<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Finn vokaler</title>
</head>

<body>
    <button onclick="compareLetters('Dette er også en test æøå')">Finn vokaler</button>
    <div id="letters">inputString</div>
    <div id="vokaler">vokaler</div>



    <script>
        function compareLetters(inputString) {
            let vovels = 0;
            var theChar;
            document.getElementById('letters').innerHTML = inputString;
            var charList = ['a', 'e', 'i', 'u', 'o', 'æ', 'ø', 'å'];
            for (let i = 0; i <= inputString.length; i++) {
                theChar = inputString.charAt(i);
                if (charList.indexOf(theChar) != -1) {
                    vovels++
                }
            }
            document.getElementById('vokaler').innerHTML = `${vovels}`;
        }
    </script>
</body>

</html>