export class Customer {

    constructor() {
        this.Addresses = new Array<Address>();
        }

    public Addresses: Array<Address>;

    public addAddress(address: Address) void{
        this.Addresses.push(address);
    }
}

var url: string = `${this.urlBase}api/customer/${id}`;
var customerObservable: Observable<Customer> = this.authHttp.get<Customer>(url);

customerObservable.subscribe(customer => {
    // Addresses is undefined!
    customer.Addresses.push(new Address());
    // addAddress is undefined!
    customer.addAddress(new Address());
});