how to get the type of request's callback about nodejs

where to get the type of cb function? and I see this type is incomingmessage, and is the son class of stream.readable, but why and how to get them? here is the link of github: github.com/nodejs/node/blob/v0.12/lib/http.js (48-50 line) github.com/nodejs/node/blob/v0.12/lib/_http_client.js (92-94 line)

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