old way of doing rxjs

app.service.ts

//import { Observable } from 'rxjs/Observable';
import { Observable } from 'rxjs';

//import { Subject } from 'rxjs/Subject';
import { Subject } from 'rxjs';

//import { Subscription } from 'rxjs/Subscription';
import { Subscription } from 'rxjs';

//import { BehaviorSubject } from 'rxjs/BehaviorSubject';
import { BehaviorSubject } from 'rxjs';

// you can combine all in one statement
import { Subscription, Subject, Observable, BehaviorSubject, of } from 'rxjs';

//you can't chain takewhile, map, catch anymore, you should import instead
//import 'rxjs/add/operator/map';
//import 'rxjs/add/operator/takeWhile'
import { takeWhile, map, catchError } from 'rxjs/operators';