rxjs/src/internal/operators/concatMap.ts

Reactive Extensions for modern JavaScript

import { mergeMap } from './mergeMap';
import { ObservableInput, OperatorFunction, ObservedValueOf } from '../types';
import { isFunction } from '../util/isFunction';

/* tslint:disable:max-line-length */
export function concatMap<T, O extends ObservableInput<any>>(
  project: (value: T, index: number) => O
): OperatorFunction<T, ObservedValueOf<O>>;
/** @deprecated The `resultSelector` parameter will be removed in v8. Use an inner `map` instead. Details: https://rxjs.dev/deprecations/resultSelector */
export function concatMap<T, O extends ObservableInput<any>>(
  project: (value: T, index: number) => O,
  resultSelector: undefined
): OperatorFunction<T, ObservedValueOf<O>>;
/** @deprecated The `resultSelector` parameter will be removed in v8. Use an inner `map` instead. Details: https://rxjs.dev/deprecations/resultSelector */
export function concatMap<T, R, O extends ObservableInput<any>>(
  project: (value: T, index: number) => O,
  resultSelector: (outerValue: T, innerValue: ObservedValueOf<O>, outerIndex: number, innerIndex: number) => R
): OperatorFunction<T, R>;
/* tslint:enable:max-line-length */

/**
 * Projects each source value to an Observable which is merged in the output
 * Observable, in a serialized fashion waiting for each one to complete before
 * merging the next.
 *
 * <span class="informal">Maps each value to an Observable, then flattens all of
 * these inner Observables using {@link concatAll}.</span>
 *
 * ![](concatMap.png)
 *
 * Returns an Observable that emits items based on applying a function that you
 * supply to each item emitted by the source Observable, where that function
 * returns an (so-called "inner") Observable. Each new inner Observable is
 * concatenated with the previous inner Observable.
 *
 * __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: `concatMap` is equivalent to `mergeMap` with concurrency parameter set
 * to `1`.
 *
 * ## Example
 *
 * For each click event, tick every second from 0 to 3, with no concurrency
 *
 * ```ts
 * import { fromEvent, concatMap, interval, take } from 'rxjs';
 *
 * const clicks = fromEvent(document, 'click');
 * const result = clicks.pipe(
 *   concatMap(ev => 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 concatMapTo}
 * @see {@link exhaustMap}
 * @see {@link mergeMap}
 * @see {@link switchMap}
 *
 * @param project A function that, when applied to an item emitted by the source
 * Observable, returns an Observable.
 * @return A function that returns an Observable that emits the result of
 * applying the projection function (and the optional deprecated
 * `resultSelector`) to each item emitted by the source Observable and taking
 * values from each projected inner Observable sequentially.
 */
export function concatMap<T, R, O extends ObservableInput<any>>(
  project: (value: T, index: number) => O,
  resultSelector?: (outerValue: T, innerValue: ObservedValueOf<O>, outerIndex: number, innerIndex: number) => R
): OperatorFunction<T, ObservedValueOf<O> | R> {
  return isFunction(resultSelector) ? mergeMap(project, resultSelector, 1) : mergeMap(project, 1);
}