Error encountered in Typescript when returning subject.asObservable()

I have a question that I hope you can assist me with. Thank you.

Ever since the upgrade to Typescript 2.9.1, the compiler has been flagging an error for not returning the expected type in a method.

CURRENT CODE:

public getCardPairingSession(sessionId:string):Observable<FundSourceCardSessionInterface> {
    const subject = new Subject();
    
    this.apiService
      .get(this.buildApiPath('card-session/' + sessionId))
      .subscribe(
        (response) => {
          subject.next(response.result);
        },
        (error) => {
          subject.error(error);
        }
      );
/* <-- ERROR: Type 'Observable<{}>' is not assignable to type 'Observable<FundSourceCardSessionInterface>' */
    return subject.asObservable();
  }

MY SOLUTION:

Up until now, I have been casting my return value like this, although it feels strange to me.

return  <Observable<FundSourceCardSessionInterface>> subject.asObservable();

QUESTION:

If everything was working fine with Typescript 2.3.4 before, why am I facing this issue now? Why do I need to cast the return if I have already specified the expected return type at the beginning of the method? It seems redundant:

public getCardPairingSession(sessionId:string):Observable<FundSourceCardSessionInterface> {

Answer №1

The issue is clear: Your function declares a return type of

Observable<FundSourceCardSessionInterface>
, but you are actually returning Observable<{}>. It seems that TypeScript previously inferred the return type as Observable<any>, which can be assigned to
Observable<FundSourceCardSessionInterface>
. However, an empty object {} is not compatible with FundSourceCArdSessionInterface.

To fix this, adjust how you declare your Subject:

const subject = new Subject<FundSourceCardSessionInterface>();`

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