docs: function copyFolder(ffs, from, to) {
      checkFolder(ffs, to);
      let tasks = [];
      ffs.readdirSync(from).forEach(child => {
          if (ffs.statSync(from + '/' + child).isDirectory()) {
              copyFolder(ffs, from + '/' + child, to + '/' + child);
          }
          else {
              copyFile(ffs, from + '/' + child, to + '/' + child);
          }
      });
  }