code: function groupByLimit(coll, limit, iteratee, callback) {
      var _iteratee = wrapAsync(iteratee);
      return mapLimit(coll, limit, (val, iterCb) => {
          _iteratee(val, (err, key) => {
              if (err) return iterCb(err);
              return iterCb(err, {key, val});
          });
      }, (err, mapResults) => {
          var result = {};
          // from MDN, handle object having an `hasOwnProperty` prop
          var {hasOwnProperty} = Object.prototype;
  
          for (var i = 0; i < mapResults.length; i++) {
              if (mapResults[i]) {
                  var {key} = mapResults[i];
                  var {val} = mapResults[i];
  
                  if (hasOwnProperty.call(result, key)) {
                      result[key].push(val);
                  } else {
                      result[key] = [val];
                  }
              }
          }
  
          return callback(err, result);
      });
  }
docs: The same as [`groupBy`]{@link module:Collections.groupBy} but runs a maximum of `limit` async operations at a time.
  
  @name groupByLimit
  @static
  @memberOf module:Collections
  @method
  @see [async.groupBy]{@link module:Collections.groupBy}
  @category Collection
  @param {Array|Iterable|AsyncIterable|Object} coll - A collection to iterate over.
  @param {number} limit - The maximum number of async operations at a time.
  @param {AsyncFunction} iteratee - An async function to apply to each item in
  `coll`.
  The iteratee should complete with a `key` to group the value under.
  Invoked with (value, callback).
  @param {Function} [callback] - A callback which is called when all `iteratee`
  functions have finished, or an error occurs. Result is an `Object` whoses
  properties are arrays of values which returned the corresponding key.
  @returns {Promise} a promise, if no callback is passed