NestJS handle HttpModule errors - rxjs

Question

NestJS has the following example for using their HttpModule:
#Injectable()
export class CatsService {
constructor(private readonly httpService: HttpService) {}
findAll(): Observable<AxiosResponse<Cat[]>> {
return this.httpService.get('http://localhost:3000/cats');
}
}
My question is, how does the client code (most likely a Controller) handle this response? How does it treat Observables so that Cat[] may be accessed. Or what if the Http request throws an error such as a 404.
How does a NestJS client (Controller) in this case interact with the findAll() method provided by the service?

Answer 1

I am not too familiar with NestJs, but if you want to run an observable http request generally you do the following to consume and catchError
this.catService.findAll().pipe(
// axio wraps the result in data
map(res=>res.data),
catchError(e=>{
...handle error 404
return of(e)
})).subscribe()

Answer 2

If, for whatever reason, the URL you provide to the HttpService returns a 404, that error will propagate back through the service, to the controller, and eventually to the client that called the original URL. Under the hood, NestJS will subscribe to all returned Observables so that you don't need to worry about it, you can just return the call directly from your Controller. So in the example above, say we have a controller that looks like this:
#Controller('cats')
export class CatsController {
constructor(private readonly catsService: CatsService) {}
#Get()
findAllCats(): Observable<Cat[]> {
return this.catsService.findAll();
}
}
And CatsService looks like this
#Injectable()
export class CatsService {
constructor(private readonly httpService: HttpService) {}
findAll(): Observable<Cat[]> {
return this.httpService.get('http://localhost:3000/cats');
}
}
Assuming you are calling off to another server (i.e. this server is not running on port 3000) and /cats is not a valid endpoint and that server returns a 404 to you, the httpService's response will bubble up through the CatsService to the CatsController where NestJS will handle the subscription, and send the response back to the client. If you are looking to do some custom error handling, that will need to be handled in a different way. A great way to test how the HttpService responds to things is to create a simple endpoint and call off to a bad URL (like https://google.com/item which is a 404)