[RxJS] SwitchAll() - switch between two observables display always latest value

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Converts a higher-order Observable into a first-order Observable producing values only from the most recent observable sequence

import './style.css';

import { fromEvent, of, merge, empty, concat, defer } from 'rxjs';
import {
  delay,
  map,
  mergeMap,
  tap,
  debounceTime,
  distinctUntilChanged,
  mapTo,
  filter,
  share,
  switchAll
} from 'rxjs/operators';
import { format } from 'date-fns';

// track in progress saves
let savesInProgress = 0;

// references
const input = document.getElementById('note-input');
const saveIndicator = document.querySelector('.save-indicator');

// streams
const keyup$ = fromEvent(input, 'keyup');

// fake save request
const saveChanges = value => {
  return of(value).pipe(delay(1500));
};

/**
 * Trigger a save when the user stops typing for 200ms
 * After new data has been successfully saved, so a saved
 * and last updated indicator.
 */
const inputToSave$ = keyup$.pipe(
  debounceTime(200),
  // @ts-ignore
  map(e => e.target.value),
  distinctUntilChanged(),
  share()
);

const savesInProgress$ = inputToSave$.pipe(
  mapTo(of('Saving')),
  tap(_ => {
    savesInProgress++;
    console.log('savesInProgress++', savesInProgress);
  })
);

const savesCompleted$ = inputToSave$.pipe(
  mergeMap(saveChanges),
  tap(_ => {
    savesInProgress--;
    console.log('savesInProgress++', savesInProgress);
  }),
  // ignore if additional saves are in progress
  filter(_ => !savesInProgress),
  mapTo(
    concat(
      // display saved for 2s
      of('Saved!'),
      empty().pipe(delay(2000)),
      // then last updated time, defer for proper time
      defer(() =>
        of(`Last updated: ${format(Date.now(), 'MM/DD/YYYY hh:mm:ss')}`)
      )
    )
  )
);

merge(savesInProgress$, savesCompleted$)
  .pipe(
    /*
   If new save comes in when our completion observable is running, we want to switch to it for a status update.
  */
    switchAll()
  )
  .subscribe(status => {
    saveIndicator.innerHTML = status;
  });

<iframe height="240" src="https://stackblitz.com/edit/rxjs-3txbvy?file=index.ts" style="width: 100%; height: 500px" width="320"></iframe>

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