diff --git a/analysis_options.yaml b/analysis_options.yaml
index 3d63a9f..5df03dc 100644
--- a/analysis_options.yaml
+++ b/analysis_options.yaml
@@ -11,8 +11,7 @@ linter:
     cancel_subscriptions: true
     close_sinks: true
     hash_and_equals: true
-    iterable_contains_unrelated_type: true
-    list_remove_unrelated_type: true
+    collection_methods_unrelated_type: true
     test_types_in_equals: true
     unrelated_type_equality_checks: true
     valid_regexps: true
diff --git a/lib/src/engine/socket.dart b/lib/src/engine/socket.dart
index a9af997..34f83cd 100644
--- a/lib/src/engine/socket.dart
+++ b/lib/src/engine/socket.dart
@@ -53,11 +53,11 @@ class Socket extends EventEmitter {
     opts = opts ?? <dynamic, dynamic>{};
 
     if (uri.isNotEmpty) {
-      var _uri = Uri.parse(uri);
-      opts['hostname'] = _uri.host;
-      opts['secure'] = _uri.scheme == 'https' || _uri.scheme == 'wss';
-      opts['port'] = _uri.port;
-      if (_uri.hasQuery) opts['query'] = _uri.query;
+      var uri0 = Uri.parse(uri);
+      opts['hostname'] = uri0.host;
+      opts['secure'] = uri0.scheme == 'https' || uri0.scheme == 'wss';
+      opts['port'] = uri0.port;
+      if (uri0.hasQuery) opts['query'] = uri0.query;
     } else if (opts.containsKey('host')) {
       opts['hostname'] = Uri.parse(opts['host']).host;
     }
@@ -101,7 +101,7 @@ class Socket extends EventEmitter {
 
 
     this.opts['path'] =
-        this.opts['path'].toString().replaceFirst(RegExp(r'\/$'), '') +
+        this.opts['path'].toString().replaceFirst(RegExp(r'/$'), '') +
             (this.opts['addTrailingSlash'] ? '/' : '');
 
     if (opts['query'] is String) {
@@ -155,9 +155,9 @@ class Socket extends EventEmitter {
       ...this.opts,
       'query': query,
       'socket': this,
-      'hostname': this.hostname,
-      'secure': this.secure,
-      'port': this.port,
+      'hostname': hostname,
+      'secure': secure,
+      'port': port,
       ...transportOptions,
     };
 
@@ -170,7 +170,7 @@ class Socket extends EventEmitter {
   /// @api private
   void open() {
     dynamic transport;
-    if (this.opts['rememberUpgrade'] != null &&
+    if (opts['rememberUpgrade'] != null &&
         priorWebsocketSuccess &&
         transports.contains('websocket')) {
       transport = 'websocket';
@@ -325,7 +325,7 @@ class Socket extends EventEmitter {
     once('close', onclose);
     once('upgrading', onupgrade);
 
-    if (this.upgrades!.indexOf('webtransport') != -1 && name != 'webtransport') {
+    if (upgrades!.contains('webtransport') && name != 'webtransport') {
       // favor WebTransport
       Timer(Duration(milliseconds: 200), () {
         if (!failed) {
@@ -351,7 +351,7 @@ class Socket extends EventEmitter {
     // we check for `readyState` in case an `open`
     // listener already closed the socket
     if ('open' == readyState &&
-        this.opts['upgrade'] == true &&
+        opts['upgrade'] == true &&
         transport?.name == 'polling') {
       _logger.fine('starting upgrade probes');
       for (var i = 0, l = upgrades!.length; i < l; i++) {
diff --git a/lib/src/engine/transport.dart b/lib/src/engine/transport.dart
index 627417e..1652d5b 100644
--- a/lib/src/engine/transport.dart
+++ b/lib/src/engine/transport.dart
@@ -1,6 +1,6 @@
-/// Copyright (C) 2019 Potix Corporation. All Rights Reserved
-/// History: 2019-01-21 12:27
-/// Author: jumperchen<jumperchen@potix.com>
+// Copyright (C) 2019 Potix Corporation. All Rights Reserved
+// History: 2019-01-21 12:27
+// Author: jumperchen<jumperchen@potix.com>
 import 'dart:convert';
 
 import 'package:logging/logging.dart';
@@ -19,8 +19,7 @@ abstract class Transport extends EventEmitter {
   String? readyState;
   Socket? socket;
 
-  Transport(Map opts) {
-    this.opts = opts;
+  Transport(this.opts) {
     query = opts['query'];
     readyState = '';
     socket = opts['socket'];
@@ -105,7 +104,7 @@ abstract class Transport extends EventEmitter {
   }
   get name;
 
