ㄧ.分離式下拉式選單

<div class="component" id="demo">
  <div class="btn-group">
    <a href="#" class="btn btn-success">Dropdown</a>
    <button class="btn btn-success dropdown-toggle dropdown-toggle-split" data-toggle="dropdown">
      <span class="sr-only">Dropdown</span> 
    </button>
    <div class="dropdown-menu">
      <div class="dropdown-header">Web language</div>
      <a href="#" class="dropdown-item">HTML</a>
      <a href="#" class="dropdown-item">CSS</a>
      <a href="#" class="dropdown-item">JavaScript</a>
      <div class="dropdown-divider"></div>
      <a href="#" class="dropdown-item">More</a>
    </div>
  </div>
</div>





.demo.component{
  padding: 50px;
}

重點：

1.將按鈕上的a跟button調換
2.button添加.dropdown-toggle-split
3.button在添加子元素span.sr-only，將內容的文字放入是不會顯示文字的
4.最後將.Dropdown改成.btn-group

codepen
參考：bootstrap4