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NG_VALIDATORS

const

An InjectionToken for registering additional synchronous validators used with AbstractControls.

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

See also

Usage notes

Providing a custom validator

The following example registers a custom validator directive. Adding the validator to the existing collection of validators requires the multi: true option.

@Directive({
  selector: '[customValidator]',
  providers: [{provide: NG_VALIDATORS, useExisting: CustomValidatorDirective, multi: true}]
})
class CustomValidatorDirective implements Validator {
  validate(control: AbstractControl): ValidationErrors | null {
    return { 'custom': true };
  }
}

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https://v7.angular.io/api/forms/NG_VALIDATORS