flatMap
Projects each source value to an Observable which is merged in the output Observable.
flatMap<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 = Number.POSITIVE_INFINITY): OperatorFunction<T, ObservedValueOf<O> | R>Parameters
Returns
OperatorFunction<T, ObservedValueOf<O> | R>: 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.
Description
Maps each value to an Observable, then flattens all of
these inner Observables using mergeAll.
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
import { of, interval } from 'rxjs';
import { mergeMap, map } from 'rxjs/operators';
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 with respective ascending integers