如何让下拉按钮的弹出按钮在选择一个项目后自动关闭？

<select id="dropdown">
  <option value="project1">项目1</option>
  <option value="project2">项目2</option>
  <option value="project3">项目3</option>
</select>

<div id="popup">
  <!-- 弹出按钮的内容 -->
</div>

var dropdown = document.getElementById("dropdown");
var popup = document.getElementById("popup");

dropdown.addEventListener("change", function() {
  if (dropdown.value !== "") {
    popup.style.display = "block";
  } else {
    popup.style.display = "none";
  }
});

#popup {
  position: absolute;
  top: 30px;
  left: 0;
  display: none;
  /* 其他样式设置 */
}