P43

// In Firefox, press Ctrl+Shift+K then copy, paste and run.

function encode(input)
{
    var output = "";

    for(var i = 0, length = input.length; i < length; i++)
    {
        var ch = input.charCodeAt(i);
        var chNew = ch;

        if(ch >= 65 && ch <= 90)
        {
            chNew = 90 - ch + 65;
        }
        else if(ch >= 97 && ch <= 122)
        {
            chNew = 122 - ch + 97;
        }
        else if(ch >= 48 && ch <= 57)
        {
            chNew = 57 - ch + 48;
        }
        output += String.fromCharCode(chNew);
    }

    return output;
}

encode("AbCzZ 123409");