-  void pause(onPause()) {}
+  void pause(Function() onPause) {}
 
   String createUri(String schema, Map<String, dynamic> query) {
     return '$schema://${_hostname()}${_port()}${opts["path"]}${_query(query)}';
diff --git a/lib/src/engine/transport/io_websocket_transport.dart b/lib/src/engine/transport/io_websocket_transport.dart
index 4590c32..c2735cc 100644
--- a/lib/src/engine/transport/io_websocket_transport.dart
+++ b/lib/src/engine/transport/io_websocket_transport.dart
@@ -120,10 +120,10 @@ class IOWebSocketTransport extends Transport {
   /// @api private
   String uri() {
     var query = this.query ?? {};
-    var schema = this.opts['secure'] ? 'wss' : 'ws';
+    var schema = opts['secure'] ? 'wss' : 'ws';
     // append timestamp to URI
-    if (this.opts['timestampRequests'] == true) {
-      query[this.opts['timestampRequests']] =
+    if (opts['timestampRequests'] == true) {
+      query[opts['timestampRequests']] =
           DateTime.now().millisecondsSinceEpoch.toRadixString(36);
     }
 
diff --git a/lib/src/engine/transport/polling_transport.dart b/lib/src/engine/transport/polling_transport.dart
index 85de5bd..aad9970 100644
--- a/lib/src/engine/transport/polling_transport.dart
+++ b/lib/src/engine/transport/polling_transport.dart
@@ -1,14 +1,14 @@
-///
-/// polling_transport.dart
-///
-/// Purpose:
-///
-/// Description:
-///
-/// History:
-///   26/04/2017, Created by jumperchen
-///
-/// Copyright (C) 2017 Potix Corporation. All Rights Reserved.
+//
+// polling_transport.dart
+//
+// Purpose:
+//
+// Description:
+//
+// History:
+//   26/04/2017, Created by jumperchen
+//
+// Copyright (C) 2017 Potix Corporation. All Rights Reserved.
 import 'dart:async';
 import 'dart:html';
 
@@ -39,8 +39,8 @@ class PollingTransport extends Transport {
   String? name = 'polling';
 
   bool polling = false;
-  dynamic? pollXhr;
-  dynamic? cookieJar;
+  dynamic pollXhr;
+  dynamic cookieJar;
   late bool xd;
 
   ///
@@ -83,6 +83,7 @@ class PollingTransport extends Transport {
   ///
   /// @param {Function} callback upon buffers are flushed and transport is paused
   /// @api private
+  @override
   void pause(onPause) {
     var self = this;
 
@@ -222,11 +223,11 @@ class PollingTransport extends Transport {
   /// @api private
   String uri() {
     final query = this.query ?? {};
-    var schema = this.opts['secure'] ? 'https' : 'http';
+    var schema = opts['secure'] ? 'https' : 'http';
 
     // cache busting is forced
-    if (this.opts['timestampRequests'] != null) {
-      query[this.opts['timestampRequests']] =
+    if (opts['timestampRequests'] != null) {
+      query[opts['timestampRequests']] =
           DateTime.now().millisecondsSinceEpoch.toRadixString(36);
     }
 
@@ -241,11 +242,11 @@ class PollingTransport extends Transport {
     opts = opts ?? {};
     final mergedOpts = {
       ...opts,
-      xd: this.xd,
-      cookieJar: this.cookieJar,
+      xd: xd,
+      cookieJar: cookieJar,
       ...this.opts
     };
-    return Request(this.uri(), mergedOpts);
+    return Request(uri(), mergedOpts);
   }
   ///
   /// Sends data.
@@ -253,7 +254,6 @@ class PollingTransport extends Transport {
   /// @param {String} data to send.
   /// @param {Function} called upon flush.
   /// @api private
-  @override
   void doWrite(data, fn) {
     var isBinary = data is! String;
     var req = request({'method': 'POST', 'data': data, 'isBinary': isBinary});
@@ -267,7 +267,6 @@ class PollingTransport extends Transport {
   /// Starts a poll cycle.
   ///
   /// @api private
-  @override
   void doPoll() {
     _logger.fine('xhr poll');
     var req = request();
@@ -290,11 +289,9 @@ class Request extends EventEmitter {
   int? index;
   StreamSubscription? readyStateChange;
 
-  Request(uri, Map opts) {
-    this.opts = opts;
-    this.method = opts['method'] ?? 'GET';
-    this.uri = uri;
-    this.data = opts['data'];
+  Request(this.uri, this.opts) {
+    method = opts['method'] ?? 'GET';
+    data = opts['data'];
 
