code: function (callback) {
                  var REDUCE_INITIAL_VALUE_INDEX = 1, initialValue = arguments[REDUCE_INITIAL_VALUE_INDEX], thisLength = this.length, index = 0, value;
                  if (undefined === initialValue) {
                      value = this[index++];
                  } else {
                      value = initialValue;
                  }
                  for (; index < thisLength; ++index) {
                      value = callback(value, this[index], index, this);
                  }
                  return value;
              }
docs: Introduced with JavaScript 1.8