core-js/internals/array-reduce.js

'use strict';
var aCallable = require('../internals/a-callable');
var toObject = require('../internals/to-object');
var IndexedObject = require('../internals/indexed-object');
var lengthOfArrayLike = require('../internals/length-of-array-like');

var $TypeError = TypeError;

var REDUCE_EMPTY = 'Reduce of empty array with no initial value';

// `Array.prototype.{ reduce, reduceRight }` methods implementation
var createMethod = function (IS_RIGHT) {
  return function (that, callbackfn, argumentsLength, memo) {
    var O = toObject(that);
    var self = IndexedObject(O);
    var length = lengthOfArrayLike(O);
    aCallable(callbackfn);
    if (length === 0 && argumentsLength < 2) throw new $TypeError(REDUCE_EMPTY);
    var index = IS_RIGHT ? length - 1 : 0;
    var i = IS_RIGHT ? -1 : 1;
    if (argumentsLength < 2) while (true) {
      if (index in self) {
        memo = self[index];
        index += i;
        break;
      }
      index += i;
      if (IS_RIGHT ? index < 0 : length <= index) {
        throw new $TypeError(REDUCE_EMPTY);
      }
    }
    for (;IS_RIGHT ? index >= 0 : length > index; index += i) if (index in self) {
      memo = callbackfn(memo, self[index], index, O);
    }
    return memo;
  };
};

module.exports = {
  // `Array.prototype.reduce` method
  // https://tc39.es/ecma262/#sec-array.prototype.reduce
  left: createMethod(false),
  // `Array.prototype.reduceRight` method
  // https://tc39.es/ecma262/#sec-array.prototype.reduceright
  right: createMethod(true)
};