<select id="mySelect">
</select>

<script>
  var select = document.getElementById("mySelect");

  // 创建一个分组1
  var group1 = document.createElement("select");
  group1.label = "分组1";

  // 创建选项1
  var option1 = document.createElement("option");
  option1.value = "1";
  option1.text = "选项1";
  group1.appendChild(option1);

  // 创建选项2
  var option2 = document.createElement("option");
  option2.value = "2";
  option2.text = "选项2";
  group1.appendChild(option2);

  // 将分组1添加到<select>中
  select.appendChild(group1);

  // 创建一个分组2
  var group2 = document.createElement("select");
  group2.label = "分组2";

  // 创建选项3
  var option3 = document.createElement("option");
  option3.value = "3";
  option3.text = "选项3";
  group2.appendChild(option3);

  // 创建选项4
  var option4 = document.createElement("option");
  option4.value = "4";
  option4.text = "选项4";
  group2.appendChild(option4);

  // 将分组2添加到<select>中
  select.appendChild(group2);
</script>