Decoding a url-encoded windows-1251 (cp1251) strin

I have faced a problem, unfortunately, I have not found a correct solution: I need to decode url-slice that is encoded with windows-1251 (cp1251).

I know there are theese methods - decodeURI() and decodeURIComponent(), but they work for UTF-8 only (as I have understood). A solution that I found uses deprecated methods escape() and unescape().

For example, there is sequence:

%EF%F0%EE%E3%F0%E0%EC%EC%E8%F0%EE%E2%E0%ED%E8%E5 (программирование)

The methods decodeURI() and decodeURIComponent() will cause an exception.

Will be grateful for the help.

There's no built-in support for the percent-encoding scheme with legacy charsets in the browser, as far as I can see. You'll have to:

find the %-escapes representing the win-1251 octets,
decode the win-1251 octets to the corresponding characters (JS String)

Below is one way to do it. For the #1 I assume that only 3-character upper-case escapes need decoding, and the rest of the string is already ASCII, so I just use inputStr.replace(/%([0-9A-Z]{2})/g,replacerFunction) for this.

For the actual decoding you need a mapping from the win-1251 octets to JS characters. In the example below I build the mapping using TextDecoder.decode() API, just for fun (and in case someone finds this answer while trying to convert between different charsets in JS). (Note: it isn't universally supported as of this time -- only Gecko/Blink support it).

There's also https://github.com/mathiasbynens/windows-1251 , which I initially wanted to use for this answer, but it turned out to be easier to just build the decoding map by hand.

var decodeMap = {};
var win1251 = new TextDecoder("windows-1251");
for (var i = 0x00; i < 0xFF; i++) {
  var hex = (i <= 0x0F ? "0" : "") +      // zero-padded
            i.toString(16).toUpperCase();
  decodeMap[hex] = win1251.decode(Uint8Array.from([i]));
}
// console.log(decodeMap);
// {"10":"\u0010", ... "40":"@","41":"A","42":"B", ... "C0":"А","C1":"Б", ...


// Decodes a windows-1251 encoded string, additionally
// encoded as an ASCII string where each non-ASCII character of the original
// windows-1251 string is encoded as %XY where XY (uppercase!) is a
// hexadecimal representation of that character's code in windows-1251.
function percentEncodedWin1251ToDOMString(str) {
  return str.replace(/%([0-9A-F]{2})/g,
    (match, hex) => decodeMap[hex]);
}

console.log(percentEncodedWin1251ToDOMString("%EF%F0%EE%E3%F0%E0%EC%EC%!%E8%F0%EE%E2%E0%ED%E8%E5a"))