postman websocket api, "connected" - but no messages or acknowledgement from server - testing

Question

I can not get postman to connect to the server using websockets.
const port =5001;
io.on("connection", (socket) => {
console.log("Client connected",socket.id);
socket.emit("handshake","connected to backend");
socket.on("test", (data)=>{
console.log("test data is:",data);
socket.emit("test", "server heard you!")
});
}
in postman the request address is:
ws://localhost:5001/socket.io/?transport=websocket
the symptoms are: postman says it's connected. but if I try to send anything - it disconnects after a timeout.
if I set the reconnection attempts to 1, it will automatically reconnect when it disconnects...
but I don't think it's actually connecting - because nothing is happening on the server (no new client connected message)
the format of messages I have also experimented with, to no avail.
42["test","i hear you"]
42[test,i hear you]
["test":"i hear you"]
{"test":"I hear you"}
42{"test":"I hear you"}
{"event":"test","data":"I hear you"}
42{"event":"test","data":"I hear you"}
42["event","test","data","I hear you"]
["event","test","data","I hear you"]
I have inspected the console results, and have not found leads there yet. what could I be missing?

Answer 1

You are using socket.io as WebSocket and that does not work because socket.io is not an implementation of websocket.
From official socket.io documentation:
Socket.IO is NOT a WebSocket implementation. Although Socket.IO indeed uses WebSocket as a transport when possible, it adds additional metadata to each packet. That is why a WebSocket client will not be able to successfully connect to a Socket.IO server, and a Socket.IO client will not be able to connect to a plain WebSocket server either.
// WARNING: the client will NOT be able to connect!
const socket = io("ws://echo.websocket.org");
Source: https://socket.io/docs/v4#What-Socket-IO-is-not

Answer 2

Postman v8.10.0 added support for Socket.IO, read more.
Just enter ws://localhost:5001 as the connection URL and hit Connect.
Also, you can configure the client version (default: v3), handshake path (default: /socket.io), and other reconnection configurations in the request settings.

Answer 3

Because you don not add listener. Add listener "handshake" to postman. You will receive message.
This is my code:
io.on('connection', () => {
console.log('user connected');
setInterval(() => {
io.emit('msg', { data: [1, 2, 3] });
}, 5000);
});