Beautiful Code with async and callbacks function

data-statistics.js

var database = require('../../../models');
var async = require('async');

var VisitorApi = (function() {
    /**
     * Public Functions
     */
    var _countVisitorsByOperator = function(operator,value,callback) {
        var query = {};
        query[operator] = value;
        database.getCollection('Visitor').count(query).exec(function(err, visitors) {
            if(err) return callback(err);
            return callback(null,visitors);
        });
    };
    var _gettatisticByCriteria = function(criteria, done) {
        database.getCollection('Visitor').distinct(criteria, function(err, systems) {
            if(err) return done(err);
            if(systems) {
                async.map(systems, function(system,callback) {
                    _countVisitorsByOperator(criteria,system, function(err, result) {
                        if(err) return callback(err);
                        var statObj = {};
                        statObj[system] = result;
                        return callback(null,statObj);
                    })
                }, function(err, data) {
                    return done(err, data);
                });
            } else {
                return done(null, {});
            }
        });
    };
    /**
     * Private Functions
     */
    return {
        getStatisticsByCriteria : function(req, res, next) {
            var criteria = req.query.criteria;
            _gettatisticByCriteria(criteria, function(err, data) {
                if(err) return res.status(500).send({message: err.message});
                return res.status(200).send(data);
            })

        }
    }
})();