Repeat request (Angular2 - http.get) n seconds after finished

Question

I played around with angular2 and got stuck after a while.

Using http.get works fine for a single request, but I want to poll live-data every 4 seconds, after tinkering for quite a while and reading a lot of reactivex stuff i ended up with:

Observable.timer(0,4000)
  .flatMap(
    () => this._http.get(this._url)
       .share()
       .map(this.extractData)
       .catch(this.handleError)
  )
  .share(); 

Is there a simple way to start a (4 second) interval after the http.get-observable has emitted the result of the request? (Or will I end up in observable-hell?)

Timeline i want:

Time(s): 0 - - - - - 1 - - - - - 2 - - - - - 3 - - - - - 4 - - - - - 5 - - - - - 6
Action:  Request - - Response - - - - - - - - - - - - - - - - - - - -Request-... 
Wait:                | wait for 4 seconds -------------------------> |

Answer 1

Update to RxJS 6

import { timer } from 'rxjs';
import { concatMap, map, expand, catchError } from 'rxjs/operators';

pollData$ = this._http.get(this._url)
  .pipe(
    map(this.extractData),
    catchError(this.handleError)
  );

pollData$.pipe(
  expand(_ => timer(4000).pipe(concatMap(_ => pollData$)))
).subscribe();

---

I'm using RxJS 5 and I'm not sure what the RxJS 4 equivalent operators are. Anyway here is my RxJS 5 solution, hope it helps:

var pollData = this._http.get(this._url)
            .map(this.extractData)
            .catch(this.handleError);
pollData.expand(
  () => Observable.timer(4000).concatMap(() => pollData)
).subscribe();

The expand operator will emit the data and recursively start a new Observable with each emission