export declare function groupBy<K, V>(array: V[], groupFn: (V: any) => K): Map<K, V[]>;
export declare function groupByWithCreator<K, V, C>(array: V[], groupFn: (V: any) => K, createCollectionFn: () => C, addCollectionFn: (C: any, V: any) => void): Map<K, C>;
export declare function countBy<V>(events: V[], mapper: (V: any) => string | number): {
    [key: string]: number;
};
export declare function tests(): {
    countBy: (assert: any) => void;
};