JSFiddle - React, Tailwind, and code Playground

HTML

<select>
    <optgroup label="group1">
        <option>option 1</option>
        <option>option 2</option>
    </optgroup>
    <optgroup label="group2">
        <option>option 3</option>
        <option>option 4</option>
    </optgroup>
</select>

JavaScript

var o = {};

$('optgroup').each(function() {
    o[this.label] = $(this).find('option').map(function() {
        return $(this).text();
    }).get();
});

console.log(JSON.stringify(o));