1  /**
 2   * https://tc39.es/ecma262/multipage/fundamental-objects.html#sec-object.groupby
 3   */
 4  export function objectGroupBy<T, K extends PropertyKey>(
 5    items: Iterable<T>,
 6    keySelector: (item: T, index: number) => K,
 7  ): Partial<Record<K, T[]>> {
 8    const result = Object.create(null) as Partial<Record<K, T[]>>
 9    let index = 0
10    for (const item of items) {
11      const key = keySelector(item, index++)
12      if (result[key] === undefined) {
13        result[key] = []
14      }
15      result[key].push(item)
16    }
17    return result
18  }