--- title: Constraint validation API slug: Web/API/Constraint_validation tags: - API - Constraint validation - Landing - NeedsTranslation - Reference - TopicStub translation_of: Web/API/Constraint_validation ---
{{apiref()}}

The Constraint Validation API enables checking values that users have entered into form controls, before submitting the values to the server.

Concepts and usage

Certain HTML form controls, such as {{HTMLElement("input")}}, {{HTMLElement("select")}} and {{HTMLElement("textarea")}}, can restrict the format of allowable values, using attributes like required and pattern to set basic constraints.

However, you may want to impose more complex constraints, or to provide clearer reporting of validation failures than the defaults. This can be done using the Constraint Validation API.

Note: Client-side constraint validation doesn't remove the need for validation on the server side. Even though client-side validation can prevent many common kinds of invalid values, invalid ones can still be sent by older browsers or by attackers trying to trick your web application. Therefore, you need to also validate input values on the server side, in a way that is consistent with what is done on the client side. Client side validation is a tool for giving quick feedback to the user. You should not rely on it to completely sanitize data received by the server.

Validation of constraints through the constraint validation API is done either on a single form element or at the form level, on the {{ HTMLElement("form") }} element itself.

Constraint validation interfaces

ValidityState
The ValidityState interface represents the validity states that a form control element can have, with respect to its defined constraints. Together, they help explain whether and why an element's value fails to validate.
invalid event
This event type is fired at a form control element if the element does not satisfy its constraints.

Extensions to other interfaces

The constraint validation API extends the interfaces for the form-associated elements listed below with a number of new properties and methods (elements that can have a form attribute that indicates their form owner):

Properties

validity
A read-only properity that returns a ValidityState object, whose properties represent validation errors for the value of that element.
validationMessage
A read-only property that returns an empty string if the element is not a candidate for constraint validation, or if the element's value is valid. If the element's value is not valid, it returns a localized validation message. This will be displayed in the UI if the element is the only form control with a validity problem; if a custom error message is set using setCustomValidity(), this will be shown.
willValidate
A read-only boolean property that returns true if the element is a candidate for constraint validation; and false otherwise. Elements with the HTMLObjectElement interface are never candidates for constraint validation. Others may be barred from constraint validation depending on specific conditions.

Methods

checkValidity()
Checks the element's value against its constraints. If the value is invalid, it fires an invalid event at the element and returns false; otherwise it returns true.
reportValidity()
Checks the element's value against its constraints and also reports the validity status; if the value is invalid, it fires an invalid event at the element, returns false, and then reports the validity status to the user in whatever way the user agent has available. Otherwise, it returns true.
setCustomValidity(message)
Sets a custom error message string to be shown to the user upon submitting the form, explaining why the value is not valid — when a message is set, the validity state is set to invalid. To clear this state, invoke the function with an empty string passed as its argument. In this case the custom error message is cleared, the element is considered valid, and no message is shown.

Examples

Take the following form:

<form>
  <label for="name">Enter username (upper and lowercase letters): </label>
  <input type="text" name="name" id="name" required pattern="[A-Za-z]+">
  <button>Submit</button>
</form>

The basic HTML form validation features will cause this to produce a default error message if you try to submit the form with either no valid filled in, or a value that does not match the pattern.

If you wanted to instead display custom error messages, you could use JavaScript like the following:

const nameInput = document.querySelector('input');
const form = document.querySelector('form');

nameInput.addEventListener('input', () => {
  nameInput.setCustomValidity('');
  nameInput.checkValidity();
});

nameInput.addEventListener('invalid', () => {
  if(nameInput.value === '') {
    nameInput.setCustomValidity('Enter your username!');
  } else {
    nameInput.setCustomValidity('Usernames can only contain upper and lowercase letters. Try again!');
  }
});

The example renders like so:

{{EmbedLiveSample('Examples')}}

In brief:

Note: Firefox supported a proprietary error attribute — x-moz-errormessage — for many versions, which allowed you set custom error messages in a similar way. This has been removed as of version 66 (see bug 1513890).

Specifications

Specification Status Comment
{{ SpecName('HTML WHATWG', 'forms.html#the-constraint-validation-api', 'constraint validation API') }} {{Spec2('HTML WHATWG')}}  
{{ SpecName('HTML5.1', 'sec-forms.html#the-constraint-validation-api', 'constraint validation API') }} {{Spec2('HTML5.1')}} No change from the previous snapshot {{SpecName('HTML5 W3C')}}.
{{ SpecName('HTML5 W3C', 'forms.html#the-constraint-validation-api', 'constraint validation API') }} {{Spec2('HTML5 W3C')}} First snapshot of  {{SpecName('HTML WHATWG')}} containing this interface.

See also