onExit() is similar to the onLoad() callback. onExit() is called on each module just before the process exits. This can be used to check state in unit tests, but not to perform I/O. The process will forcibly exit as soon as all of the onExit callbacks are made.
If users do not send transfer-encoding or content-length headers, then I will not add any additional. Content-Length: 0 is assumed if there aren't other headers and chunked encoding is rare.
This is a boolean value which allows one to detect if the socket was closed due to errors. There is not yet a way to look up the actual error code.