...
  app.post('/testapi', function (req, res) {

   request({
       "url": 'https://webservice.com',
       "method": "POST",
       "body" :req.body
     },
       function(err, response, body){
        if (err) {
            res.send(err);
        } else {
            res.send(body);
            logSomeData(req.body);
        }
    });
});

function logSomeData(body) {
    var currYear = new Date().getFullYear(),
            currMonth = new Date().getMonth(),
            fileName = __dirname + '/logs/' + currYear + '_' + currMonth + '.json';

    parseString(body, function (err, result) {
        jsonfile.readFile(fileName, function(err, obj) {

            var logData = {test : "test"};

            obj.push(logData);

        jsonfile.writeFile(fileName, obj, {spaces: 2}, function(err) {
                console.error(err)
            });
        });
    });
}