The argument type of request callback in nodejs

var http = require('http');
var req = http.request({host:"www.google.com"})
req.on("response",function(res){
    // ***IncomingMessage***
})

why the type argument of callback is IncomingMessage? how to get from native code? This is the link of github: https://github.com/nodejs/node/blob/v0.12/lib/http.js

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