lodash/internal/createAggregator.js

define(['./baseCallback', './baseEach', '../lang/isArray'], function(baseCallback, baseEach, isArray) {

  /**
   * Creates a `_.countBy`, `_.groupBy`, `_.indexBy`, or `_.partition` function.
   *
   * @private
   * @param {Function} setter The function to set keys and values of the accumulator object.
   * @param {Function} [initializer] The function to initialize the accumulator object.
   * @returns {Function} Returns the new aggregator function.
   */
  function createAggregator(setter, initializer) {
    return function(collection, iteratee, thisArg) {
      var result = initializer ? initializer() : {};
      iteratee = baseCallback(iteratee, thisArg, 3);

      if (isArray(collection)) {
        var index = -1,
            length = collection.length;

        while (++index < length) {
          var value = collection[index];
          setter(result, value, iteratee(value, index, collection), collection);
        }
      } else {
        baseEach(collection, function(value, key, collection) {
          setter(result, value, iteratee(value, key, collection), collection);
        });
      }
      return result;
    };
  }

  return createAggregator;
});