Angular conditional validator

conditional-validator.ts

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

/**
 * Validates control with `validator` if `predicate` executed on `provider` control value returns `true`
 *
 * @usageNotes
 *
 * ### Create a form where `option` is required only when `condition` is `true`
 * ```
 * const form = new FormGroup({
 *   condition: new FormControl(false),
 *   option: new FormControl(
 *     null,
 *     conditionalValidator<boolean>('condition', (value) => !!value, Validators.required)
 *   ),
 * });
 * ```
 */
export function conditionalValidator<T extends any>(
  provider: string,
  predicate: (value: T) => boolean,
  validator: ValidatorFn | null
): (control: AbstractControl) => ValidationErrors | null {
  return (control: AbstractControl) => {
    if (!control.parent || !control.parent.get(provider)) {
      return null;
    }

    const m = control.parent.get(provider)!;

    if (predicate(m.value) && !!validator) {
      return validator(control);
    }

    return null;
  };
}