mock-price-api-service.ts

const BASE_PRICES = {
  bitcoin: 11000,
  litecoin: 130,
  ethereum: 300
};

const randomize = (price: number) => {
  return (Math.random() * 2) + price;
};

@Injectable()
export class MockPriceApiService implements PriceApi {
  static latency = 250;
  private unsubsciptions$ = new Subject();

  public getPrice(currency: string): Observable<string> {
    return timer(0, MockPriceApiService.latency).pipe(
      delay(1000),
      map(() => BASE_PRICES[currency]),
      map((price: number) => randomize(price)),
      map((price: number) => price.toFixed(5)),
      takeUntil(this.unsubsciptions$),
    )
  }

  public unsubscribe() {
    this.unsubsciptions$.next();
  }
}