rxjs/src/internal/operators/concatMapTo.ts

Reactive Extensions for modern JavaScript

import { concatMap } from './concatMap';
import { ObservableInput, OperatorFunction } from '../types';

/* tslint:disable:max-line-length */
export function concatMapTo<T>(observable: ObservableInput<T>): OperatorFunction<any, T>;
/** @deprecated */
export function concatMapTo<T>(observable: ObservableInput<T>, resultSelector: undefined): OperatorFunction<any, T>;
/** @deprecated */
export function concatMapTo<T, I, R>(observable: ObservableInput<I>, resultSelector: (outerValue: T, innerValue: I, outerIndex: number, innerIndex: number) => R): OperatorFunction<T, R>;
/* tslint:enable:max-line-length */

/**
 * Projects each source value to the same Observable which is merged multiple
 * times in a serialized fashion on the output Observable.
 *
 * <span class="informal">It's like {@link concatMap}, but maps each value
 * always to the same inner Observable.</span>
 *
 * ![](concatMapTo.png)
 *
 * Maps each source value to the given Observable `innerObservable` regardless
 * of the source value, and then flattens those resulting Observables into one
 * single Observable, which is the output Observable. Each new `innerObservable`
 * instance emitted on the output Observable is concatenated with the previous
 * `innerObservable` instance.
 *
 * __Warning:__ if source values arrive endlessly and faster than their
 * corresponding inner Observables can complete, it will result in memory issues
 * as inner Observables amass in an unbounded buffer waiting for their turn to
 * be subscribed to.
 *
 * Note: `concatMapTo` is equivalent to `mergeMapTo` with concurrency parameter
 * set to `1`.
 *
 * ## Example
 * For each click event, tick every second from 0 to 3, with no concurrency
 * ```javascript
 * const clicks = fromEvent(document, 'click');
 * const result = clicks.pipe(
 *   concatMapTo(interval(1000).pipe(take(4))),
 * );
 * result.subscribe(x => console.log(x));
 *
 * // Results in the following:
 * // (results are not concurrent)
 * // For every click on the "document" it will emit values 0 to 3 spaced
 * // on a 1000ms interval
 * // one click = 1000ms-> 0 -1000ms-> 1 -1000ms-> 2 -1000ms-> 3
 * ```
 *
 * @see {@link concat}
 * @see {@link concatAll}
 * @see {@link concatMap}
 * @see {@link mergeMapTo}
 * @see {@link switchMapTo}
 *
 * @param {ObservableInput} innerObservable An Observable to replace each value from
 * the source Observable.
 * @return {Observable} An observable of values merged together by joining the
 * passed observable with itself, one after the other, for each value emitted
 * from the source.
 * @method concatMapTo
 * @owner Observable
 */
export function concatMapTo<T, I, R>(
  innerObservable: ObservableInput<I>,
  resultSelector?: (outerValue: T, innerValue: I, outerIndex: number, innerIndex: number) => R
): OperatorFunction<T, R> {
  return concatMap(() => innerObservable, resultSelector);
}