<div id="wrapper">
        <textarea id="text" cols="30" rows="10"></textarea>
        <p id="result"></p>
    </div>

    <button type="button" id="button-szyfruj">Szyfruj</button>
    <button type="button" id="button-deszyfruj">Deszyfruj</button>

    <script>
        const encrypt = (str, amount) => {

            let result = '';

            for (let i = 0; i < str.length; i ++) {

                let currentLetter = str.charCodeAt(i);

                let c = (97 + i - 97 + amount) % 26 + 97;

                result += String.fromCharCode(c);

            }

            return result;

        };

        const decrypt = (str, amount) => {
          return encrypt(str, 26 - amount % 26);

        };

        document.getElementById('button-szyfruj').onclick = () => {

            document.getElementById('result').innerHTML = '';

            let zaszyfrowane = encrypt(document.getElementById('text').value, 1);

            document.getElementById('result').innerHTML = zaszyfrowane;

        }

        document.getElementById('button-deszyfruj').onclick = () => {

            document.getElementById('result').innerHTML = '';

            let zaszyfrowane = decrypt(document.getElementById('text').value, 1);

            document.getElementById('result').innerHTML = zaszyfrowane;

        }

    </script>