code: function promisify(err, value, callback) {
    if (err) { throw err; }
    if (isPromiseLike(value)) {
      value.then(function(v) {
        callback(null, v);
      }, function(err) {
        callback(err);
      });
    } else {
      callback(null, value);
    }
  }
docs: When the result was "promise", resolve its value
  @private