@angular/forms

NG_ASYNC_VALIDATORS

constant

An InjectionToken for registering additional asynchronous validators used with AbstractControls.

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

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});  }}