Why engine.io never try the second transport

[SijieDingEsna]

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You want to:

• report a bug
  I have a socket.io server which apply socket.io + express and it only supports websocket. If transports is set as ['polling', 'websocket'] in client side, engine.io-client always try polling infinity and never try websocket.
• request a feature

Current behaviour

Steps to reproduce (if the current behaviour is a bug)

Expected behaviour

Once the socket.io server side return status 400 and the payload is {"code":0,"message":"Transport unknown"}. The client side should be smart enough to try the second protocol. Because your document suggestion is polling is at first and the default transports is ['polling', 'websocket']. So I think under such transport setting of client, engine.io-client should be tolerable enough to handle such condition.

Setup

• OS:
• browser:
• engine.io version:

Other information (e.g. stacktraces, related issues, suggestions how to fix)

[darrachequesne]

For future readers:

The tryAllTransports option makes the client test each transport:

import { io } from "socket.io-client";

const socket = io({
  tryAllTransports: true
});

Reference: https://socket.io/docs/v4/client-options/#tryalltransports

Implemented in socketio/engine.io-client@579b243, included in engine.io-client@6.6.0 and socket.io-client@4.8.0.