NodeJs - calling one method from another in server controller

With the following controller, how can I call one method from another in the same controller?

Specifically, calling login() within a successful signup(), while retaining the same functionality for login() when it is used by a form?

The line this.login(newUser) does not work, nor does plain old login(newUser)

In both scenarios, I get the error:

 TypeError: Cannot call method 'login' of undefined

---

var mongoskin = require('mongoskin');

module.exports = {
login: (function (req, res) {
    req.db.collection('auth').findOne({_id: mongoskin.helper.toObjectID(req.body.id)},
        function (err, results) {
            // log person in and send results to client
        }
    )
}),

signup: (function (req, res) {
    var user = req.body;
    req.db.collection('auth').insert(user, function (err, newUser) {
            // after adding user, automatically log them in

            // does not work:
            //login(newUser, function (err) {
               // do something
            //})

            // does not work:
            this.login(newUser, function (err) {
               // do something
            })
        }
    )
})
}