JSFiddle - React, Tailwind, and code Playground

HTML

<select id="s_um" name="s_um">
                <option id="um_1" value="0"> 0 </option>
                <option id="um_2" value="1"> 1 </option>
                <option id="um_3" value="2"> 2 </option>
                <option id="um_4" value="3"> 3 </option>
                </select>

<select id="s_dois" name="s_dois">
                <option id="dois_1" value="0"> 0 </option>
                <option id="dois_2" value="1"> 1 </option>
                <option id="dois_3" value="2"> 2 </option>
                <option id="dois_4" value="3"> 3 </option>
                </select>

JavaScript

$('#s_dois').change(function() {

var soma = parseInt($('#s_um option:selected').val()) + parseInt($('#s_dois option:selected').val());

alert(soma);

})