Simple as cake modal

index.html

<div class="modal" data-modal-id="foo">
  <span onclick="Modal.close('foo')">&times;</span>
  <p>Lorem ipsum..</p>
</div>

<script>
  Modal.open('foo')
</script>

Modal.js

var Modal = {
  open: function(id) { 
    document.querySelector('.modal[data-modal-id="' + id + '"]').classList.add('active')
  },
  close: function(id) {
    document.querySelector('.modal[data-modal-id="' + id + '"]').classList.remove('active')
  }
}