import { Operator } from '../Seq';
/**
 * Returns a sequence grouped by a specified key.
 *
 * ```typescript
 * const source1 = [
 *   {groupKey: 1, value: "test1"},
 *   {groupKey: 3, value: "test2"},
 *   {groupKey: 1, value: "test3"},
 *   {groupKey: 1, value: "test4"},
 *   {groupKey: 3, value: "test5"},
 *   {groupKey: 2, value: "test6"}
 * ]
 *
 * const result1 = from(source1).pipe(groupBy(one => one.groupKey),).toArray();
 *
 *
 * // result1: [
 * //   {key: 1, values: [
 * //     {groupKey: 1, value: "test1"},
 * //     {groupKey: 1, value: "test3"},
 * //     {groupKey: 1, value: "test4"}
 * //   ]},
 * //   {key: 3, values: [
 * //     {groupKey: 3, value: "test2"},
 * //     {groupKey: 3, value: "test5"}
 * //   ]},
 * //   {key: 2, values: [
 * //     {groupKey: 2, value: "test6"}
 * //   ]}
 * // ]
 *
 * const source2 = [
 *   {groupKey: {key: 1}, value: "test1"},
 *   {groupKey: {key: 2}, value: "test2"},
 *   {groupKey: {key: 1}, value: "test3"},
 *   {groupKey: {key: 1}, value: "test4"},
 * ]
 *
 * const result2 = from(source2).pipe(groupBy(one => one.groupKey,one => one.value,k => k.key),).toArray();
 *
 * // result2: [
 * //   {key: {key: 1}, values: ["test1","test3","test4"]},
 * //   {key: {key: 2}, values: ["test2"]}
 * // ];
 * ```
 *
 * For more information on *keySelector* and *comparableValueForKey*, please refer to [Equality Strategy](/#equality-strategy).
 *
 * The implementation of *defaultSelector* is as follows.
 * ```typescript
 * export const defaultSelector = (target: any): any => target;
 * ```
 *
 * @param target Sequence to be removed.
 * @param keySelector Function to return the object used to check Equality..
 * @param elementSelector Function to return the object to be enumerated in the Value property of the grouped result.
 * @param comparableValueForKey This function returns an object that is unique to the key selected by *keySelector*.
 * It is recommended to return a string or number.
 * @returns Operator function.
 * @typeParam T Source element type.
 * @typeParam TValue The type that will be enumerated in the Value property of the grouped result.
 * @typeParam TKey key type.
 * @typeParam TComparableValue The type of the return value returned by *comparableValueForKey*.
 * @category Operators
 */
export declare const groupBy: <T, TComparableValue, TKey = T, TValue = T>(keySelector: (target: T) => TKey, elementSelector?: (target: T) => TValue, comparableValueForKey?: ((key: TKey) => TComparableValue) | undefined) => Operator<T, {
    key: TKey;
    values: readonly TValue[];
}>;