code: function _browseDir (dir, fileList) {
    fileList || (fileList = [])
  
    try {
      fs.readdirSync(dir)
        .map(item => _buildFullPath(dir, item))
        .map(item => _addFileOrContinueBrowsing(item, fileList))
    } catch (e) {
      throw new Error(e.message)
    }
  
    return fileList
  }
docs: explore directories and store files in an array
  @param {String} dir       directory absolute path
  @param {Array.<String>} fileList   store all files
  @param {Array.<String>} fileList   return all files