How to get "Data" field from xhr.responseText?

You first need to parse responseText in JSON, for this you should use JSON.parse(). Then you can access it using key.

var json = JSON.parse(xhr.responseText);
var yourData = json.Data; // or json["Data"]

use JSON.parse(), like:

var data=xhr.responseText;
var jsonResponse = JSON.parse(data);
console.log(jsonResponse["Data"]);