Resolving the Error: "Type 'Customer | undefined' is not compatible with type 'Customer'" in Angular

I encountered an issue with the following code:

...
export class ListCustomersComponent implements OnInit {

  customers: Array<Customer> = [];
  showCustomer?: Customer;
  isSelected: boolean = false;
  deletedCustomer?: Customer;
  returnedMessage?: string;

  constructor(private customerService: CustomerService,
                private messageService: MessageService) { }

     
  updateCustomer() {
    this.customerService.updateCustomer(this.showCustomer!)
                      .subscribe((message: Message) => {
                        console.log(message);
                        // update customers list
                        this.customers.map(x => {
                          if(x.id == this.showCustomer!.id){
            ISSUE------>    x = this.showCustomer;
                          }
                        });

                        
  }

The problem lies in the updateCustomer() function at the line x = this.showCustomer;. How can I resolve this error? Thank you for your help.

Answer №1

When Typescript warns that a value may be undefined, using an exclamation point can bypass this warning. It is important to ensure that your logic guarantees there will always be a value present. Otherwise, if no value exists, x will default to undefined.

To suppress the warning in this scenario, you must include an exclamation mark after the x = this.showCustomer line. By appending !, Typescript recognizes the value as being not null.

this.customerService.updateCustomer(this.showCustomer!)
                      .subscribe((message: Message) => {
                        console.log(message);
                        // update customers list
                        this.customers.map(x => {
                          if(x.id == this.showCustomer!.id){
                             x = this.showCustomer!; // Added ! after showCustomer to assert non-null
                          }
                        });

For further information about the use of !, refer to the official documentation.

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