Vue Custom Directives: how to share a variable between hooks?

I want to share a variable among custom directive's hooks.

Example:

Vue.directive('demo',{
  bind: function(el, binding, vnode) {
    const index = setInterval(/* ... */) //I have an "index" here
  },
  unbind: function(el, binding, vnode) {
    clearInterval(index) // I want to use "index" here
  }
})

How can I pass my value from bind to unbind?

P.S. The only workaround in my mind is to modify el in a way to attach an html-attribute "my-custom-index" in bind and to read it in unbind. But it's soooo hacky...

It seems to be a way to go to use html-attributes:

Vue.directive('demo',{
  bind: function(el, binding, vnode) { 
    //save
    el.setAttribute('x-my-attr', index);
  },
  unbind: function(el, binding, vnode) {
    //read
    const index = el.getAttribute('x-my-attr'); // The value is a string!
  }
})