Create Tags

createTags.js

function createTags(element) {
  var all = document.getElementsByTagName(element);
  var elements = []
for (var i=0, max=all.length; i < max; i++) {
     var el = all[i].getAttribute('data-tags')
     elements.push(el.split(','));
}

function flatten(arr) {
  return arr.reduce(function (flat, toFlatten) {
    return flat.concat(Array.isArray(toFlatten) ? flatten(toFlatten) : toFlatten);
  }, []);
}
elements = flatten(elements);

elements = elements.filter(function(item, pos) {
  return elements.indexOf(item) == pos;
})

elements = elements.sort()
console.log(elements);
}

createTags('li')