TYPESCRIPT

angular rxjs mergemap

 this.http
      .get<any[]>('https://jsonplaceholder.typicode.com/posts')
      .pipe(
        mergeMap((val) => from(val))
      )
      .subscribe((data) => {
        console.log(data);
      });

rxjs mergemap foreach

public getCombinedData(): Observable<any> {
  return this.getMultipleRelationData().pipe(
      mergeMap((result: any) =>
          // `from` emits each contact separately
          from(result.contact).pipe(
              // load each contact
              mergeMap(
                  contact =>
                      this.getSignleData(contact._id).pipe(
                          map(detail => ({ ...contact, relationship: detail })),
                      ),
                  // collect all contacts into an array
                  toArray(),
                  // add the newly fetched data to original result
                  map(contact => ({ ...result, contact })),
              ),
          ),
      ),
  );
}