<input id="tal1" type="text" placeholder="Skriv ett nummer">
<input id="tal2" type="text" placeholder="Skriv ett nummer">
<input id="tal3" type="text">
<button type="button" onclick="skrivUt()">skriv ut summan</button>
<script>
const skrivUt = () =>{
const tal1 = document.getElementById("tal1").value;
const tal2 = document.getElementById("tal2").value;
const summa = (parseFloat(tal1)) + (parseFloat(tal2));
document.getElementById("tal3").value = summa;
};
</script>