nsITCPSocketEvent is the event object which is passed as the
first argument to all the event handler callbacks. It contains
the socket that was associated with the event, the type of event,
and the data associated with the event (if any).
The socket object which produced this event.
The type of this event. One of:
open
error
data
drain
close
The data related to this event, if any. In the ondata callback,
data will be the bytes read from the network; if the binaryType
of the socket was “arraybuffer”, this value will be of type ArrayBuffer;
otherwise, it will be a normal JavaScript string.
In the onerror callback, data will be a string with a description
of the error.
In the other callbacks, data will be an empty string.