lib/index.js
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;
              });
    }
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;
              });
    }
for ES6 notation