NodeJS: Request and Respose Objects

# req Objects
- req
- req.body, req.queries, req.validationerrors
- res.headers, req.users, req.file, req.assert
- req.params, req.checkbody

#res Objects
- res.send, res.end, res.json, res.render, res.status, res.sendStatus

#
- .limit(4) //limit only upto 4 docs
- skips(3) //skip first 3 docs
- exe //query build and then execute, it is also callback function of mongoose.
- sort({_id: -1}) //for decending order
- populate('userId') //populating data inside the reference.

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