How to concatenate streams in rxjs?

Question

I am trying to get data from the server using rxjs:

class HolidaysService {
    constructor() {
        this.restService = newRestService();
    }

    public getHolidaysByYear(year: number): Observable<Date[]> {
        const endpoint = `/get${year}`;
        return this.restService.GET(endpoint, { cache: true })
            .pipe(map(dates: any)=> dates.map((item: string) => new Date(item)))
    }

    public getHolidaysByYears(years: number[]): Observable<Date[]> {
        const requests = years.reduce((acc,v)=>acc.concat(this.getHolidaysByYear(v)),[])
        return forkJoin(...requests).pipe(scan((acc,v)=>acc.concat(v), []))
    }

}

// We call it:
const holidayService = new HolidaysService();
const substriction1 = holidayService.getHolidaysByYear(2020)
    .subscribe(
        res => console.log('res', res),
        err => console.warn(err)
    );
substriction1.unsunscribe();

The console displays res and the data itself. Now I want to get information for several years using rxjs-analogue of promise.all:

const substriction2 = holidayService.getHolidaysByYears([2019,2020])
    .subscribe(
        res => console.log('res', res),
        err => console.warn(err)
    );
substriction2.unsunscribe();

And nothing works. I do not see res in the console and data. What could be the mistake?

Answer 1

This is how you would write Promise.all analogue in the RxJS with forkJoin :

public getHolidaysByYears(years: number[]): Observable<Date[]> {
    const requests = years.map(year => this.getHolidaysByYear(year));
    return forkJoin(requests).pipe(concatAll());
}

Note that I am using concatAll operator, because forkJoin(requests) will give you Observable<Date[][]> , that is Array of holiday dates per year. Since your return type is Observable<Date[]> , that's why I am using concatAll operator to concatenate the resulting Arrays and get the Observable<Date[]> instead of Observable<Date[][]> .