export type DialogFocusFallback = () => HTMLElement | null;
type DialogFocusTarget = HTMLElement | null;

function isUsableFocusTarget(target: DialogFocusTarget): target is HTMLElement {
  const ownerDocument = target?.ownerDocument;

  return Boolean(
    target?.isConnected &&
    ownerDocument &&
    target !== ownerDocument.body &&
    target !== ownerDocument.documentElement,
  );
}

/**
 * Return focus to the control that opened a dialog while it still exists. If a
 * successful action removed that control, move focus to a persistent settings
 * target instead of leaving it on the document body.
 */
export function restoreDialogFocus(
  previouslyFocused: HTMLElement | null,
  getFallback?: DialogFocusFallback,
): void {
  let target = previouslyFocused;
  if (!isUsableFocusTarget(target)) {
    target = getFallback?.() ?? null;
  }

  if (!isUsableFocusTarget(target)) {
    return;
  }

  target.focus({ preventScroll: true });
}