ElementsByClassName('close')[0];
        // When the user clicks the button, open the popup 
        btn.onclick = function () {
          popup.style.display = 'block';
        }
        // When the user clicks on <span> (x), close the popup
        span.onclick = function () {
          popup.style.display = 'none';
        }
        // When the user clicks anywhere outside of the popup, close it
        window.onclick = function (event) {
          if (event.target == popup) {
            popup.style.dis