/**
 * Creates an array of unique values, in order, from all given arrays using a provided mapping function to determine equality.
 *
 * @template T - The type of elements in the array.
 * @template U - The type of mapped elements.
 * @param {T[]} arr1 - The first array.
 * @param {T[]} arr2 - The second array.
 * @param {(item: T) => U} mapper - The function to map array elements to comparison values.
 * @returns {T[]} A new array containing the union of unique elements from `arr1` and `arr2`, based on the values returned by the mapping function.
 *
 * @example
 * // Custom mapping function for numbers (modulo comparison)
 * const moduloMapper = (x) => x % 3;
 * unionBy([1, 2, 3], [4, 5, 6], moduloMapper);
 * // Returns [1, 2, 3]
 *
 * @example
 * // Custom mapping function for objects with an 'id' property
 * const idMapper = (obj) => obj.id;
 * unionBy([{ id: 1 }, { id: 2 }], [{ id: 2 }, { id: 3 }], idMapper);
 * // Returns [{ id: 1 }, { id: 2 }, { id: 3 }]
 */
declare function unionBy<T, U>(arr1: readonly T[], arr2: readonly T[], mapper: (item: T) => U): T[];

export { unionBy };