<script>
    function myFunction() {
        var button = document.getElementsByClassName("dia");
        button.classList.add('active');
    }

function myFunction() {
    var button = document.getElementsByClassName("dia")[0];
    button.classList.add('active');
}

function myFunction() {
  var button = document.getElementsByClassName("dia");
  button.addClass("active");
}

function myFunction() {
  $('.dia').addClass("active");
}