code: function clean(directoryPath, isRoot, done) {
    fs.stat(directoryPath, function(err, stat) {
      if (err) return done(isRoot ? undefined : err);
      if (stat.isFile()) return fs.unlink(directoryPath, done);
      fs.readdir(directoryPath, function(err, relativePaths) {
        if (err) return done(err);
        each(relativePaths, function(relativePath, next) {
          var absolutePath = path.join(directoryPath, relativePath);
          fs.stat(absolutePath, function(err, stats) {
            if (err) return next(err);
            if (!stats.isDirectory()) return fs.unlink(absolutePath, next);
            clean(absolutePath, false, next);
          });
        }, function(err) {
          if (err) return done(err);
          fs.rmdir(directoryPath, done);
        });
      });
    });
  }
docs: Cleans the files and directories in the directory path.
  @param {string} directoryPath
  @param {function(Error=)} done