save/upload file from server in chromeapp

Hi I am creating chrome app using rediumjs.I want to download epub file from server BLOB. for that I written this code:

 var url="http://10.11.9.1/TapRoot/Books/1408fd6cce7447e224cf/Extracted/toc.json";
      var xhr = new XMLHttpRequest();
      xhr.open("GET",url, true);
      xhr.responseType = 'json';
      xhr.onreadystatechange = function() {
        if (xhr.readyState == 4 && xhr.status == 200) {
          var blob = this.response;
          console.log(blob);
          //Here I want to save this file in my local drive.
          // saveAs(xhr.response, '../test123.epub');
         // saveFile(this.response, '../temp/file.json');
        }
      };
      xhr.send();