Angular/Typescript handle result from subscription after getting the value

Question

I hope there is no answer to this question, I couldn't find anything helpful for my case. I'm kinda new to angular and typescript so: how can I extract the value returned by a subscription when calling a rest call? The code looks like this:

    static utilAccountName(....., queryService: QueryService) {
        ...
        let value = true;
        let name;
        do {
            ....
           // the name is built and should be changed here until there shouldn't be in the backend
            value = this.theValueExists(name: string, queryService: QueryService);
        } while (value);
        return name;
    }

    private static theValueExists(name: string, queryService: QueryService): boolean {
        let val;
        queryService.findValueByName(name)
            .subscribe(
            (res) => {
                val= res!= null;
            }, () => {
                val= false;
            }
        );
        return val;

    }

   //the function from the queryService looks like this
    findValueByName(name: string): Observable<MyObject> {
        return this.httpClient
            .get<MyObject>('/rest/byName/' + name)
            .map(result => {
                if (<any>result) {
                    return new MyObject().deserialize(result);
                }
            })
    }

The problem that I encounter is that the val from theValueExists is returned an undefined and I need for it to return the value true or false after the call to the backend is done and the result is in. Is there a way to do this?

MoxxiManagarm · Accepted Answer · 2020-10-05 11:36:04Z

1

findValueByName is an asynchronous operation. Withint theValueExits you do return val; before the subsciption block is executed.

queryService.findValueByName(name)
            .subscribe(
            (res) => {
                val= res!= null;
            }, () => {
                val= false;
            }
        );

The best approach would be to define further observables based on the others. theValueExists will become

private theValueExists(name: string, queryService: QueryService): Observable<boolean> {
  return queryService.findValueByName(name).pipe(
    map(res => res != null),
    catchError(err => of(false)),
  );

And utilAccountName would also need to handle thoseobservables with respective rxjs utils.

edited Oct 5, 2020 at 11:36

answered Oct 5, 2020 at 11:05

MoxxiManagarm

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2 Comments

Catalin Over a year ago

Can you explain to me what are you doing with pipe? I never seen anything like this and it seems odd that you have , and after that nothing. And it still requires Observable<MyObject>. Is some part of your answer missing?

MoxxiManagarm Over a year ago

pipeis a method of Observable and is required to chain rxjs operators to transform the stream of data. Please get in touch with rxjs basics.

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