How is encoding handled correctly during copy-paste between programs

To figure out how this code works, I would recommend expanding it, and assigning intermediate local variables to store various results, so you can watch how they change as you step through.

Before assuming that you're looking at you're looking at user defined parameters or functions, you might want to look the functions up in a standard library. If you do look them up, you'll notice all the functions used are standard library functions for Javascript.

For example, with the code you've got, I'd do something like this:

function dF(s) {
    var s1 = unescape(s.substr(0, s.length - 1));
    var t = '';
    for (i = 0; i < s1.length; i++) {
        var cCode = s1.charCodeAt(i);
        var key = s.substr(s.length - 1, 1);
        var temp= String.fromCharCode( cCode - key );
        t += temp;
    }
    var unescaped = unescape(t);
    document.write(unescaped);
}

NOTE:

This question smells a lot like a homework question. It's generally frowned upon to ask us to do your homework for you, and it's usually good form to at least warn that it's homework you're doing.

Files generally indicate their encoding with a file header. There are many examples here. However, even reading the header you can never be sure what encoding a file is really using.

For example, a file with the first three bytes 0xEF,0xBB,0xBF is probably a UTF-8 encoded file. However, it might be an ISO-8859-1 file which happens to start with the characters . Or it might be a different file type entirely.

Notepad++ does its best to guess what encoding a file is using, and most of the time it gets it right. Sometimes it does get it wrong though - that's why that 'Encoding' menu is there, so you can override its best guess.

For the two encodings you mention:

• The "UCS-2 Little Endian" files are UTF-16 files (based on what I understand from the info here) so probably start with 0xFF,0xFE as the first 2 bytes. From what I can tell, Notepad++ describes them as "UCS-2" since it doesn't support certain facets of UTF-16.
• The "UTF-8 without BOM" files don't have any header bytes. That's what the "without BOM" bit means.