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API > @angular/forms

AsyncValidator

An interface implemented by classes that perform asynchronous validation.

      
      interface AsyncValidator extends Validator {
  validate(control: AbstractControl<any, any>): Promise<ValidationErrors | null> | Observable<ValidationErrors | null>

  // inherited from forms/Validator
  validate(control: AbstractControl<any, any>): ValidationErrors | null
  registerOnValidatorChange(fn: () => void)?: void
}

Methods

validate()

Method that performs async validation against the provided control.

 
      
      validate(control: AbstractControl<any, any>): Promise<ValidationErrors | null> | Observable<ValidationErrors | null>
Parameters
control AbstractControl<any, any>

The control to validate against.
Returns

Promise<ValidationErrors | null> | Observable<ValidationErrors | null>: A promise or observable that resolves a map of validation errors if validation fails, otherwise null.

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.

      
      import { of } from 'rxjs';

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