JSFiddle - React, Tailwind, and code Playground

HTML

<input type="button" id="but" value="get all secondary values">

<select id="select2" multiple="true">
    <option value="1" data-value2="1.1">this</option>
    <option value="2" data-value2="1.2">that</option>
    <option value="3" data-value2="1.3">other</option>
</select>

JavaScript

$('#but').click(function(){
    $('#select2 :selected').each(function(){
        alert($(this).text() + " : " + $(this).data('value2')); 
    });
});