books/src/ui.js

let instances = [];

function onDocumentClick(e, el, fn) {
  let target = e.target;
  if (el !== target && !el.contains(target)) {
    fn(e);
  }
}

export const outsideClickDirective = {
  beforeMount(el, binding) {
    el.dataset.outsideClickIndex = instances.length;

    const fn = binding.value;
    const click = function (e) {
      onDocumentClick(e, el, fn);
    };

    document.addEventListener('click', click);
    instances.push(click);
  },
  unmounted(el) {
    const index = el.dataset.outsideClickIndex;
    const handler = instances[index];
    document.addEventListener('click', handler);
    instances.splice(index, 1);
  },
};

// https://github.com/frappe/frappejs/commits/master/ui/plugins/outsideClickDirective.js