JSFiddle - React, Tailwind, and code Playground

HTML

<form>
           <select id="dia"></select>
    </form>

JavaScript

window.onload = fillDropDown;

function fillDropDown() {

    var ddl = document.getElementById("dia");
    var theOption;
    var x;
    var i;

    for (i = 1; i < 32; i++) {
        x = i + 1;
        theOption = document.createElement('option');
        theOption.label = x;
        theOption.value = x;
        ddl.add(theOption, null);
    }


}