JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" id="total_devis" />
<button name="click" onclick="calculTotal();">Click!!!</button>
<select id="id_tax">
  <option value="0">NingĂşn tax</option>
  <option value="1">IVA ES 10% (10.000%)</option>
  <option>IVA ES 21% (21.000%)</option>
  <option>IVA ES 4% (4.000%)</option>
</select>

JavaScript

function getTax(){
    if (false)       
        return 0;
    var taxesArray = ["0","10.000","21.000","4.000",];
    var selectedTax = document.getElementById("id_tax");
    var taxId = selectedTax.options[selectedTax.selectedIndex].value;
    console.log(taxId);
    return taxesArray[taxId];
}

function calculTotal (){
    var tax = getTax();
    console.log("tax: "+tax);
    var tot_out_ht = 23;
    var tot_ttc = tot_out_ht * ((tax/100) + 1);
    console.log("tot_ttc: "+tot_ttc);
    var shipp = 6.80;
    var total = tot_ttc + shipp >= 0 ? Math.round(total = (tot_ttc + shipp)*100)/100   : 0.00;
    document.getElementById("total_devis").value = total;
}