// Created on savesnippets.com · https://savesnippets.com/5LKzl5aWsMKMTg
function groupBy(array, keyFn) {
  if (typeof Object.groupBy === 'function') {
    return Object.groupBy(array, keyFn);
  }
  return array.reduce((acc, item) => {
    const key = typeof keyFn === 'function' ? keyFn(item) : item[keyFn];
    (acc[key] ??= []).push(item);
    return acc;
  }, {});
}

// Usage
const people = [
  { name: 'Alice', dept: 'eng' },
  { name: 'Bob',   dept: 'design' },
  { name: 'Carol', dept: 'eng' },
];
console.log(groupBy(people, 'dept'));
// { eng: [Alice, Carol], design: [Bob] }