Hello,

I’m wondering what the first 2 indices of the arrays sent/received over Phoenix socket connections are. Here are some examples from my Network tab in the browser dev tools:

(send)    ["3","3","room:36","phx_join",{}]
(receive) ["3","3","room:36","phx_reply",{"response":{"some_join_response_key": "some_join_response_value"},"status":"ok"}]
(send)    [null,"4","phoenix","heartbeat",{}]
(receive) [null,"4","phoenix","phx_reply",{"response":{},"status":"ok"}]
(send)    ["3","5","room:36","event_initiated_from_this_window", <data...>]
(receive) ["3",null,"room:36","event_initiated_from_other_window", <data...>]
(receive) [null,null,"room:36","presence_diff",{"joins":{"2": <presence info>},"leaves":{}}]

The second looks like the heartbeat counter. What I’m confused about is what the "3" means in the first index, and under what conditions do each of them get set to null.

I was trying to find some answers in the source code, but I couldn’t find anywhere parsing a message as an array. This line in socket.js seems most relevant, but not sure how it ties in since it expects an object.

FWIW, I ask because I’m starting to write front-end tests for a React app which gets its data from a Phoenix channel. To do this, I’m mocking a WebSocket connection, sending the client data, and asserting what gets rendered. So mostly what I’m wondering is whether it makes any difference to pass bogus values for those fields in a mock setting. Also, if there is any advice on that use case as well, I’d love to hear

Thanks!