[RxJS] Append values to a stream using startWith and endWith (countdown$)

// begin lesson code
import { interval, fromEvent, of } from 'rxjs';
import { scan, mapTo, takeWhile, takeUntil, tap, startWith, endWith } from 'rxjs/operators';

/*
 * CODE FOR FOR FIRST SECTION OF LESSON
 */
const numbers$ = of(1,2,3);

numbers$.pipe(
  /*
   * startWith lets you seed a stream with 1:M values.
   * On subscription, these values will be emitted
   * immediately, followed by any future values from
   * the source.
   */
  startWith('a', 'b', 'c'),
  /*
   * You can also end a stream with any number of values,
   * emitted on completion.
   */
  endWith('d', 'e', 'f')
).subscribe(console.log);
/*
a
b
c
1
2
3
d
e
f
*/

const countdown = document.getElementById('countdown');
const message = document.getElementById('message');
const abortButton = document.getElementById('abort');

// streams
const counter$ = interval(1000);
const abort$ = fromEvent(abortButton, 'click');

const COUNTDOWN_FROM = 10;

counter$
  .pipe(
    mapTo(-1),
    scan((accumulator, current) => {
      return accumulator + current;
    }, COUNTDOWN_FROM),
    takeWhile(value => value >= 0),
    takeUntil(abort$),
    /*
     * With startWith, we can seed the stream with
     * the starting countdown value.
     */
    startWith(COUNTDOWN_FROM)
  )
  .subscribe((value: any) => {
    countdown.innerHTML = value;
    if (!value) {
      message.innerHTML = 'Liftoff!';
    }
  });