check

A custom synchronous check. The function returns true if the value is valid.

Signature

function check<T>(fn: (value: T, info: Info) => boolean, msg?: Message): Action<T>

interface Info {
  parent: unknown; // the object containing the field (raw input)
  root: unknown;   // all data passed to validate
}

Parameters

Parameter	Type	Description
fn	(value: T, info: Info) => boolean	The check. value is already validated and transformed
msg	Message	Custom error message. A string with {params} or a function

Error codes

Code	When	Default message
invalid	fn returned false	Invalid value

You can change the texts: see Messages & i18n.

Notes

• To reuse a check, wrap it in a function — see Custom rules & schemas.
• For database or API calls use checkAsync.
• An exception inside fn is not turned into a validation error, it is rethrown.

Examples

Age 18+

const adult = check<Date>((birth) => {
  const d = new Date();
  d.setFullYear(d.getFullYear() - 18);
  return birth <= d;
}, '18+ only');

object({ birthday: pipe(coerce.date(), adult) });

Depends on another field

object({
  delivery: oneOf(['pickup', 'courier']),
  address: optional(pipe(string(), check((v, { parent }) => parent.delivery === 'courier', 'Address is only for courier delivery'))),
});

See also

• checkAsync
• Custom rules & schemas
• sameAs