Map Reduce in JavaScript

mapReduce.js

Array.prototype.mapReduce = function(keyGrouper, reducer) {
  const emit = (key, value) => ({key, value});
  const obj = this
    .map(value => keyGrouper(emit, value))
    .reduce((prev, {key, value}) => {
      prev[key] = Array.isArray(prev[key]) ? prev[key] : [];
      return prev[key].push(value) && prev;
    }, {});
  return Object.keys(obj).map(key => reducer(key, obj[key]));
};

const arr = [
  { gender: 'male',   country: 'italy',   balance: 120},
  { gender: 'male',   country: 'usa',     balance: 320},
  { gender: 'female', country: 'brazil',  balance: 100},
  { gender: 'male',   country: 'usa',     balance: 80},
  { gender: 'female', country: 'canada',  balance: 130},
];
const groupByGenderAndCountry = input => input.mapReduce(
  (emit, item) => emit(`${item.gender}-${item.country}`, item.balance),
  (key, values) => ({
    key,
    value: values.reduce((sum, cur) => sum + cur, 0)
  })
);
const countByGender = input => input.mapReduce(
  (emit, item) => emit(`${item.gender}`, 1),
  (key, values) => ({
    key,
    value: values.reduce((sum, cur) => sum + cur, 0)
  })
);

console.log(groupByGenderAndCountry(arr));
// [ 
//   { key: 'male-italy', value: 120 },
//   { key: 'male-usa', value: 400 },
//   { key: 'female-brazil', value: 100 },
//   { key: 'female-canada', value: 130 } 
// ]

console.log(countByGender(arr));
// [
//  { key: 'male', value: 3 }, 
//  { key: 'female', value: 2 }
// ]