code: function createAggregator(setter) {
      return function(collection, callback, thisArg) {
        var result = {};
        callback = createCallback(callback, thisArg, 3);
  
        var index = -1,
            length = collection ? collection.length : 0;
  
        if (typeof length == 'number') {
          while (++index < length) {
            var value = collection[index];
            setter(result, value, callback(value, index, collection), collection);
          }
        } else {
          forOwn(collection, function(value, key, collection) {
            setter(result, value, callback(value, key, collection), collection);
          });
        }
        return result;
      };
    }
docs: Creates a function that aggregates a collection, creating an object composed
  of keys generated from the results of running each element of the collection
  through a callback. The given `setter` function sets the keys and values
  of the composed object.
  
  @private
  @param {Function} setter The setter function.
  @returns {Function} Returns the new aggregator function.