import './style.css';
​
import { Observable, of, empty, fromEvent, from } from 'rxjs';
import {
  delay,
  switchMapTo,
  concatAll,
  count,
  scan,
  withLatestFrom,
  share
} from 'rxjs/operators';
​
const requestOne = of('first').pipe(delay(500));
const requestTwo = of('second').pipe(delay(800));
const requestThree = of('third').pipe(delay(1100));
const requestFour = of('fourth').pipe(delay(1400));
const requestFive = of('fifth').pipe(delay(1700));
​
const loadButton = document.getElementById('load');
const progressBar = document.getElementById('progress');
const content = document.getElementById('data');
​
// update progress bar as requests complete
const updateProgress = progressRatio => {
  console.log('Progress Ratio: ', progressRatio);
  progressBar.style.width = 100 * progressRatio + '%';
  if (progressRatio === 1) {
    progressBar.className += ' finished';
  } else {
    progressBar.className = progressBar.className.replace(' finished', '');
  }
};
// simple helper to log updates
const updateContent = newContent => {
  content.innerHTML += newContent;
};
​
const displayData = data => {
  updateContent(`<div class="content-item">${data}</div>`);
};
​
// simulate 5 separate requests that complete at variable length
const observables: Array<Observable<string>> = [
  requestOne,
  requestTwo,
  requestThree,
  requestFour,
  requestFive
];
​
const array$ = from(observables);
const requests$ = array$.pipe(concatAll());
const clicks$ = fromEvent(loadButton, 'click');
​
const progress$ = clicks$.pipe(
  switchMapTo(requests$),
  share()
);
​
const count$ = array$.pipe(count());
​
const ratio$ = progress$.pipe(
  scan(current => current + 1, 0),
  withLatestFrom(count$, (current, count) => current / count)
);
​
clicks$.pipe(switchMapTo(ratio$)).subscribe(updateProgress);
​
progress$.subscribe(displayData);

<div class="progress-container">
  <div class="progress" id="progress"></div>
</div>
​
<button id="load">
  Load Data
</button>
​
<div id="data"></div>