implementation.js

function groupBy(key) {
  return function group(array) {
    return array.reduce((acc, obj) => {
      const property = obj[key];
      acc[property] = acc[property] || [];
      acc[property].push(obj);
      return acc;
    }, {});
  };
}

const groupByYear = groupBy("year");
groupByyear(albums);

const groupByArtist = groupBy("artist");
groupByArtist(albums);