How to create an optgroup with a label on the select?

Question

JavaScript

Hello.

On the form I create a form element of type select

var select = document.createElement("select");
        select.name="main_select_region";
        //select.className="selectpicker show-tick form-control";
        select.setAttribute('class', 'selectpicker');
        select.setAttribute('data-live-search', 'true');
        select.setAttribute('data-size', '15');

I dynamically add to it in a cycle from JSON

for (var i = 0; i < jsondata.regions.length; i++) {
select.options[i]=new Option(jsondata.regions[i].name, jsondata.regions[i].name);
...

How to add to it ? <optgroup label="Группа1">