The validity read-only property of the ElementInternals interface returns a ValidityState object which represents the different validity states the element can be in, with respect to constraint validation.

A ValidityState object.

NotSupportedError DOMException

Thrown if the element does not have its formAssociated property set to true.

The following example shows a custom checkbox component with formAssociated set to true, the value of validity.valid is logged to the console.

class CustomCheckbox extends HTMLElement {
  static formAssociated = true;

  constructor() {
    super();
    this.internals_ = this.attachInternals();
  }

  // …
}

window.customElements.define("custom-checkbox", CustomCheckbox);

let element = document.getElementById("custom-checkbox");
console.log(element.internals_.validity.valid);