/**
 * Calls a defined callback function on each element of an array, and
 * returns a new array of groups keyed by the return value of the callback
 * function, associated with an array of elements that have the same key.
 *
 * @param keyExtractFn - A function that accepts up to three arguments. The
 *  groupBy method calls the keyFn function one time for each
 *  element in the array.
 * @param keyComparerFn - An optional function that accepts two arguments.
 *  The groupBy method calls the keyComparerFn function to determine the
 *  equality of keys. Triple equal `===` comparison is used by default.
 *
 * @returns An array of group elements, each with an array of elements
 *  matching each key.
 */
export declare const groupBy: <T, K>(keyExtractFn: import("../../types/Callback.js").Callback<T, K>, arg2?: import("../../types/Comparer.js").Comparer<K> | undefined) => (arr: readonly T[]) => import("../../types/Group.js").Group<K, T>[];