Implement groupBy(arr, keyFn)
Write groupBy(arr, keyFn) that buckets an array into an object, keyed by keyFn(item), where each bucket is an array of the matching items in their original order.
groupBy(
[{ role: "eng" }, { role: "sales" }, { role: "eng" }],
(p) => p.role
);
// { eng: [{role:"eng"}, {role:"eng"}], sales: [{role:"sales"}] }