How to make Google Analytics authentication on Node.js/Backbone.js

I've found this method to make authentication and collect data from Google Analytics, but when I press the button, which will run the following method, I'm getting an exception: Uncaught TypeError: undefined is not a function

            var ga = document.createElement('script');
            ga.type = 'text/javascript';
            ga.async = true;
            ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js';
            var s = document.getElementsByTagName('script')[0];
            s.parentNode.insertBefore(ga, s);
            console.log(ga);
            var profile = '';
            var username = '';
            var password = '';
            var GA = new ga.GA({
                user: username,
                password: password
            });
            var dimensions = [
                'ga:day'
            ];
            var metrics = [
                'ga:pageviews',
                'ga:visitors',
                'ga:sessions'
            ];
            GA.login(function (err, token) {
                var options = {
                    'ids': 'ga:' + profile,
                    'start-date': '2014-08-01',
                    'end-date': '2014-08-31',
                    'dimensions': dimensions.join(','),
                    'metrics': metrics.join(','),
                    'sort': 'ga:day'
                };

                GA.get(options, function (err, entries) {
                    if (!err) {
                        console.log('date,pageviews,visitors,sessions');
                        entries.forEach(function (entry) {
                            var buf = entry.dimensions[0]['ga:day'];

                            metrics.forEach(function (metric) {
                                buf += ',' + entry.metrics[0][metric];
                            });

                            console.log(buf);

                        });
                    } else {
                        console.log(err);
                    }