var groupBy = (parties, property) => {
  return parties.reduce((grouped, party) => {
    (grouped[party[property]] = grouped[party[property]] || []).push(party);
    return grouped;
  }, {});
};

groupBy([{ name: 2, group: 'a' }, { name: 1, group: 'a' }, { name: 1, group: 'b' }], 'group')