callback

Validates a field using a custom synchronous function.

Options

Option	Type	Default	Description
callback	(input: ValidatorInput) => { valid: boolean, message?: string }	required	Custom validation function
message	string	locale default	Custom error message

Playground

Valid values

Value	Notes
Callback returns { valid: true }	Validation passes

Invalid values

Value	Reason
Callback returns { valid: false }	Validation fails with default or inline message

Notes

• Empty string ("") always returns valid: true — validators only run on non-empty values. Combine with notEmpty to require a value.
• The input object has: value (string), elements (HTMLElement[]), options (validator options), field (field name).
• The callback can return { valid: false, message: 'Custom message' } to override the configured message.
• For async operations, use promise instead.