index.html

<!DOCTYPE html>
<html>
<head>
  <title>Custom Directives</title>
  <script src="https://unpkg.com/vue"></script>
</head>
<body>
  <div id="app">
    <h2>Click Outside</h2>
    <div v-click-outside="clickedOutside" class="box">
      Here is some text
    </div>
  </div>

  <script type="text/javascript">
    new Vue({
      el: "#app",
      data: {},
      directives: {
        clickOutside: {
          bind (el, binding) {
            el.__ClickOutsideHandler__ = event => {
              if (!(el === event.target || el.contains(event.target))) {
                binding.value(event)
              }
            }
            document.body.addEventListener('click', el.__ClickOutsideHandler__)
          },
          unbind (el) {
            document.body.removeEventListener('click', el.__ClickOutsideHandler__)
          }
        }
      },
      methods: {
        clickedOutside () {
          alert('you clicked outside 👏')
        }
      }
    })
  </script>
  <style type="text/css">
    .box {
      border: 2px solid purple;
      width: 200px;
      height: 100px;
    }
  </style>
</body>
</html>