Bootstrap 4下拉导航栏使用加号和减号更改插入符号5

<div class="dropdown">
  <button class="btn btn-secondary dropdown-toggle" type="button" id="dropdownMenuButton" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
    Dropdown button
  </button>
  <div class="dropdown-menu" aria-labelledby="dropdownMenuButton">
    <a class="dropdown-item" href="#">Action</a>
    <a class="dropdown-item" href="#">Another action</a>
    <a class="dropdown-item" href="#">Something else here</a>
  </div>
</div>

/* 隐藏默认的下拉箭头 */
.dropdown-toggle::after {
  display: none;
}

/* 添加加号和减号的样式 */
.dropdown-toggle.collapsed::after {
  content: '+';
}

.dropdown-toggle:not(.collapsed)::after {
  content: '-';
}

$(document).ready(function(){
  $('.dropdown-toggle').on('click', function(e){
    $(this).toggleClass('collapsed');
  });
});