Jquery控制菜单，在所有菜单上显示箭头，即使是没有子菜单的菜单也是如此

<script src="https://cdn.jsdelivr.net/npm/jquery@3.6.0/dist/jquery.min.js"></script>

<ul id="menu">
  <li>菜单1</li>
  <li>菜单2
    <ul>
      <li>子菜单1</li>
      <li>子菜单2</li>
    </ul>
  </li>
  <li>菜单3</li>
</ul>

$(document).ready(function() {
  $('#menu li').append('<span class="arrow"></span>');
});

#menu li .arrow {
  display: inline-block;
  width: 0;
  height: 0;
  border-style: solid;
  border-width: 4px 4px 0 4px;
  border-color: #000 transparent transparent transparent;
  margin-left: 5px;
}

$(document).ready(function() {
  $('#menu li').hover(
    function() {
      $(this).children('ul').show();
    },
    function() {
      $(this).children('ul').hide();
    }
  );
});