code: function fsReaddirSync(root, files, fp) {
    if (typeof root !== 'string') {
      throw new TypeError('fsReaddir.sync: expect `root` to be string');
    }
  
    fp = fp || '';
    files = files || [];
  
    var dir = path.join(root, fp);
    if (fs.statSync(dir).isDirectory()) {
      fs.readdirSync(dir).forEach(function(name) {
        fsReaddirSync(root, files, path.join(dir, name));
      });
    } else {
      files.push(fp);
    }
  
    return files;
  }
docs: `fs-readdir-recursive` without filter feature
  
  @param  {String} `root`
  @param  {Array} `files`
  @param  {String} `fp`
  @return {Array}