reduce_array-to-object-map.js

const counties = [
  { name: 'Clackamas', state: 'Oregon', population: 380000 },
  { name: 'King', state: 'Washington', population: 2000000 },
  { name: 'Kitsap', state: 'Washington', population: 250000 },
  { name: 'Multinomah', state: 'Oregon', population: 750000 },
  { name: 'Snohomish', state: 'Washington', population: 720000 },
  { name: 'Wasco', state: 'Oregon', population: 25000 },
];

const oldStateCountyMap = counties.reduce((acc, county) => {
  const currentCounty = { name: county.name, population: county.population };

  if (acc[county.state]) {
    acc[county.state] = acc[county.state].concat(currentCounty)
  } else {
    acc[county.state] = [currentCounty]
  }
  return acc;
}, {}); // Setting our initialValue as an object to map against

const newStateCountyMap = counties.reduce((acc, { name, state, population }) => {
  const currentCounty = { name, population };

  acc[state] = acc[state] ? [...acc[state], currentCounty] : [currentCounty];

  return acc;
}, {});

// Both return:
// {
//   'Oregon': [
//     { name: 'Multinomah', population: 750000 },
//     { name: 'Clackamas', population: 380000 },
//     { name: 'Wasco', population: 25000 },
//   ],
//   'Washington': [
//     { name: 'King', population: 2000000 },
//     { name: 'Snohomish', population: 720000 },
//     { name: 'Kitsap', population: 250000 },
//   ]
// }