Type 'null' is not assignable to type 'ValidationErrors'

Question

So, my issue is I am trying to build a custom validator on Angular 15, and I get an error message that tells this:

Type 'Observable<{ titleAlreadyExists: boolean; } | null>' is not assignable to type 'Observable'. Type '{ titleAlreadyExists: boolean; } | null' is not assignable to type 'ValidationErrors'. Type 'null' is not assignable to type 'ValidationErrors'."

This is my validator:

alreadyExistingTitle(alreadyExistingTitles: String[]): AsyncValidatorFn {
  return (control: AbstractControl): Observable<ValidationErrors> => {
    return of(alreadyExistingTitles.includes(control.value))
      .pipe(
        map((result: boolean) =>
          result ? { titleAlreadyExists: true } : null
        )
      );
};

what if you change it to: result ? { titleAlreadyExists: true } : { titleAlreadyExists: false }; ? — Rick
– Rick, Commented Jan 30, 2023 at 23:48
Well, from what I understand from this official source (angular.io/guide/form-validation#defining-custom-validators), the validator "takes an Angular control object and returns either null if the control value is valid or a validation error object." — Tamsin-chan
– Tamsin-chan, Commented Jan 30, 2023 at 23:55
I'm just going by the error message which says it can't be null. maybe try it. — Rick
– Rick, Commented Jan 31, 2023 at 0:07

Yong Shun · Accepted Answer · 2023-01-31 00:06:26Z

2

Modify the return type from Observable<ValidationErrors> to Observable<ValidationErrors | null>.

alreadyExistingTitle(alreadyExistingTitles: String[]): AsyncValidatorFn {
  return (control: AbstractControl): Observable<ValidationErrors | null> => {
    return of(alreadyExistingTitles.includes(control.value)).pipe(
      map((result: boolean) => (result ? { titleAlreadyExists: true } : null))
    );
  };
}

answered Jan 31, 2023 at 0:06

Yong Shun

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1 Comment

Tamsin-chan Over a year ago

Thank you for your answer. I don't have the compile error anymore.

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