import { Subject } from 'rxjs';
import { ajax as rxAjax } from 'rxjs/ajax';
import { merge } from 'rxjs/operators';

let form_data = new FormData();
form_data.append( 'key', 'value' );

const progressSubscriber = new Subject();
let request$ = rxAjax({
  url: 'http://localhost.com/some-api',
  method: 'POST',
  crossDomain: true,
  withCredentials: true,
  body: form_data,
  progressSubscriber
});

progressSubscriber
  .pipe( merge(request$) )
  .subscribe( data =>{

    if ( data.hasOwnProperty('status') ) {
      console.log('XHR is DONE');
    }

  });