getCustomer(id: number | string): Observable<Customer> {
   const customer$ = of(CUSTOMERS.find(customer => customer.id === +id));
   return delay.call(customer$, FETCH_LATENCY);
 }

 getCustomers(): Observable<Customer[]>  {
   return delay.call(of(CUSTOMERS), FETCH_LATENCY);
 }

 getContact(id: number | string): Observable<Contact> {
   const contact$ = of(CONTACTS.find(contact => contact.id === +id));
   return delay.call(contact$, FETCH_LATENCY);
 }

 getContacts(): Observable<Contact[]>  {
   return delay.call(of(CONTACTS), FETCH_LATENCY);
 }

 getItem(id: number | string): Observable<Item> {
   const item$ = of(ITEMS.find(item => item.id === +id));
   return delay.call(item$, FETCH_LATENCY);
 }

 getItems(): Observable<Item[]>  {
   return delay.call(of(ITEMS), FETCH_LATENCY);
 }