TL;DR: the code

SolidJS use:validation directive

Validator function

Both customValidity and validation directives must / can be used with custom validator(s).

A validator is a sync or async function that takes an HTMLElement that implements the Constraint Validation API and returns an (i18n) error validation message or an empty string when valid.

Refered here as HTMLElement_Validation, the element can be either a HTMLButtonElement, HTMLFieldSetElement, HTMLInputElement, HTMLOutputElement, HTMLSelectElement, or HTMLTextAreaElement.

/**
 * validator function
 * @param {HTMLElement_Validation} element The element to validate
 * @returns {string|Promise<string>} Empty string `""` when the element is valid, the i18n custom error `validationMessage` otherwise.
 */
type validator = (element: HTMLElement_Validation) => string | Promise<string>;

Example of a sync validator that constraint an input element to have a value with a non empty string:

function hasNonEmptyValue(element: HTMLElement_Validation): string {
  const value = (element as HTMLInputElement).value.trim();
  const errored = value === "";
  return errored ? "Text must not be empty" : "";
}

Usage of the customValidity directive

The reportValidity function is called upon the reportOn event type i.e change (default), blur, input.

In the example below, we use the directive on an input HTMLInputElement.

• With validators only

<input use:customValidity={[validator, validator]}>

• With validators and event options

<input use:customValidity={[validator, validator, {
  reportOn: "change" // default
}]}>

Usage of the validationMessage directive

This directive is useful to synchronize an element validationMessage with a Signal<string>.

A validationMessageSetter must be Type

Setter<string>

A validationMessageOptions must be Type

{
  resetOn?: keyof HTMLElementEventMap,
  setOn?: keyof HTMLElementEventMap,
}

• With validationMessageSetter only

<input use:validationMessage={validationMessageSetter}>

• With validationMessageSetter and options

<input use:validationMessage={[validationMessageSetter, {
  resetOn?: "change", // default
  setOn?: "invalid" // default
}]}>

Usage of the validation directive

This directive is the combination of the customValidty and validationMessage directive.

A validationOptions must be Type

{
  resetOn?: keyof HTMLElementEventMap,
  setOn?: keyof HTMLElementEventMap,
  reportOn?: keyof HTMLElementEventMap,
}

• With validationMessageSetter only

<input use:validation={validationMessageSetter}>

• With validationMessageSetter and options

<input use:validation={[validationMessageSetter, {
  resetOn?: "change", // default
  setOn?: "invalid", // default
}]}>

• With validationMessageSetter, validators and options

<input use:validation={[validationMessageSetter, [validator, validator], {
  resetOn?: "change", // default
  setOn?: "invalid", // default
  reportOn?: "change", // default
}]}>