Creating an Observable with no data in Angular 6 using Rxjs 6

Question

Creating an Observable with no data in Angular 6 using Rxjs 6

Currently, I am diving into the world of Angular 6 with RxJS 6 and have stumbled upon a question regarding the scenario where I need to return a null or empty Observable in case the response fails or encounters an exception. In this situation, my assumption is that the remote API will provide an object of type IOptionResponse, which consists of a message string indicating either 'SUCCESS' or 'FAILED', along with a model that is an array of 'IOption' objects.

export interface IOptionResponse {
    message: string;
    model: IOption[];
}

One of my service methods aims to return an Observable containing an array of IOption elements, which represents the "model" from the result of the remote API.

loadIOptionMembersRelationship(): Observable<IOption[]> {
    return this.httpClient.get<IOptionResponse>('${environment.apiUrl}/api/member/XXX')
        .map(
            (response) => {
                console.log(response);
                // If the response message indicates success, return the IOption[] model
                if (response.message == responseMessage.Success) {
                    return response.model;
                }
                else {
                    // Here lies the challenge of returning a null or empty Observable
                    // What would be the correct approach to handle this scenario?
                }
            }
        );
}

I came across a similar post on StackOverflow, but none of the suggested solutions seem to work. It's unclear whether it's due to outdated information, changes in RxJS 6, or potential TypeScript issues...

typescript rxjs angular6 rxjs6

Answer 1

Answer №1

To receive a void Observable array with Rxjs6, follow these steps...

import { of } from 'rxjs';

Next, specify the location to send back your empty Observable array of <IOption[]> type ...

return of<IOption[]>([]);

Answer 2

To receive a void Observable array with Rxjs6, follow these steps...

import { of } from 'rxjs';

Next, specify the location to send back your empty Observable array of <IOption[]> type ...

return of<IOption[]>([]);

Answer 3

Answer №2

Are you concerned about FAILED outcomes? If not (a common scenario when aiming to produce an empty Observable), consider applying a filter to exclude those cases. This way, there's no need to explicitly emit an empty Observable:

 fetchMembersData(): Observable<Member[]> {
    return this.httpClient.get<MemberResponse>('${environment.apiUrl}/api/member/XXX')
        .filter(response => response.result === responseResult.SUCCESS) //only emit successful responses
        .map(response => response.data);
}

By explicitly setting an empty Observable, the stream will be terminated - which may or may not suit your requirements based on the specific implementation.

Answer 4

Are you concerned about FAILED outcomes? If not (a common scenario when aiming to produce an empty Observable), consider applying a filter to exclude those cases. This way, there's no need to explicitly emit an empty Observable:

 fetchMembersData(): Observable<Member[]> {
    return this.httpClient.get<MemberResponse>('${environment.apiUrl}/api/member/XXX')
        .filter(response => response.result === responseResult.SUCCESS) //only emit successful responses
        .map(response => response.data);
}

By explicitly setting an empty Observable, the stream will be terminated - which may or may not suit your requirements based on the specific implementation.

Answer 5

Answer №3

None of the solutions mentioned in the post will activate the "next" callback, causing your workflow to fail. Instead, you can simply use

return;

However, by doing this, you will lose the error information. The best approach would be:

throw new Error("Failed to retrieve the model...");

Answer 6

None of the solutions mentioned in the post will activate the "next" callback, causing your workflow to fail. Instead, you can simply use

return;

However, by doing this, you will lose the error information. The best approach would be:

throw new Error("Failed to retrieve the model...");

Answer 7

Answer №4

function myFunction() {
    return new Observable<SomeType[]>((observer) => {
      observer.next(null); //when you want to return NULL/Empty.
      observer.next(ActualReturnValue); //When you want to return actual value.
    });
  }

how to use the function

this.myFunction().subscribe(response=> {
  if(response)
        //DO SOMETHING
  else
        //DO SOMETHING ELSE
        
},error => {
    console.log(error);        
});

Answer 8

function myFunction() {
    return new Observable<SomeType[]>((observer) => {
      observer.next(null); //when you want to return NULL/Empty.
      observer.next(ActualReturnValue); //When you want to return actual value.
    });
  }

how to use the function

this.myFunction().subscribe(response=> {
  if(response)
        //DO SOMETHING
  else
        //DO SOMETHING ELSE
        
},error => {
    console.log(error);        
});

Answer 9

Answer №5

One way to manipulate the data emitted by a new observable using map() is to base it on the source observable. However, in your case, you seem to be attempting to return an empty Observable when your Observable should actually be returning a value of type IOptionsResponse. As a result, based on your code, your Observable seems to be of type

Observable<IOptionsResponse | Observable<IOptionsResponse>>

. In this scenario, you may want to consider simply returning null or undefined instead of an empty Observable in your else condition.

Answer 10

One way to manipulate the data emitted by a new observable using map() is to base it on the source observable. However, in your case, you seem to be attempting to return an empty Observable when your Observable should actually be returning a value of type IOptionsResponse. As a result, based on your code, your Observable seems to be of type

Observable<IOptionsResponse | Observable<IOptionsResponse>>

. In this scenario, you may want to consider simply returning null or undefined instead of an empty Observable in your else condition.

Creating an Observable with no data in Angular 6 using Rxjs 6

Answer №1

Answer №2

Answer №3

Answer №4

Answer №5

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