elegantly reduce objects (aka collator/collate)

reduce-aka-collate.ts

// Elegantly reduce array of objects into an object labeled by the value of a specific property
// Source: https://vmarchesin.medium.com/using-array-prototype-reduce-in-objects-using-javascript-dfcdae538fc8

interface Person {
  name: string;
  year: number;
  sign: "Aries" | "Taurus" | "Gemini" | "Cancer" | "Leo" | "Virgo" | "Libra" | "Scorpio" | "Sagittarius" | "Capricorn" | "Aquarius" | "Pisces";
}

const people: Person[] = [
  {name: "Gifford", year: 1988, sign: "Taurus"},
  {name: "Liz", year: 1994, sign: "Taurus"},
  {name: "Heather", year: 1987, sign: "Pisces"}
]

// Make an object whos keys are zodiac signs (e.g. "Taurus") and values are an array of objects matching that zodiac sign
const organized = people.reduce((map, person) => ({
  ...map,
  [person.sign]: [...map[person.sign] || [], person],
}), {} as Record<string, Person[]>)

console.log(organized)
// {
//   "Taurus": [
//     {
//       "name": "Gifford",
//       "year": 1988,
//       "sign": "Taurus"
//     },
//     {
//       "name": "Liz",
//       "year": 1994,
//       "sign": "Taurus"
//     }
//   ],
//   "Pisces": [
//     {
//       "name": "Heather",
//       "year": 1987,
//       "sign": "Pisces"
//     }
//   ]
// }