/**
 * Flattens an array up to the specified depth.
 *
 * @template T - The type of elements within the array.
 * @template D - The depth to which the array should be flattened.
 * @param {T[]} arr - The array to flatten.
 * @param {D} depth - The depth level specifying how deep a nested array structure should be flattened. Defaults to 1.
 * @returns {Array<FlatArray<T[], D>>} A new array that has been flattened.
 *
 * @example
 * const arr = flatten([1, [2, 3], [4, [5, 6]]], 1);
 * // Returns: [1, 2, 3, 4, [5, 6]]
 *
 * const arr = flatten([1, [2, 3], [4, [5, 6]]], 2);
 * // Returns: [1, 2, 3, 4, 5, 6]
 */
declare function flatten<T, D extends number = 1>(arr: readonly T[], depth?: D): Array<FlatArray<T[], D>>;

export { flatten };