     create();
   }
diff --git a/lib/src/engine/transport/transports.dart b/lib/src/engine/transport/transports.dart
index d98f925..c602383 100644
--- a/lib/src/engine/transport/transports.dart
+++ b/lib/src/engine/transport/transports.dart
@@ -1,6 +1,6 @@
-/// Copyright (C) 2017 Potix Corporation. All Rights Reserved
-/// History: 2017-04-26 12:27
-/// Author: jumperchen<jumperchen@potix.com>
+// Copyright (C) 2017 Potix Corporation. All Rights Reserved
+// History: 2017-04-26 12:27
+// Author: jumperchen<jumperchen@potix.com>
 import 'package:socket_io_client/src/engine/transport.dart';
 import 'package:socket_io_client/src/engine/transport/polling_transport.dart';
 import 'package:socket_io_client/src/engine/transport/websocket_transport.dart';
diff --git a/lib/src/engine/transport/websocket_transport.dart b/lib/src/engine/transport/websocket_transport.dart
index bf81736..4f7ff0d 100644
--- a/lib/src/engine/transport/websocket_transport.dart
+++ b/lib/src/engine/transport/websocket_transport.dart
@@ -7,7 +7,6 @@ import 'dart:html';
 import 'package:logging/logging.dart';
 import 'package:socket_io_client/src/engine/transport.dart';
 import 'package:socket_io_common/src/engine/parser/parser.dart';
-import 'package:socket_io_client/src/engine/parseqs.dart';
 
 class WebSocketTransport extends Transport {
   static final Logger _logger =
@@ -25,9 +24,9 @@ class WebSocketTransport extends Transport {
   @override
   void doOpen() {
     var uri = this.uri();
-    var protocols = this.opts['protocols'];
-    if (this.opts.containsKey('extraHeaders')) {
-      this.opts['headers'] = this.opts['extraHeaders'];
+    var protocols = opts['protocols'];
+    if (opts.containsKey('extraHeaders')) {
+      opts['headers'] = opts['extraHeaders'];
     }
 
     try {
@@ -40,7 +39,7 @@ class WebSocketTransport extends Transport {
       supportsBinary = false;
     }
 
-    ws!.binaryType = this.socket!.binaryType;
+    ws!.binaryType = socket!.binaryType;
 
     addEventListeners();
   }
@@ -111,10 +110,10 @@ class WebSocketTransport extends Transport {
   /// @api private
   String uri() {
     var query = this.query ?? {};
-    var schema = this.opts['secure'] ? 'wss' : 'ws';
+    var schema = opts['secure'] ? 'wss' : 'ws';
     // append timestamp to URI
-    if (this.opts['timestampRequests'] == true) {
-      query[this.opts['timestampRequests']] =
+    if (opts['timestampRequests'] == true) {
+      query[opts['timestampRequests']] =
           DateTime.now().millisecondsSinceEpoch.toRadixString(36);
     }
 
diff --git a/lib/src/manager.dart b/lib/src/manager.dart
index c77207d..eb2f8b8 100644
--- a/lib/src/manager.dart
+++ b/lib/src/manager.dart
@@ -1,6 +1,6 @@
-/// Copyright (C) 2017 Potix Corporation. All Rights Reserved
-/// History: 2017-04-26 15:27
-/// Author: jumperchen<jumperchen@potix.com>
+// Copyright (C) 2017 Potix Corporation. All Rights Reserved
+// History: 2017-04-26 15:27
+// Author: jumperchen<jumperchen@potix.com>
 import 'dart:async';
 import 'dart:math' as math;
 
@@ -169,7 +169,7 @@ class Manager extends EventEmitter {
       if (callback != null) callback();
     });
 
-    var onError;
+    Function(dynamic error) onError;
     // emit `connect_error`
     var errorSub = util.on(socket, 'error', onError = (error) {
       _logger.fine('error');
diff --git a/lib/src/socket.dart b/lib/src/socket.dart
index ec500f7..0fdc4cf 100644
--- a/lib/src/socket.dart
+++ b/lib/src/socket.dart
@@ -1,14 +1,14 @@
-///
-/// socket.dart
-///
-/// Purpose:
-///
-/// Description:
-///
-/// History:
-///   26/04/2017, Created by jumperchen
-///
-/// Copyright (C) 2017 Potix Corporation. All Rights Reserved.
+//
+// socket.dart
+//
+// Purpose:
+//
+// Description:
+//
+// History:
+//   26/04/2017, Created by jumperchen
+//
+// Copyright (C) 2017 Potix Corporation. All Rights Reserved.
 import 'dart:async';
 import 'dart:typed_data';
 
@@ -44,7 +44,7 @@ class Socket extends EventEmitter {
   dynamic auth;
   List receiveBuffer = [];
   List sendBuffer = [];
-  List _queue = [];
+  final List _queue = [];
   int _queueSeq = 0;
 
   String nsp;
@@ -54,8 +54,8 @@ class Socket extends EventEmitter {
   List? subs;
   Map flags = {};
   String? query;
-  List _anyListeners = [];
-  List _anyOutgoingListeners = [];
+  final List _anyListeners = [];
+  final List _anyOutgoingListeners = [];
 
   Socket(this.io, this.nsp, this._opts) {
     if (_opts != null) {