Rx.ReplaySubject() demo

<!DOCTYPE html>
<html>

<head>
  <script src="https://npmcdn.com/@reactivex/rxjs@5.0.0-beta.6/dist/global/Rx.umd.js"></script>
  <script src="https://ajax.googleapis.com/ajax/libs/jquery/2.2.4/jquery.min.js"></script>
  <script src="script.js"></script>
</head>

<body>
</body>

</html>

let subject = new Rx.ReplaySubject();
let array = ['5', 'd', '5', '9', '3', '2', 'x', '4', '8'];

let requestStream = Rx.Observable.interval(400).take(array.length).map(i => array[i])
  .map(x => parseInt(x))
  .filter(x => {
    return !isNaN(x)
  })
  .map(x => `/api/myurl/${x}`);

setTimeout(function() {
  subject.subscribe(function(val) {
    $('body').append('<div>' + val + '</div>');
  });
  subject.next('hi there');
}, 2000);

requestStream.subscribe(function(val) {
  subject.next(val);
});

Rx.ReplaySubject() demo

With ReplaySubject we receive items that were emitted before we subscribed.