module.exports = {
    startup: function(dbToUse) {
        mongoose.connect(dbToUse);
        mongoose.connection.on('open', function() {
            console.log("MongoDB connection established.");
        });
    },

    saveUser: function(userInfo, callback) {
        var newUser = new User({
            displayName: userInfo.displayName
        , email: userInfo.email
        , password: userInfo.password
        });
}

// app.js

db = new DB.startup(conn);

db.saveUser()

TypeError: Object [object Object] has no method 'saveUser'