What do these numbers mean in socket.io payload?

When I am using native websocket API I can see just a payload in my chrome console for sockts:

But when I use socket.io with their emit event, I can see some strange numbers before my actual payload. I do understand that colors mean that you either send or received the data, but what does the numbers like 42, 3, 2, 430, 420, 5 mean.

Is there a place I can get a full list of these numbers with descriptions?

The code which generates it is kind of big, so I just post small snippets.

Client side always look like this:

socket.emit('joinC', room, function(color){ ... });

Server side looks like this:

io.sockets.in(room).emit('moveS', {...});

Websockets allow you to send data back and forth over a full-duplex communication channel.

Socket.IO on the other hand is a realtime application framework that uses websockets as transport adding features like namespacing connections, rooms, fallback to other transports etc. To build all those features, the messages exchanged back and forward must cary some semantics so that Socket.IO knows what they mean (i.e. what kind of message is it, event, error etc) and what to do with them. For that it uses a protocol that frames the message with some codes that identify it's semantic. That's what you are seeing with those numbers.

Unfortunately the Socket.IO documentation is very terse and it's hard to understand exactly how those codes are combined and parsed. To get their exact meaning I think one needs to look at the Socket.IO source code.

EDIT from a socket.io Github issue:

This is handled in socket.io-parser and engine.io-parser, which are implementations of socket.io-protocol and engine.io-protocol respectively. You can find the protocol description for socket.io here and for engine.io here.

The encoding sections in these documents are of interest when looking at the actual data that is sent through the transports. The socket.io-protocol handles encoding of metadata, like namespaes to an engine.io-protocol handleable format.