//#region src/array/array-union.d.ts
/**
 * Returns the union of two arrays — all unique elements from both `firstArray`
 * and `secondArray`, preserving order of first appearance. The strategy used
 * for the equality check depends on the optional third argument:
 *
 * - When omitted, items are compared using `SameValueZero` (the same semantics
 *   as `Set.prototype.has`).
 * - When a binary function `(a, b) => boolean` is passed, it is used as a
 *   custom comparator (delegates to `unionWith`).
 * - When a unary function `(value) => key` is passed, both arrays are mapped
 *   through it and the resulting keys are compared (delegates to `unionBy`).
 * @param firstArray - The first array. Not mutated.
 * @param secondArray - The second array. Not mutated.
 * @returns A new array containing all unique elements from both arrays.
 * @example
 * // Basic equality (SameValueZero)
 * arrayUnion([1, 2, 3], [3, 4, 5]);
 * // [1, 2, 3, 4, 5]
 * @example
 * // With a mapper — compare by a derived key
 * arrayUnion(
 *   [{id: 1}, {id: 2}],
 *   [{id: 2}, {id: 3}],
 *   (item) => item.id,
 * );
 * // [{id: 1}, {id: 2}, {id: 3}]
 * @example
 * // With a custom comparator
 * arrayUnion(
 *   [{id: 1}, {id: 2}],
 *   [{id: 2}, {id: 3}],
 *   (a, b) => a.id === b.id,
 * );
 * // [{id: 1}, {id: 2}, {id: 3}]
 */
declare function arrayUnion<T>(firstArray: readonly T[], secondArray: readonly T[]): T[];
declare function arrayUnion<T>(firstArray: readonly T[], secondArray: readonly T[], areItemsEqual: (x: T, y: T) => boolean): T[];
declare function arrayUnion<T>(firstArray: readonly T[], secondArray: readonly T[], mapper: (value: T) => unknown): T[];
//#endregion
export { arrayUnion };