SOURCE

function heidiDecode(hex) {
    var str = '';
    var shift = parseInt(hex.substr(-1));
    hex = hex.substr(0, hex.length - 1);
    for (var i = 0; i < hex.length; i += 2) {
        str += String.fromCharCode(parseInt(hex.substr(i, 2), 16) - shift);
    }
    return str;
}
console.log(heidiDecode("657768696A6B4"));