Webpagetest returning value "undefined" in function callback

HI Guys,

Can some one help me on how to get test id in callback method.

Below is the script

it(‘WebpageTest Homepage’, function(done) {

var script = wpt.scriptToString([
 {navigate: config.homepage}, 
wpt.runTest(script, {
    label: 'homepagetest',
    runs: 1,
pollResults: 5,
firstViewOnly : true,
connectivity: "3GFast",
specs:path.join(__dirname, 'specs.json'),},

function(err, res) { 

    assert.equal(err, 0);


in console log im getting value of “res” or “res.data” as undefined. can some one please suggest here if im doing anything wrong. Test was running fine with out any issues.
I wanted to capture testId from callback function(res.data.testId).