whole_func_string: function (...rest) {
        const _key = _hash(rest, opt.id);
        return cacheObj.get(opt, _key)
                .then(res => {
                  if (typeof res == 'undefined' || res === null) {  
                    res = _update.bind(this)(opt, func, rest);
                  }
                  return res;
                });
      }
func_code_string: function (...rest) {
        const _key = _hash(rest, opt.id);
        return cacheObj.get(opt, _key)
                .then(res => {
                  if (typeof res == 'undefined' || res === null) {  
                    res = _update.bind(this)(opt, func, rest);
                  }
                  return res;
                });
      }
func_documentation_string: for ES6 notation