TypeError in engine/socket.dart on Flutter Web

[djuelg]

Hi,

I'm using your Dart Socket.IO client (v3.1.2) in a Flutter Web application alongside a Socket.IO server. Whenever the server restarts, the client throws the following exception, which breaks the reconnection logic:

TypeError: Instance of 'JSArray<dynamic>': type 'List<dynamic>' is not a subtype of type 'List<String>'

The issue seems to stem from this line in engine/socket.dart :

transports = opts['transports'] ?? ['polling', 'websocket', 'webtransport'];

On Flutter Web, opts['transports'] becomes a JSArray<dynamic>, which gets converted into a List<dynamic> instead of the expected List<String>. This causes the type mismatch.

A potential fix could be to cast the list explicitly like this:

transports = (opts['transports'] as List<dynamic>?)?.cast<String>() ?? ['polling', 'websocket', 'webtransport'];

The logs look like this:

FINE: 2025-06-16 10:13:36.587: socket_io_client:engine.Socket: socket close with reason: "transport close"
FINE: 2025-06-16 10:13:36.587: socket_io_client:Manager: closed due to transport close
FINE: 2025-06-16 10:13:36.588: socket_io_client:Manager: cleanup
FINE: 2025-06-16 10:13:36.588: socket_io_client:Manager: will wait %dms before reconnect attempt
FINE: 2025-06-16 10:13:37.163: socket_io_client:Manager: attempting reconnect
FINE: 2025-06-16 10:13:37.164: socket_io_client:Manager: readyState closed
FINE: 2025-06-16 10:13:37.164: socket_io_client:Manager: opening http://localhost:6095
DartError: TypeError: Instance of 'JSArray<dynamic>': type 'List<dynamic>' is not a subtype of type 'List<String>'
dart-sdk/lib/_internal/js_dev_runtime/private/ddc_runtime/errors.dart 307:3  throw_
dart-sdk/lib/_internal/js_dev_runtime/private/profile.dart 117:39            _failedAsCheck
dart-sdk/lib/_internal/js_shared/lib/rti.dart 1554:3                         _generalAsCheckImplementation
packages/socket_io_client/src/engine/socket.dart 82:5                        new
packages/socket_io_client/src/manager.dart 173:28                            open
packages/socket_io_client/src/manager.dart 434:9                             <fn>
dart-sdk/lib/_internal/js_dev_runtime/private/isolate_helper.dart 47:11      internalCallback

BR