Returning data from on "close" event

Hi everyone!

I have run into this problem and I am out of ideas.
So in my project, the idea is that the device can register itself in my system.
The device need to send the get request, and in return the access key is send back to the device.
But I have problem to get the access key because of the asynchronous nature of the request.

My code is:

 var accessKey;

function registerDevice() {
  var options = {
    host: this.server_address, // host name
    port: 8080,            // (optional) port, defaults to 80
    path: '/sensor/register',           // path sent to server
    method: 'GET',
    headers: {
      'Accept' : 'application/json',
      'Content-Type' : 'application/json',  }
  };
  var resData = "";
  this.http.request(options, function (res) {
      res.on('data', function (data) {
        resData += data;
      });
      res.on('close', function() {
       resData = JSON.parse(resData);
    });
    }).end();
   return setTimeout(function () {
     resData.access_key;
     console.log('timeout', resData.access_key);
     return resData.access_key;
   },10000);
}

 accessKey = registerDevice();
 console.log(accessKey);

And after running it I always get the undefined object.
I need the access key for later use, to add it to the header in other requests, but I have no idea for now, how to return it from the registerDevice(). Also the console.log() with 'timeout' is returning the valid access key.

Any help will be mostly appreciated.