An InjectionToken for registering additional asynchronous validators used with AbstractControls.

const NG_ASYNC_VALIDATORS: InjectionToken<(Function | Validator)[]>;

See also

• NG_VALIDATORS

Usage notes

Provide a custom async validator directive

The following example implements the AsyncValidator interface to create an async validator directive with a custom error key.

@Directive({
  selector: '[customAsyncValidator]',
  providers: [{provide: NG_ASYNC_VALIDATORS, useExisting: CustomAsyncValidatorDirective, multi:
true}]
})
class CustomAsyncValidatorDirective implements AsyncValidator {
  validate(control: AbstractControl): Promise<ValidationErrors|null> {
    return Promise.resolve({'custom': true});
  }
}