUpgrades(["polling", "websocket"])).to.eql([
         "polling",
       ]);
       socket.close();
@@ -29,7 +29,6 @@ describe("Socket", function () {
     socket.on("error", (error) => {
       errorMessage = error;
     });
-    socket.open();
     setTimeout(() => {
       expect(errorMessage).to.be("No transports available");
       socket.close();
@@ -135,7 +134,6 @@ describe("Socket", function () {
       socket.on("error", (error) => {
         errorMessage = error;
       });
-      socket.open();
       clock.tick(1); // Should trigger error emit.
       expect(errorMessage).to.be("No transports available");
       clock.uninstall();