JSFiddle - React, Tailwind, and code Playground

HTML

<select name="PERIOD" style="font-size:10px;">
  <option value="">Не выбрано</option>
  <optgroup label="Все периоды"> 
      <option value="1">05.01.2012</option>
      <option value="2">12.02.2012</option>
      <option value="3">26.06.2012</option>
  </optgroup>
</select>    
<button type="button" onclick="javascript: createpost();">Сформировать</button>

JavaScript

function createpost() {
    var id_period = $('select[name="PERIOD"]').val();
    var sel=$('select[name=PERIOD]')[0];
    console.log(sel.options[sel.selectedIndex].text);
}