code: function findCommonPrefix(extractDir, callback) {
      fs.readdir(extractDir, function (err, files) {
          ignoredFolders.forEach(function (folder) {
              var index = files.indexOf(folder);
              if (index !== -1) {
                  files.splice(index, 1);
              }
          });
          if (err) {
              callback(err);
          } else if (files.length === 1) {
              var name = files[0];
              if (fs.statSync(path.join(extractDir, name)).isDirectory()) {
                  callback(null, name);
              } else {
                  callback(null, "");
              }
          } else {
              callback(null, "");
          }
      });
  }
docs: Finds the common prefix, if any, for the files in a package file.
  
  In some package files, all of the files are contained in a subdirectory, and this function
  will identify that directory if it exists.
  
  @param {string} extractDir directory into which the package was extracted
  @param {function(Error, string)} callback function to accept err, commonPrefix (which will be "" if there is none)