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router.get('/analytics.js', function (req, res, next) {
    var fileUrl = 'http://www.google-analytics.com/analytics.js';
    var filePath = path.resolve('/content/analytics.js');

    // ensure file exists and is less than 1 hour old
    fs.stat(filePath, function (err, stats) {
        if (err) {
            // file doesn't exist so download and create it
            updateFileAndReturn();
        } else {
            // file exists so ensure it's not stale
            if (moment().diff(stats.mtime, 'minutes') > 60) {
                updateFileAndReturn();
            } else {
                returnFile();
            }
        }
    });

    // update file from remote url then send to client
    function updateFileAndReturn() {
        request(fileUrl, function (error, response, body) {
            fs.writeFileSync(filePath, body);
            returnFile();
        });
    }

    // send file to client
    function returnFile() {
        res.set('Cache-Control', 'public, max-age=' + oneWeekSeconds);
        res.sendFile(filePath);
    }
});