Validator

An interface implemented by classes that perform synchronous validation.

interface Validator {
  validate(control: AbstractControl): ValidationErrors | null
  registerOnValidatorChange(fn: () => void)?: void
}

Methods

validate()
Method that performs synchronous validation against the provided control.
validate(control: AbstractControl): ValidationErrors | null Parameters control AbstractControl The control to validate against. Returns ValidationErrors | null: A map of validation errors if validation fails, otherwise null.

registerOnValidatorChange()
Registers a callback function to call when the validator inputs change.
registerOnValidatorChange(fn: () => void)?: void Parameters fn () => void The callback function Returns void

Usage notes

Provide a custom validator

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

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