code: function reduce(collection, callback, accumulator, thisArg) {
        var noaccum = arguments.length < 3;
  
        callback = lodash.createCallback(callback, thisArg, 4);
        if (isArray(collection)) {
          var index = -1,
              length = collection.length;
  
          if (noaccum && length) {
            accumulator = collection[++index];
          }
          while (++index < length) {
            accumulator = callback(accumulator, collection[index], index, collection);
          }
        } else {
          baseEach(collection, function(value, index, collection) {
            accumulator = noaccum
              ? (noaccum = false, value)
              : callback(accumulator, value, index, collection)
          });
        }
        return accumulator;
      }
docs: Reduces a collection to a value which is the accumulated result of running
  each element in the collection through the callback, where each successive
  callback execution consumes the return value of the previous execution. If
  `accumulator` is not provided the first element of the collection will be
  used as the initial `accumulator` value. The callback is bound to `thisArg`
  and invoked with four arguments; (accumulator, value, index|key, collection).
  
  @static
  @memberOf _
  @alias foldl, inject
  @category Collections
  @param {Array|Object|string} collection The collection to iterate over.
  @param {Function} [callback=identity] The function called per iteration.
  @param {*} [accumulator] Initial value of the accumulator.
  @param {*} [thisArg] The `this` binding of `callback`.
  @returns {*} Returns the accumulated value.
  @example
  
  var sum = _.reduce([1, 2, 3], function(sum, num) {
  return sum + num;
  });
  // => 6
  
  var mapped = _.reduce({ 'a': 1, 'b': 2, 'c': 3 }, function(result, num, key) {
  result[key] = num * 3;
  return result;
  }, {});
  // => { 'a': 3, 'b': 6, 'c': 9 }