whole_func_string: function addFeatureItem(logger, existed, name, params) {
      if (!existed[name]) {
          existed[name] = true;
          let feature = {name};
          params && (feature.params = params);
          return feature;
      }
  
      logger.debug('duplicated feature declaration', name);
      return false;
  }
func_code_string: function addFeatureItem(logger, existed, name, params) {
      if (!existed[name]) {
          existed[name] = true;
          let feature = {name};
          params && (feature.params = params);
          return feature;
      }
  
      logger.debug('duplicated feature declaration', name);
      return false;
  }
func_documentation_string: Add API feature item.
  Return the feature item to add, or return false if existed.
  
  @inner
  @param {Object} logger logger the logger utilities
  @param {Object} existed the added feature item map
  @param {string} name the feature item name to add
  @param {Object=} params the feature item params, optional
  @return {Object|boolean}