Opening multiple modal boxes on one page

I have a page that needs to have different modal boxes open when their corresponding link on the page is clicked. I've got the structure and javascript to work for ONE window, but I can't get it to work for more than one. I am thinking I need to loop through each modal box...but can't quite figure out the syntax.

This is for a client's WordPress site, and I'm using Advanced Custom Fields to populate the content. The page isn't published yet, so here's a codepen that shows my code: http://codepen.io/FelixB/pen/EPvEVG

The code:

var modal = document.getElementsByClassName('modal-window');

var btn = document.getElementsByClassName("click-to-open");

var span = document.getElementsByClassName("close")[0];

for (var i = 0; i < btn.length; i++) {
  var thisBtn = btn[i]
  thisBtn.onclick = function() {
    alert("hello");

    //modal.style.display = "block";
  }
}

span.onclick = function() { 
  modal.style.display = "none";
}

window.onclick = function(event) {

}

You must make modals unique to be able to select them later. One way to do this is through id.

<div id="modal-window-one" class="modal-window modal"></div>
<div id="modal-window-two" class="modal-window modal"></div>

You need to define which button which window should open. One possible solution for this is through data-attributes

<div class="click-to-open" data-modal="modal-window-one"> //will open modal one
<div class="click-to-open" data-modal="modal-window-two"> //will open modal two

And then - event:

var btn = document.getElementsByClassName("click-to-open");

for (var i = 0; i < btn.length; i++) {
  var thisBtn = btn[i];
  thisBtn.addEventListener("click", function(){
    var modal = document.getElementById(this.dataset.modal);
    modal.style.display = "block";
}, false);

Again - this is one possible solution.