Simple implementation of the Hare-Niemeyer (or Hamilton or largest remainder) method for allocating parliament seats proportionally

hareNiemeyer.js

const results = [
  { party: 'socialists', votes: 130755 },
  { party: 'conservatives', votes: 102068 },
  { party: 'liberals', votes: 34012 },
  { party: 'greens', votes: 31090 },
  { party: 'crazypeople', votes: 11111 }
];

const hareNiemeyer = (results, seats) => {

  const totalVotes = results.reduce((total, party) => {
    return total + party.votes;
  }, 0);

  const _results = results.map(party => {
    return Object.assign({}, party, {
      expectedSeats: seats * party.votes / totalVotes,
      roundedSeats: Math.floor(seats * party.votes / totalVotes),
      finalSeats: Math.floor(seats * party.votes / totalVotes)
    });
  }).sort((a, b) => b.expectedSeats % 1 - a.expectedSeats % 1);

  const totalRoundedSeats = _results.reduce((total, party) => {
    return total + party.roundedSeats;
  }, 0);

  const remainingSeats = seats - totalRoundedSeats;

  for (let i = 0; i < remainingSeats; i++) {
    _results[i].finalSeats += 1;
  }

  return _results;
};

console.log(hareNiemeyer(results, 420));

// [{
//   party: 'conservatives',
//   votes: 102068,
//   expectedSeats: 138.71704267463986,
//   roundedSeats: 138,
//   finalSeats: 139
// },
// {
//   party: 'socialists',
//   votes: 130755,
//   expectedSeats: 177.70453927697744,
//   roundedSeats: 177,
//   finalSeats: 178
// },
// {
//   party: 'greens',
//   votes: 31090,
//   expectedSeats: 42.2533297091601,
//   roundedSeats: 42,
//   finalSeats: 42
// },
// {
//   party: 'liberals',
//   votes: 34012,
//   expectedSeats: 46.22451753193803,
//   roundedSeats: 46,
//   finalSeats: 46
// },
// {
//   party: 'crazypeople',
//   votes: 11111,
//   expectedSeats: 15.100570807284589,
//   roundedSeats: 15,
//   finalSeats: 15
// }]