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

Certain HTML form controls, such as <input>, <select> and <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.

Validation of constraints through the constraint validation API is done either on a single form element or at the form level, on the <form> element itself.

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.

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):

• HTMLButtonElement
• HTMLFieldsetElement
• HTMLInputElement
• HTMLObjectElement
• HTMLOutputElement
• HTMLSelectElement
• HTMLTextAreaElement

Properties

validity

A read-only property 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() HTMLFormElement method

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.

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 value 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');

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:

In brief:

• We check the valid state of the input element every time its value is changed by running the checkValidity() method via the input event handler.
• If the value is invalid, an invalid event is raised, and the invalid event handler function is run. Inside this function we work out whether the value is invalid because it is empty, or because it doesn't match the pattern, using an if() block, and set a custom validity error message.
• As a result, if the input value is invalid when the submit button is pressed, one of the custom error messages will be shown.
• If it is valid, it will submit as you'd expect. For this to happen, the custom validity has to be cancelled, by invoking setCustomValidity() with an empty string value. We therefore do this every time the input event is raised. If you don't do this, and a custom validity was previously set, the input will register as invalid, even if it currently contains a valid value on submission.

• <input>
• <select>
• <textarea>
• ValidityState's properties: badInput, customError, patternMismatch, rangeOverflow, rangeUnderflow, stepMismatch, tooLong, tooShort, typeMismatch, valid, and valueMissing.