Javascript version of Caesar shift

caesarShift.js

function crypt(message, shiftText, isDecrypt) {
    'use strict';
    return caesarShift(message.toUpperCase(), getShift(shiftText, isDecrypt));
}
function getShift(shiftText, isDecrypt) {
    var shift;
    if (isNaN(shiftText) && !/^-?\d+$/.test(shift)) {
        console.log("Shift is not an integer");
        return 0;
    }
    shift = parseInt(shiftText, 10);
    shift = (shift < 0) ? -1 * shift : shift;
    shift = (shift >= 26) ? shift % 26 : shift;
    if (isDecrypt) {
        shift = (26 - shift) % 26;
    }
    return shift;
}
function caesarShift(text, shift) {
    var result = "",
        i,
        c,
        aCode = "A".charCodeAt(0),
        zCode = "Z".charCodeAt(0);
    for (i = 0; i < text.length; i++) {
        c = text.charCodeAt(i);
        if (c >= aCode && c <=  zCode) {
            // encrypt or decrypt
            result += String.fromCharCode((c - aCode + shift) % 26 + aCode);
        } else {
            // copy
            result += text.charAt(i);
        }
    }
    return result;
}