Error Encyclopedia

Wrong Async Validator Return Type

Async validators must return a promise or an observable, and emit/resolve them whether the validation fails or succeeds. In particular, they must implement the AsyncValidatorFn API

export function isTenAsync(control: AbstractControl):
Observable<ValidationErrors | null> {
const v: number = control.value;
if (v !== 10) {
// Emit an object with a validation error.
return of({ 'notTen': true, 'requiredValue': 10 });
// Emit null, to indicate no error occurred.
return of(null);

Debugging the error

Did you mistakenly use a synchronous validator instead of an async validator?