import { Observable } from '../Observable';
/**
 * Emits a given value if the source Observable completes without emitting any
 * `next` value, otherwise mirrors the source Observable.
 *
 * <span class="informal">If the source Observable turns out to be empty, then
 * this operator will emit a default value.</span>
 *
 * <img src="./img/defaultIfEmpty.png" width="100%">
 *
 * `defaultIfEmpty` emits the values emitted by the source Observable or a
 * specified default value if the source Observable is empty (completes without
 * having emitted any `next` value).
 *
 * @example <caption>If no clicks happen in 5 seconds, then emit "no clicks"</caption>
 * var clicks = Rx.Observable.fromEvent(document, 'click');
 * var clicksBeforeFive = clicks.takeUntil(Rx.Observable.interval(5000));
 * var result = clicksBeforeFive.defaultIfEmpty('no clicks');
 * result.subscribe(x => console.log(x));
 *
 * @see {@link empty}
 * @see {@link last}
 *
 * @param {any} [defaultValue=null] The default value used if the source
 * Observable is empty.
 * @return {Observable} An Observable that emits either the specified
 * `defaultValue` if the source Observable emits no items, or the values emitted
 * by the source Observable.
 * @method defaultIfEmpty
 * @owner Observable
 */
export declare function defaultIfEmpty<T, R>(defaultValue?: R): Observable<T | R>;
export interface DefaultIfEmptySignature<T> {
    (defaultValue?: T): Observable<T>;
    <R>(defaultValue?: R): Observable<T | R>;
}
