interface MergeOptions<T, U> {
    by?: (first: T, second: U, key: PropertyKey) => boolean;
}
/**
 * Simple recursive merging technique for two objects,
 * including explicitely undefined values.
 */
declare function merge<T extends object, U extends object>(first: T, second: U, { by, }?: MergeOptions<T, U>): T & U;
export default merge;