import { type Iteratee } from '../internal/iteratee.js';
/**
 * Create a union of two arrays using an iteratee to determine uniqueness.
 *
 * @param arr1 - First array
 * @param arr2 - Second array
 * @param iteratee - Function or property path to derive comparison key
 * @returns New array of unique values
 *
 * @example
 * unionByFast([{ id: 1 }, { id: 2 }], [{ id: 2 }, { id: 3 }], 'id')
 * // [{ id: 1 }, { id: 2 }, { id: 3 }]
 */
export declare function unionByFast<T>(arr1: readonly T[], arr2: readonly T[], iteratee: Iteratee<T, unknown>): T[];
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