core-js/modules/esnext.iterator.reduce.js

'use strict';
var $ = require('../internals/export');
var iterate = require('../internals/iterate');
var aCallable = require('../internals/a-callable');
var anObject = require('../internals/an-object');
var getIteratorDirect = require('../internals/get-iterator-direct');

var $TypeError = TypeError;

// `Iterator.prototype.reduce` method
// https://github.com/tc39/proposal-iterator-helpers
$({ target: 'Iterator', proto: true, real: true }, {
  reduce: function reduce(reducer /* , initialValue */) {
    anObject(this);
    aCallable(reducer);
    var record = getIteratorDirect(this);
    var noInitial = arguments.length < 2;
    var accumulator = noInitial ? undefined : arguments[1];
    var counter = 0;
    iterate(record, function (value) {
      if (noInitial) {
        noInitial = false;
        accumulator = value;
      } else {
        accumulator = reducer(accumulator, value, counter);
      }
      counter++;
    }, { IS_RECORD: true });
    if (noInitial) throw new $TypeError('Reduce of empty iterator with no initial value');
    return accumulator;
  }
});