groupBy.ts

interface Group<TKey, TValue> {
  key: TKey;
  values: TValue[];
}

function groupBy<TKey, TValue>(array: TValue[], keyFn: (t: TValue) => TKey): Group<TKey,TValue>[] {
  const result: Group<TKey, TValue>[] = [];
  const map = new Map<TKey, Group<TKey, TValue>>();
  array.forEach(t => {
    const k = keyFn(t);
    if (map.has(k)) {
      map.get(k).values.push(t);
    } else {
      const g = {
        key: k,
        values: [t],
      };
      map.set(k, g);
      result.push(g);
    }
  });
  return result;
}