如何让我的模式框在弹出框后面淡入淡出？

<button id="openModal">打开模态框</button>

<div id="myModal" class="modal">
  <div class="modal-content">
    <span class="close">&times;</span>
    <p>这是一个模态框。</p>
  </div>
</div>

.modal {
  display: none;
  position: fixed;
  z-index: 1;
  left: 0;
  top: 0;
  width: 100%;
  height: 100%;
  overflow: auto;
  background-color: rgba(0, 0, 0, 0.4);
}

.modal-content {
  background-color: #fefefe;
  margin: 15% auto;
  padding: 20px;
  border: 1px solid #888;
  width: 80%;
}

.close {
  color: #aaa;
  float: right;
  font-size: 28px;
  font-weight: bold;
}

.close:hover,
.close:focus {
  color: black;
  text-decoration: none;
  cursor: pointer;
}

var modal = document.getElementById("myModal");
var btn = document.getElementById("openModal");
var span = document.getElementsByClassName("close")[0];

btn.onclick = function() {
  modal.style.display = "block";
  modal.style.opacity = 1;
};

span.onclick = function() {
  modal.style.opacity = 0;
  setTimeout(function() {
    modal.style.display = "none";
  }, 500); // 过渡时间为0.5秒
};

window.onclick = function(event) {
  if (event.target == modal) {
    modal.style.opacity = 0;
    setTimeout(function() {
      modal.style.display = "none";
    }, 500); // 过渡时间为0.5秒
  }
};