import { ArrayRemoveOrUnionFunction } from '../types';
/**
 * Returns a special value that can be used with set(), create() or update()
 * that tells the server to union the given elements with any array value
 * that already exists on the server. Each specified element that doesn't
 * already exist in the array will be added to the end. If the field being
 * modified is not already an array it will be overwritten with an array
 * containing exactly the specified elements.
 *
 * @param elements The elements to union into the array.
 * @return The FieldValue sentinel for use in a call to set(), create() or
 * update().
 */
export declare const arrayUnion: ArrayRemoveOrUnionFunction;