Capture exception thrown in prototype (Typescript) - rest

Question

Excellent Angular 2/Material Design framework, Teradata Covalent, provides a RESTService abstract class that wraps REST api calls here:
https://teradata.github.io/covalent/#/components/http
Code to incorporate the extension is easy, as follows:
export class CustomRESTService extends RESTService<any> {
constructor(private _http: Http /* or HttpInterceptorService */) {
super(_http, {
baseUrl: 'www.api.com',
path: '/path/to/endpoint',
headers: new Headers(),
dynamicHeaders: () => new Headers(),
transform: (res: Response): any => res.json(),
});
}
}
The "update" method in the RESTService abstract class is shown here:
public update(id: string | number, obj: T, transform?: IRestTransform): Observable<any> {
let requestOptions: RequestOptionsArgs = this.buildRequestOptions();
let request: Observable<Response> = this.http.patch(this.buildUrl(id), obj, requestOptions);
return request.map((res: Response) => {
if (res.status === 200) {
if (transform) {
return transform(res);
}
return this.transform(res);
} else {
return res;
}
}).catch((error: Response) => {
return new Observable<any>((subscriber: Subscriber<any>) => {
try {
subscriber.error(this.transform(error));
} catch (err) {
subscriber.error(error);
}
});
});
}
My question is: if the update method of the abstract class throws an exception, how can that be captured in the CustomRESTService class? I.e., what Typescript code might one use to display an error in the UI?
Thank you.

Answer 1

First thing's first - Why would you want to catch it inside the rest client and not inside the app's logic?
Assuming you have some good reason for doing that (some other infrastructure code that you're running in the CustomRESTClient class), I would override the update function and implement error handling there.
A simple example without observables:
abstract class Base {
update(n:number):number {
return n;
}
test():bool;
}
class Child extends Base {
update(n:number):number {
return super.update(n)*2;
}
test():bool {
return true;
}
}