groupBy.ts

export function groupBy<T, K>(array: T[], getKey: (value: T) => K): Map<K, T[]> {
  return array.reduce(
    (groups, item) => {
      const key = getKey(item);
      const innerArray = groups.get(key);
      if (innerArray === undefined) {
        groups.set(key, [item])
      } else {
        innerArray.push(item)
      }
      return groups
    },
    new Map<K, T[]>()
  )
}