/**
 * A function to group array rows by property value (or result of a function)
 *
 * Returns a `Map` whose keys are the property value (or the return from the
 * extractor function) and values are each an array of entries that produced
 * that value. For example:
 *
 * Example:
 * const books = [{ ... details ..., genre: 'mystery' }, { ... more books ... }]
 *
 * // extract the genre by providing a function that returns the genre
 * const booksByGenre = mapgroupby(books, book => book.genre)
 *
 * // extract the genre by providing the property name as a string
 * // this can also be a dot-separated path to a nested property
 * const alsoBooksByGenre = mapgroupby(books, 'genre')
 *
 * console.log(booksByGenre)
 * // Map(2) { mystery => [ ... mystery books ... ], adventure => [ ... adventure books ... ] }
 */
export declare function mapgroupby<ObjectType extends object, Key extends keyof ObjectType>(objArray: ObjectType[] | undefined, key: Key): Map<NonNullable<ObjectType[Key]>, ObjectType[]>;
export declare function mapgroupby<ObjectType, KeyType = string | number | symbol | undefined>(objArray: ObjectType[] | undefined, key: string): Map<NonNullable<KeyType>, ObjectType[]>;
export declare function mapgroupby<KeyType, ObjectType>(objArray: ObjectType[] | undefined, extractor: (obj: ObjectType) => KeyType): Map<NonNullable<KeyType>, ObjectType[]>;