InterfacegetElemetsByTagName

<!DOCTYPE html>
<html>
<head>
    <title>Davitra Eka S XI RPL 2</title>
</head>
<body>
    <input type="text" name="nilai">
    <input type="button" id="btnAmbilNilai" value="Ambil nilai" onclick="ambilNilai();">
    <input type="text" name="hasil">
    <script type="text/javascript">
        function ambilNilai()
        {
            var obj1 = document.getElementsByTagName('input')[0];
            var obj2 = document.getElementsByTagName('input')[1];
            var obj3 = document.getElementsByTagName('input')[2];
            var s1 = obj1.value;
            var s2 = obj2.value;
            obj3.value = s1 + s2 ;
        }
    </script>
</body>
</html>