decode.js

/**
 * Decode string "Москва" (ISO-8859-1) to "Москва" (UTF-8)
 * @param {string} input
 * @returns {string}
 */
export function decodeISO88591toUTF8(input) {
  let string = '';
  let i = 0;
  let currentChar = 0;
  let nextChar = 0;
  let nextCharPlus = 0;

  while (i < input.length) {
    currentChar = input.charCodeAt(i);

    if (currentChar < 128) {
      string += String.fromCharCode(currentChar);
      i++;
    } else if (currentChar > 191 && currentChar < 224) {
      nextChar = input.charCodeAt(i + 1);
      string += String.fromCharCode(
        ((currentChar & 31) << 6) | (nextChar & 63)
      );
      i += 2;
    } else {
      nextChar = input.charCodeAt(i + 1);
      nextCharPlus = input.charCodeAt(i + 2);
      string += String.fromCharCode(
        ((currentChar & 15) << 12) |
          ((nextChar & 63) << 6) |
          (nextCharPlus & 63)
      );
      i += 3;
    }
  }
  return string;
}