Function compose

packages/forms/src/validators.ts:665–673

* Merges synchronous validators into a single validator function. * See `Validators.compose` for additional information.

(validators: (ValidatorFn | null | undefined)[] | null)

 * See `Validators.compose` for additional information.
 */
function compose(validators: (ValidatorFn | null | undefined)[] | null): ValidatorFn | null {
  if (!validators) return null;
  const presentValidators: ValidatorFn[] = validators.filter(isPresent) as any;
  if (presentValidators.length == 0) return null;

  return function (control: AbstractControl) {
    return mergeErrors(executeValidators<ValidatorFn>(control, presentValidators));
  };
}

/**
 * Accepts a list of validators of different possible shapes (`Validator` and `ValidatorFn`),