// RxJS v6+
import { of } from 'rxjs';
import { concatMap, timeout, catchError, delay } from 'rxjs/operators';
​
// simulate request
function makeRequest(timeToDelay) {
  return of('Request Complete!').pipe(delay(timeToDelay));
}
​
of(4000, 3000, 2000)
  .pipe(
    concatMap(duration =>
      makeRequest(duration).pipe(
        timeout(2500),
        catchError(error => of(`Request timed out after: ${duration}`))
      )
    )
  )
  /*
   *  "Request timed out after: 4000"
   *  "Request timed out after: 3000"
   *  "Request Complete!"
   */
  .subscribe(val => console.log(val));