Keep scroll positions for routes in Angular app

app.component.ts

import { Component, OnInit, OnDestroy } from '@angular/core';
import { Router, NavigationStart, NavigationEnd } from '@angular/router';
import { Subscription } from 'rxjs';
import 'rxjs/add/operator/pairwise';

@Component({
  selector: 'app-root',
  template: `/* ... your template */`,
  styles: [`/* ... your styles */`]
})
export class AppComponent implements OnInit, OnDestroy {
  
  private _routeScrollPositions: {[url: string] : number}[] = [];
  private _subscriptions: Subscription[] = [];

  constructor(private router: Router) {}

  ngOnInit() {
    this._subscriptions.push(
      // save or restore scroll position on route change
      this.router.events.pairwise().subscribe(([prevRouteEvent, currRouteEvent]) => {
        if (prevRouteEvent instanceof NavigationEnd && currRouteEvent instanceof NavigationStart) {
          this._routeScrollPositions[prevRouteEvent.urlAfterRedirects || prevRouteEvent.url] = window.pageYOffset;
        }
        if (currRouteEvent instanceof NavigationEnd) {
          window.scrollTo(0, this._routeScrollPositions[currRouteEvent.urlAfterRedirects || currRouteEvent.url] || 0);
        }
      })
    );
  }

  ngOnDestroy() {
    this._subscriptions.forEach(subscription => subscription.unsubscribe());
  }
}

Hi, this functionality is not working for me (In Angular 5.2.6)