Tracking DOM focus

focused.js

function updateFocus(elm) {
  const focusedClass = 'focused';
  const currentlyFocused = document.querySelector('.' + focusedClass);
  if (currentlyFocused != null) {
    currentlyFocused.classList.remove(focusedClass);
  }
  if (elm) {
    elm.classList.add(focusedClass);
  }
}

document.body.addEventListener('focusin', e => {
  updateFocus(e.target);
});

document.body.addEventListener('focusout', e => {
  if (document.activeElement === document.body) {
    updateFocus(null);
  }
});