/*
 * This is a handy tool when it comes to accessibility-proofing widgets on the web.
 * I use this whenever some element with focus is removed from the screen, and I
 * elsewise don't know which element should become focused.
 *
 * When we remove elements on the screen with focus, it is important that we give
 * another element focus instead, so that screen readers can inform the user that
 * something has happened, and so that keyboard navigation functions optimally.
 *
 * For more about operating a web-page with the keyboard and focus order, see
 * WCAG 2.1: https://www.w3.org/TR/WCAG21/#focus-order
 * 
 * A use-case for this, is for instance if the user chooses to remove a widget on
 * the screen, then perhaps the previous widget (or an element within it) should
 * be focused instead.
 *
 * Another use-case would be a form, where the user can insert and remove form
 * fields. When a form field is removed, perhaps the previous field should become
 * focused? Some might want the _next_ field to be focused instead, in that case,
 * it should be fairly straightforward to alter this implementation.
 *
 */


/**
 * This find the previous focusable sibling/element in the DOM, relative to the
 * given one (E), with the following algorithm:
 *
 * For an element E:
 *
 * 0. If E is null or undefined, return;
 * 1. Get the previous sibling element (`E.previousElementSibling`), we call this `PES`.
 * 1b. If PES is not found, set the elements parent (`E.parentElement`) as `E`, and go to step 0.
 * 2. Collext `PES`'s non-disabled focusable descendants, we call this `focusableChildren`
 * 2b. If length of `focusableChildren` is 0, set PES to E and go to step 0.
 * 3 (_optional_): If one of the focusableChildren is an input, return the last input element in that list.
 * 4. Return this last element in the list focusableChildren.
 *
 * Here implemented as a recursive function:
 */
function findPreviousFocusableElement(E, preferInputElement) {
  // Step 0
  if (!E) return;

  // Step 1
  const PES = E.previousElementSibling;

  // Step 1b
  if (PES === null) {
    return findPreviousFocusableElement(E.parentElement);
  }

  // Step 2
  const focusableChildren = Array.from(
    PES.querySelectorAll('input, button, [tabindex="0"]')
  ).filter((n) => !n.disabled);

  // Step 2b
  if (focusableChildren.length === 0) {
    return findPreviousFocusableElement(PES);
  }

  // Step 3 (optional)
  if (preferInputElement) {
    const inputs = focusableChildren.filter((n) => n.nodeName === 'INPUT');
    if (inputs.length > 0) {
      return inputs[inputs.length - 1];
    }
  }

  // Step 4
  return focusableChildren[focusableChildren.length - 1];
}


/**
 * This is how you could use the above function.
 * The argument given here, would be the element about to be removed from the DOM tree.
 */
function givePreviousElementFocus(element) {
  if (!element) return; // Always start with paranoia check
  
  const last = findPreviousFocusableElement(e, true);
  
  if (last) {
    // Remember to also invoke API functions in your current CSS framework/
    // design system that is needed to visually show that this element has focus!
    // One way to make this work in many systems, would be to call `last.click()`
    // instead, but that could also have some unforeseen consequences.
    last.focus();
  }
}