AppenderModule's underlying structure is not typed:
https://github.com/log4js-node/log4js-node/blob/master/types/log4js.d.ts#L245

the Function is not typed.

Yet, the way to write a custom appender is codified as shown in the examples https://log4js-node.github.io/log4js-node/writing-appenders.html.

It makes things more difficult to write custom appender in Typescript, as we don't have type information for the intermediary functions.