As tweaked from https://semanticbits.com/route-history-service-in-angular/ and https://github.com/semanticbits/previous-url-example/blob/master/src/app/services/router-history.service.ts

router-history-service.ts

export interface RouterHistory {
  history: number[];
  currentIndex: number;
  event: NavigationStart | NavigationEnd;
  trigger: 'imperative' | 'popstate' | 'hashchange';
  id: number;
  idToRestore: number;
}

@Injectable({
  providedIn: 'root'
})
export class RouterHistoryService {
  private depth = 0;

  constructor(
    router: Router
  ) {
    router.events
      .pipe(
        filter(event => event instanceof NavigationStart || event instanceof NavigationEnd),
        scan<NavigationStart | NavigationEnd, RouterHistory>(
          (acc, event) => {
            if (event instanceof NavigationStart) {
              return {
                ...acc,
                event,
                trigger: event.navigationTrigger,
                id: event.id,
                idToRestore:
                  (event.restoredState && event.restoredState.navigationId) || undefined
              };
            }
            const history = [...acc.history];
            let currentIndex = acc.currentIndex;
            if (acc.trigger === 'imperative') {
              history.splice(currentIndex + 1);
              history.push(acc.id);
              currentIndex = history.length - 1;
            }
            if (acc.trigger === 'popstate') {
              const idx = history.findIndex(x => x === acc.idToRestore);
              if (idx > -1) {
                currentIndex = idx;
                history[idx] = acc.id;
              } else {
                currentIndex = 0;
              }
            }
            return {
              ...acc,
              event,
              history,
              currentIndex
            };
          },
          {
            event: null, history: [], trigger: null, id: 0, idToRestore: 0, currentIndex: 0
          }
        ),
        filter(
          ({event, trigger}) => event instanceof NavigationEnd && !!trigger
        )
      )
      .subscribe(({currentIndex}) => {
        this.depth = currentIndex;
        console.log('*** depth:' + this.depth);
      });
  }
}