The willValidate read-only property of the ElementInternals interface returns true if the element is a submittable element that is a candidate for constraint validation.

Elements that are barred from being candidates for constraint validation include those that have the attributes: disabled, hidden or readonly, input elements of type=button or type=reset, or any element that is a <datalist> element or has a <datalist> element ancestor.

true if the element is a candidate for constraint validation, otherwise false.

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 willValidate 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_.willValidate); // true