Angular Subscribe within Subscribe

Technically, nesting subscribe works, but there is a more elegant and systematic way of handling this. You should really learn more about your RxJS operators.

First, we use mergeMap to map over the observable values from the activatedRoute into an inner observable.

Then, we use forkJoin to combine the observables into a single value observable, thus returning the value itself on the .subscribe()

this.activatedRoute.pipe(
    tap(data => console.log(data)),
    mergeMap(data => {
      if (data.product === null) {
        this.router.navigate(['/home']);
      } else {
        const getCurrency = this.dataService.getCurrencySymbolById(data.product.currency);
        const getUsers= this.userService.getUser(data.product.createdBy);
        const getRatings = this.ratingService.getRatingByUserId(seller.id)
        return forkJoin(getCurrency, getUsers, getRatings);
      }
    })
  ).subscribe(res => {
    console.log(res[0]); // currency
    console.log(res[1]); // user
    console.log(res[2]); // ratings

  }

---

EDIT: Turns out I have misread the original question, as getRatingsByUserId is dependent on getUser. Let me make some changes. Either ways, I will leave the code above as it is, as it is good for OP's reference.

this.activatedRoute.data.pipe(
  switchMap(data => {
    this.user = data['user'];
    this.product = data['product'];
    return this.userService.getUser(this.product.createdBy);
  }),
  switchMap(data => {
    if (this.product === null) {
      this.router.navigate(['/home']);
    } else {
      this.seller = seller;
      return this.userService.getRatingByUserId(this.product.createdBy); 
    }
  })
).subscribe(res => {
 console.log(res)
 // handle the rest
})

Use switchMap to switch to a new observable.

this.activatedRoute.data.switchMap((routeData) => {
     this.user = routeData['user'];
     this.product = routeData['product'];
     return this.userService.getUser(this.product.createdBy);
    }).switchMap(seller => {
       this.seller = seller;
        return this.ratingService.getRatingByUserId(seller.id);
    }).subscribe(rating => {
      this.rating = rating;
    })