calcSum

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
    <form>


        num1: <input type="text" name="num1" /> <br>


        num2: <input type="text" name="num2" /> <br>


        sum: <input type="text" name="sum" /> <br>


        <input type="button" value="Sum" onclick="calcSum()" />


    </form>

    <script>
    function calcSum() {


        let num1 = document.getElementsByName('num1')[0].value;


        let num2 = document.getElementsByName('num2')[0].value;


        let sum = Number(num1) + Number(num2);


        document.getElementsByName('sum')[0].value = sum;
    }
    </script>
</body>
</html>