JSFiddle - React, Tailwind, and code Playground

HTML

<div class="col-md-9"> 
    <input class="form-control required" type="text" name="montantHT" id="montantHT" value="" role="input" aria-required="true" />
</div>

<select style="width:250px" class="select2 required taux" id="tauxTVA" name="tauxTVA" aria-required="true" >
    <option value='.055'>5.5</option>
    <option value='.08'>8</option>
    <option value='.105'>10.5</option>
</select><br/>

    <input class="form-control required" type="text" name="montantTVA" id="montantTVA" value="" role="input" aria-required="true" />

JavaScript

$('#tauxTVA').on('change',calcul);
function calcul(){
    var tva = + $('#tauxTVA').val();
    var ht = $('#montantHT').val();
   var result = ht*tva;
   console.log(ht);
   console.log(tva);
     console.log(result);
   $('#montantTVA').val(result);
}