Validators

Registration

import { registerDefaultValidators, registerValidator } from 'formlayer';

// Register all 12 built-in validators at once
registerDefaultValidators();

// Register a single custom validator
registerValidator({
  type: 'MyValidator',
  validate(value, options) {
    return { valid: true, message: '' };
  },
});

Validator Interface

interface Validator {
  type: string;
  validate(
    value: string,
    options: Record<string, unknown>
  ): ValidatorResult | Promise<ValidatorResult>;
}

interface ValidatorResult {
  valid: boolean;
  message: string;
}

The type must match the type field in data-validate JSON rules.

Built-in Validators

Type	Options	Passes on empty?
NotEmpty	message	No
StringLength	minimum, maximum, message	Yes
EmailAddress	message	Yes
RegularExpression	regularExpression (PCRE delimited), message	Yes
NumberRange	minimum, maximum, message	Yes
Integer	message	Yes
Float	message	Yes
Number	message	Yes
Alphanumeric	message	Yes
DateTime	message	Yes
DateRange	minimum, maximum, message	Yes
FileSize	minimum, maximum (e.g. "5M"), message	Yes

Most validators pass on empty values by design. Combine with NotEmpty for required fields.

ValidatorRule (HTML format)

interface ValidatorRule {
  type: string;
  options?: Record<string, unknown>;
}

Used in HTML as a JSON array:

<div data-validate='[
  {"type":"NotEmpty","options":{"message":"Required"}},
  {"type":"StringLength","options":{"minimum":3}}
]'>

Running Validators Programmatically

import { runValidators, runValidatorsAsync } from 'formlayer/validators';

// Synchronous (skips async validators with a warning)
const results = runValidators(rules, value);

// Async (awaits each validator)
const results = await runValidatorsAsync(rules, value);

Both return an array of failed ValidatorResult objects.