[RxJS] Wait on something to happen before proceeding

Let’s say you have an Observable emitting values over time and you want to wait for a specific value before proceeding on something else. And you want this to only trigger once.

import { Subject } from 'rxjs';
import { filter, take } from 'rxjs/operators';

const source$ = new Subject<number>();
const obs$ = source$.asObservable();

const subscription = obs$
  .pipe(
    filter((num) => num > 2),
    take(1)
  )
  .subscribe((num) => console.log('Ready!'));

source$.next(1); // No output
source$.next(2); // No output
source$.next(3); // "Ready!" output to console
source$.next(4); // No output
source$.next(3); // No output

// Make sure to to call `subscription.destroy()` to dispose properly of the subscription when done.
// For example, in an Angular component's `ngOnDestroy` lifecycle hook.