.js

/*
_.reduce = function(collection, iterator, accumulator) {
  for (var i = 0; i < collection.length; i++) {
    accumulator = iterator(accumulator, collection[i]);
  }
  if (accumulator === undefined) {
    return collection[0];
  }
  return accumulator;
};
*/

_.reduce = function(collection, iterator, accumulator) {
  if (accumulator === undefined) {
    accumulator === collection[0];
  } else {
    for (var i = 0; i < collection.length; i++) {
      accumulator = iterator(accumulator, collection[i]);
    }
  }
  return accumulator;
}