docs: function(file) {
      file = path.resolve(file);
  
      // text ?
      const textExts = ['.css', '.js', '.html', '.less', '.json'];
      const ext = path.extname(file);
      const isText = textExts.indexOf(ext) > -1;
  
      // content
      let content;
      if (isText) {
        content = fs.readFileSync(file, 'utf8');
      } else {
        content = fs.readFileSync(file);
      }
      const hash = createHash('md5').update(content).digest('hex');
      return hash.substr(0, 8);
    }