JSFiddle - React, Tailwind, and code Playground

by ewfes

HTML

<select id="summa">
<option value='1'>1</option>
<option value='2'>2</option>
<option value='3'>3</option>
<option value='4'>4</option>
<option value='5'>5</option>
<option value='6'>6</option>
<option value='7'>7</option>
<option value='8'>8</option>
<option value='9'>9</option>
<option value='10'>10</option>
</select>
<select id="summa1">
<option value='1'>1</option>
<option value='2'>2</option>
<option value='3'>3</option>
<option value='4'>4</option>
<option value='5'>5</option>
<option value='6'>6</option>
<option value='7'>7</option>
<option value='8'>8</option>
<option value='9'>9</option>
<option value='10'>10</option>
</select>

<br>Сумма<div id="itogo"></div>

JavaScript

var sum = document.getElementById('summa');
var sum = document.getElementById('summa1');
var total = document.getElementById('itogo');

totalInt = parseInt(total.innerHTML.innerHTML);

sum.onchange = function() {
    total.innerHTML = parseInt(summa1.value) + parseInt(summa.value);
}