[js: group-by] #js

group-by.ts

/**
 * @link https://qiita.com/nagtkk/items/e1cc3f929b61b1882bd1
 * @example
 *   const result = groupBy(users, user => user.role)
 *     .filter(([role, _users]) => role === 'admin')
 */
export const groupBy = <K, V>(
  array: readonly V[],
  getKey: (cur: V, idx: number, src: readonly V[]) => K
): [K, V[]][] =>
    Array.from(
      array.reduce((map, cur, idx, src) => {
        const key = getKey(cur, idx, src)
        const list = map.get(key)

        if (list) {
          list.push(cur)
        } else {
          map.set(key, [cur])
        }

        return map
      }, new Map<K, V[]>())
    )