Partial omission of emitted object if jsonification fails

[eldir]

Describe the bug
When passing an object to an emit that contains a function reference it just quietly gets omitted.

To Reproduce

function test_function() {
  return null;
}

const data = {
  key_one: "value 1",
  key_two: test_function
}

socket.emit("test", data);

Checking traffic will show that test gets a message containing only

{
  "key_one": "value 1"
}

With no error or even warning on the console.

Expected behavior
If jsonification fails (which is what I suspect is happening here) I would expect at least a warning or a full transmission failure and not quiet removal of part of the transmitted object.

Platform:

• Device: PC
• OS: Ubuntu 24.04.1 LTS
• tested on socket.io client 4.7.5 and 4.8.1

Additional context
For context we initially found this with a misconfigured value from redux. A console.log(data) will produce the full object as expected.

[darrachequesne]

Let's continue the discussion here: socketio/socket.io#5225