lookup.js

var uid = require('uid');
var data = require('data');

function hash(el) {
  var d = data(el);
  var hash = d.get('mylib:uid');
  if (hash) return hash; 
  var id = uid();
  d.set('mylib:uid', id);
  return id;
}

var lookup = {};
var el = document.querySelector('.some-el');

lookup[hash(el)] = 'some element data';