@angular/forms

NG_VALIDATORS

constant

An InjectionToken for registering additional synchronous validators used with AbstractControls.

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

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 };  }}