import type { Arr } from '@toolbox-ts/types/defs/array';
/**
 * Applies a transformation function to each element of an array,
 * filtering out any `undefined` results.
 *
 * Similar to [].map().filter() but only iterates once and expects undefined
 * to be used for filtering.
 *
 * @important Null values will not be filtered out. If you want to filter out nulls, use a transformation function that returns `undefined` for null inputs.
 *
 * @pure
 *
 * @example
 * ```ts
 * transform([1, 2, 3, 4], (n) => (n % 2 === 0 ? n * 2 : undefined))
 * // [4, 8]
 *
 * transform(['a', 'b', 'c'], (s) => s === 'b' ? undefined : s.toUpperCase())
 * // ['A', 'C']
 * ```
 */
export declare function transform<T extends Arr = Arr, U extends Arr = Arr>(arr: T, fn: (item: T[number]) => U[number] | undefined): U;
export declare function transformInPlace<T extends unknown[] = unknown[]>(arr: T, fn: (item: T[number]) => T[number] | undefined): void;