Group an array by object property

groupArray.js

const groupBy = (arr, groupFn) => {
  const grouped = {};
  for (const obj of arr) {
    const groupName = groupFn(obj);
    if (!grouped[groupName]) {
      grouped[groupName] = [];
    }
    grouped[groupName].push(obj);
  }
  return grouped;
};
const people = [
  { name: 'Matt' },
  { name: 'Sam' },
  { name: 'Jeff' },
  { name: 'Mac' },
];
const groupedByNameLength = groupBy(people, (person) => person.name.length);
/**
{
  '3': [ { name: 'Sam' }, { name: 'Mac' } ],
  '4': [ { name: 'Matt' }, { name: 'Jeff' } ]
}
 */
console.log(groupedByNameLength);

// Better version:
const groupByBest = (arr, groupFn) =>
  arr.reduce(
    (grouped, obj) => ({
      ...grouped,
      [groupFn(obj)]: [...(grouped[groupFn(obj)] || []), obj],
    }),
    {}
  );
const people = [
  { name: 'Matt' },
  { name: 'Sam' },
  { name: 'Jeff' },
  { name: 'Mac' },
];
const groupedByNameLength = groupByBest(people, (person) => person.name.length);
/**
{
  '3': [ { name: 'Sam' }, { name: 'Mac' } ],
  '4': [ { name: 'Matt' }, { name: 'Jeff' } ]
}
 */
console.log(groupedByNameLength);