import { ObjectMap } from '../object-map';
import { Mapper } from '../operators';
/**
 * Groups an `array` by the key extracted from each item by a `keyFn`.
 * @param array Array to group by key.
 * @param keyFn Function that extracts a key from each item.
 */
export declare function groupBy<T>(array: T[], keyFn: KeyFn<T>): ObjectMap<T[]>;
/**
 * Returns a function that
 * groups an `array` by the key extracted from each item by a `keyFn`.
 * @param keyFn Function that extracts a key from each item.
 */
export declare function groupBy<T>(keyFn: KeyFn<T>): typeof deferred;
/**
 * Groups an `array` by the key extracted from each item by a previously specified `keyFn`.
 * @param array Array to group by key.
 */
declare function deferred<T>(array: T[]): ObjectMap<T[]>;
declare type KeyFn<T> = Mapper<T, number | string>;
export {};