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name : mergeMap.ts
 import { ObservableInput, OperatorFunction, ObservedValueOf } from '../types';
import { map } from './map';
import { innerFrom } from '../observable/innerFrom';
import { operate } from '../util/lift';
import { mergeInternals } from './mergeInternals';
import { isFunction } from '../util/isFunction';

/* tslint:disable:max-line-length */
export function mergeMap<T, O extends ObservableInput<any>>(
  project: (value: T, index: number) => O,
  concurrent?: number
): 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 mergeMap<T, O extends ObservableInput<any>>(
  project: (value: T, index: number) => O,
  resultSelector: undefined,
  concurrent?: number
): 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 mergeMap<T, R, O extends ObservableInput<any>>(
  project: (value: T, index: number) => O,
  resultSelector: (outerValue: T, innerValue: ObservedValueOf<O>, outerIndex: number, innerIndex: number) => R,
  concurrent?: number
): OperatorFunction<T, R>;
/* tslint:enable:max-line-length */

/**
 * Projects each source value to an Observable which is merged in the output
 * Observable.
 *
 * <span class="informal">Maps each value to an Observable, then flattens all of
 * these inner Observables using {@link mergeAll}.</span>
 *
 * ![](mergeMap.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 Observable, and then merging those resulting Observables and
 * emitting the results of this merger.
 *
 * ## Example
 *
 * Map and flatten each letter to an Observable ticking every 1 second
 *
 * ```ts
 * import { of, mergeMap, interval, map } from 'rxjs';
 *
 * const letters = of('a', 'b', 'c');
 * const result = letters.pipe(
 *   mergeMap(x => interval(1000).pipe(map(i => x + i)))
 * );
 *
 * result.subscribe(x => console.log(x));
 *
 * // Results in the following:
 * // a0
 * // b0
 * // c0
 * // a1
 * // b1
 * // c1
 * // continues to list a, b, c every second with respective ascending integers
 * ```
 *
 * @see {@link concatMap}
 * @see {@link exhaustMap}
 * @see {@link merge}
 * @see {@link mergeAll}
 * @see {@link mergeMapTo}
 * @see {@link mergeScan}
 * @see {@link switchMap}
 *
 * @param project A function that, when applied to an item emitted by the source
 * Observable, returns an Observable.
 * @param concurrent Maximum number of `ObservableInput`s being subscribed to concurrently.
 * @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 merging
 * the results of the Observables obtained from this transformation.
 */
export function mergeMap<T, R, O extends ObservableInput<any>>(
  project: (value: T, index: number) => O,
  resultSelector?: ((outerValue: T, innerValue: ObservedValueOf<O>, outerIndex: number, innerIndex: number) => R) | number,
  concurrent: number = Infinity
): OperatorFunction<T, ObservedValueOf<O> | R> {
  if (isFunction(resultSelector)) {
    // DEPRECATED PATH
    return mergeMap((a, i) => map((b: any, ii: number) => resultSelector(a, b, i, ii))(innerFrom(project(a, i))), concurrent);
  } else if (typeof resultSelector === 'number') {
    concurrent = resultSelector;
  }

  return operate((source, subscriber) => mergeInternals(source, subscriber, project, concurrent));
}
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