<select>
<optgroup label="分組1">
<option value="1">選項1</option>
<option value="2">選項2</option>
</optgroup>
<option value="3">選項3</option>
</select>

var select = document.createElement("select");
var option1 = document.createElement("option");
option1.value = 1;
option1.text = "選項1";
select.appendChild(option1);
var option2 = document.createElement("option");
option2.value = 2;
option2.text = "選項2";
select.appendChild(option2);
var option3 = document.createElement("option");
option3.value = 3;
option3.text = "選項3";
select.appendChild(option3);
document.body.appendChild(select);