sum
HTML
<input id="a" type="number" value="123">
<input id="b" type="number" value="456">
<button type="button" id="run">sum</button>
<div>sum: <span id="sum"></span></div>JavaScript
// Yes I know using ids like this is bad practice
run.onclick = () => {
sum.textContent = parseFloat(a.value) + parseFloat(b.value);
};