I know you asked a while ago, but the information remains for those who are researching.
I did an analysis with reverse engineering in version 2.3.0 (socket.io) and 3.4.2 (engine.io) and got the following:
The first number is the type of communication for engine.io, using the enumerator:
| Key | Value |
|---|---|
| 0 | “open” |
| 1 | “close” |
| 2 | “ping” |
| 3 | “pong” |
| 4 | “message” |
| 5 | “upgrade” |
| 6 | “noop” |
The second number is the type of action for socket.io, using the enumerator
| Key | Value |
|---|---|
| 0 | “CONNECT” |
| 1 | “DISCONNECT” |
| 2 | “EVENT” |
| 3 | “ACK” |
| 4 | “ERROR” |
| 5 | “BINARY_EVENT” |
| 6 | “BINARY_ACK” |
There are other optional information that can be passed on, such as namespace and ID, but I will not go into that part.
After these codes he expects a Json Array, where index 0 is the name of the event and index 1 is the argument.
So the instruction 42["moveS",{"from":"g1", "to", "f3"}] is a message for engine.io (4), is an event for socket.io (2), which will emit the “moveS” action passing JSON {"from": "g1", "to", "f3"} as a parameter(Actually JSON.Parse({"from": "g1", "to", "f3"})).
Hope this helps. =D