如何基于rxjs中已有的observable创建新的observable订阅？

import { Observable } from 'rxjs';
import { map } from 'rxjs/operators';

const source$ = new Observable<number>(subscriber => {
  subscriber.next(1);
  subscriber.next(2);
  subscriber.next(3);
});

const mapped$ = source$.pipe(
  map(value => value * 2)
);

mapped$.subscribe(value => console.log(value));

import { Observable } from 'rxjs';
import { filter } from 'rxjs/operators';

const source$ = new Observable<number>(subscriber => {
  subscriber.next(1);
  subscriber.next(2);
  subscriber.next(3);
});

const filtered$ = source$.pipe(
  filter(value => value % 2 === 0)
);

filtered$.subscribe(value => console.log(value));

import { Observable, merge } from 'rxjs';

const source1$ = new Observable<number>(subscriber => {
  setTimeout(() => {
    subscriber.next(1);
    subscriber.complete();
  }, 1000);
});

const source2$ = new Observable<number>(subscriber => {
  setTimeout(() => {
    subscriber.next(2);
    subscriber.complete();
  }, 2000);
});

const merged$ = merge(source1$, source2$);

merged$.subscribe(value => console.log(value));