How do I check if a form element (input or button, etc) is disabled by itself or a parent fieldset in Javascript?

Question

According to the specs:

A form control is disabled if any of the following are true:

• the element is a button, input, select, textarea, or form-associated custom element, and the disabled attribute is specified on this element (regardless of its value); or

• the element is a descendant of a fieldset element whose disabled attribute is specified, and is not a descendant of that fieldset element's first legend element child, if any.

The element itself if not disabled, the disabled property is still false. How do I determine if an element is disabled by its parent?

console.log(document.querySelector("#txt").disabled)
console.log(document.querySelector("#btn").disabled)

<fieldset disabled>
  <input id="txt" />
</fieldset>

<fieldset disabled>
  <button type="button" id="btn">Button</button>
</fieldset>

Answer 1

Beside moving up the DOM tree to find a disabled fieldset, I found out CSS :disabled selector works as well:

const txts = [1,2,3].map(q => document.querySelector("#txt" + q));

for (const t of txts) {
  console.log('t.matches(:disabled):', t.matches(":disabled"));
}

function isDisabled(el) {
  if (el.disabled) { return true; }

  while (el = el.parentElement) {
    if (el instanceof HTMLFieldSetElement && el.disabled) { return true; }
  }
  
  return false;
}

for (const t of txts) {
  console.log('isDisabled(t):', isDisabled(t));
}

<fieldset disabled>
  <input id="txt1" />
</fieldset>

<fieldset>
  <input id="txt2" disabled />
</fieldset>

<fieldset>
  <input id="txt3" />
</fieldset>