What are validators in reactive forms? What do they accept and return?

Validators in Angular reactive forms are functions that check the correctness of a form's value or its field.

• What they accept: The validator receives the current state of the form control (e.g., FormControl) as an object.

• What they return:

  • If the value is valid — they return null.
  • If there are errors — they return an object with error keys and additional information, for example { required: true }.

A simple validator example that checks if the value is not empty:

import { AbstractControl, ValidationErrors } from '@angular/forms';

function notEmptyValidator(control: AbstractControl): ValidationErrors | null {
  return control.value && control.value.trim() !== '' ? null : { empty: true };
}

Thus, validators allow centralized and declarative validation of form data and error display management.