javascript lightbox code

<div class="row">
    <div class="column">
        <img src="" onclick="openModal();currentSlide(1)" class="hover-shadow">
    </div>
    <div class="column">
        <img src="" onclick="openModal();currentSlide(2)" class="hover-shadow">
    </div>
    <div class="column">
        <img src="" onclick="openModal();currentSlide(3)" class="hover-shadow">
    </div>
    <div class="column">
        <img src="" onclick="openModal();currentSlide(4)" class="hover-shadow">
    </div>
</div><!-- close row -->
<div id="Modal" class="modal">
    <span class="close cursor" onclick="closeModal()">&times;</span><!-- the "x" icon to close the lightbox once opened -->
    <div class="modal-content">
        <div class="mySlides">
            <div class="numbertext">1 / 4</div><!-- numbertext class is for counting the slides, as an indicator for the user -->
            <img src="" class="photo">
        </div>
        <div class="mySlides">
            <div class="numbertext">2 / 4</div>
            <img src="" class="photo">
        </div>
        <div class="mySlides">
            <div class="numbertext">3 / 4</div>
            <img src="" class="photo">
        </div>
        <div class="mySlides">
            <div class="numbertext">4 / 4</div>
            <img src="" class="photo"><br>
        </div>
        <!-- thumbnail controls -->
        <div class="column">
        <img src="" onclick="currentSlide(1)" alt="" class="thumb">
        </div>
        <div class="column">
            <img src="" onclick="currentSlide(2)" alt="" class="thumb">
        </div>
        <div class="column">
            <img src="" onclick="currentSlide(3)" alt="" class="thumb">
        </div>
        <div class="column">
            <img src="" onclick="currentSlide(4)" alt="" class="thumb">
        </div>
    </div>
</div>
<script>
function openModal() {
    document.getElementById('Modal').style.display = "block";
}
function closeModal() {
    document.getElementById('Modal').style.display = "none";
}
var slideIndex = 1;
showSlides(slideIndex);

function plusSlides(n) {
  showSlides(slideIndex += n);
}

function currentSlide(n) {
  showSlides(slideIndex = n);
}

function showSlides(n) {
  var i;
  var slides = document.getElementsByClassName("mySlides");
  var dots = document.getElementsByClassName("demo");
  var captionText = document.getElementById("caption");
  if (n > slides.length) {slideIndex = 1}
  if (n < 1) {slideIndex = slides.length}
  for (i = 0; i < slides.length; i++) {
      slides[i].style.display = "none";
  }
  for (i = 0; i < dots.length; i++) {
      dots[i].className = dots[i].className.replace(" active", "");
  }
  slides[slideIndex-1].style.display = "block";
  dots[slideIndex-1].className += " active";
  captionText.innerHTML = dots[slideIndex-1].alt;
}
</script>