How to subscribe and test Observable.of rxjs Angular 9

Question

I'm trying to test a function which returns a rxjs observable, but I cannot subscribe to it in any way. None of the subscribe method that I've tried are working.

"rxjs": "6.5.4","@angular/core": "9.0.1"

Function that i'm trying to test:

isNameValid(c: AbstractControl) {
    const searchResult = this.roles.filter((elem: any) => elem.name?.toLowerCase() === c.value?.toLowerCase());

    if (searchResult.length > 0) {
      c.get('name')?.setErrors({ nameTaken: true });
      return of({ matchError: 'nameTaken' });
    }
    return of(true);
  }

test:

it('should return invalid error if supplied name is taken', () => {
    const nameControl = new FormControl(mock[0].name);
    const isValid = component.isNameValid(nameControl);
    isValid.subscribe
      ({
        next: (result: any) => {
        console.log(result);
        },
        error: (err: any) => {
        console.log(err);
        },
        complete: () => {
        console.log('complete');
        }
        });
  });

What is the correct syntax of subscribing to the observable returned by isNameValid ? Right now, with the pasted syntax(or anything else that I've tried) I cannot get the error free way of subscribing.

Should I write the test differently?

The error that I'm getting is:

Expected 2-3 arguments, but got 1.ts(2554)
Observable.d.ts(51, 39): An argument for 'error' was not provided.

I've also tried:

isValid.subscribe(response => {console.log(response}) which gives the same error.

Answer 1

There seems to be some TypeScript problem not being able to infer return value correctly.

To fix the problem, please provide type definition explicitly for isNameValid method:

isNameValid(c: AbstractControl): Observable<boolean | { matchError: string }> {
  // ...
}

Btw, you can pass an object to subscribe() , this is completely valid.