Post deobfuscated Javascript code

Question 1

This is how it looks like after processing by the jsbeautifier:

function ____(_O0) {
    eval(unescape(_O0))
}
var l2 = window.opera ? 1 : 0;

function l3(l4) {
    l5 = /zc/g;
    l6 = String.fromCharCode(0);
    l4 = l4.replace(l5, l6);
    var l7 = new Array(),
        l8 = _1 = l4.length,
        l9, lI, il = 16256,
        _1 = 0,
        I = 0,
        li = '';
    do {
        l9 = l4.charCodeAt(_1);
        lI = l4.charCodeAt(++_1);
        l7[I++] = lI + il - (l9 << 7)
    } while (_1++ < l8);
    var l1 = new Array(),
        l0 = new Array(),
        Il = 128;
    do {
        l0[Il] = String.fromCharCode(Il)
    } while (--Il);
    Il = 128;
    l1[0] = li = l0[l7[0]];
    ll = l7[0];
    _l = 1;
    var l_ = l7.length - 1;
    while (_l < l_) {
        switch (l7[_l] < Il ? 1 : 0) {
            case 0:
                l0[Il] = l0[ll] + String(l0[ll]).substr(0, 1);
                l1[_l] = l0[Il];
                if (l2) {
                    li += l0[Il]
                };
                break;
            default:
                l1[_l] = l0[l7[_l]];
                if (l2) {
                    li += l0[l7[_l]]
                };
                l0[Il] = l0[ll] + String(l0[l7[_l]]).substr(0, 1);
                break
        };
        Il++;
        ll = l7[_l];
        _l++
    };
    if (!l2) {
        return (l1.join(''))
    } else {
        return li
    }
};
var lO = '';
for (ii = 0; ii < OO0O.length; ii++) {
    lO += l3(OO0O[ii])
};
if (naa) {
    document.write('<scr' + 'ipt>' + lO + '</sc' + 'ript>')
};

The next step would be to go through every variable and function and give them better names (left as an exercise for the reader).

Of course this code is not complete - eg. some functions never get called, some arrays are never populated etc.

What this code does is put every element of the OO0O array through the l3() function (that decrypt the text that was encrypted using some naive home-brew encryption algorithm), joins them together and evaluates it all by putting <script>result</script> into the DOM using document.write. Since the OO0O array is not defined in the code you quoted we can't know anything what it does, because the actual (encrypted) code is in the OO0O array.

Actually you don't really need to know how the encryption algorithm works at all since all you need to do is step through this code with a debugger and examine the l0 variable just before the call to document.write - and there you will have all of the actual code in a plain unencrypted form just before it is evaluated.

You can even put:

document.write = console.log;

just before that script is called and see all of the code printed to your console instead of being evaluated. I did it with the code that you posted in the pastebin in the update to your question and there is so many "undefined" strings in it that I think it's broken or incomplete or something went wrong with the charset encoding.

Where did you get it from, anyway?

Question 2

You can prettify your code:

http://jsbeautifier.org/

But then you're on your own really. Any sense of legibility, variable/function names, etc. are lost forever. The only way around this is to get the original source.