caesar cipher

<!DOCTYPE html>
<html>

<body>
    <form action="#" method="get" onsubmit="return false;" _lpchecked="1">
        <label for="text">Text:</label>
        <textarea id="text" cols="50" rows="10">THE QUICK BROWN FOX JUMPED OVER THE LAZY DOG.</textarea>
        <br>
        <label for="shift">Shift:</label>
        <input type="text" id="shift" value="13" />
        <br>
        <input type="button" value="Encrypt" onclick="doCrypt(false);" />
        <input type="button" value="Decrypt" onclick="doCrypt(true);" />
    </form>
    <script>
    function doCrypt(isDecrypt) { // this func figures out the shift value, and also the text
        var shift = parseInt(document.getElementById("shift").value); // find how many chars to shift by.
        //var shift = shiftText, 10; // parsing the number from the text, is now done by the prev line
        if (shift < 0 || shift >= 26) { //make sure shift from 0-26
            alert("Shift is out of range");
            return; //break the function
        }
        if (isDecrypt) // if its already decrypted
            shift = (26 - shift) % 26; // figure out what the shift was.
        var textElem = document.getElementById("text"); // figure out what the text actually is.
        textElem.value = caesarShift(textElem.value, shift); // un-decrypt it.
    }

    function caesarShift(text, shift) {
        var result = ""; // used to display the en/de crypted text
        for (var i = 0; i < text.length; i++) { // a for loop to make sure that the text is fully changed.
            var c = text.charCodeAt(i); // used to determine what the character is according to the computer
            if (c >= 65 && c <= 90) result += String.fromCharCode((c - 65 + shift) % 26 + 65); // Uppercase
            else if (c >= 97 && c <= 122) result += String.fromCharCode((c - 97 + shift) % 26 + 97); // Lowercase
            else result += text.charAt(i); // Copy
        }
        return result; // display the result in the text box that text was entered into
    }
    </script>
</body>

</html>