function utf8_decode (str_data) {
  // http://kevin.vanzonneveld.net
  // +   original by: Webtoolkit.info (http://www.webtoolkit.info/)
  // +      input by: Aman Gupta
  // +   improved by: Kevin van Zonneveld (http://kevin.vanzonneveld.net)
  // +   improved by: Norman "zEh" Fuchs
  // +   bugfixed by: hitwork
  // +   bugfixed by: Onno Marsman
  // +      input by: Brett Zamir (http://brett-zamir.me)
  // +   bugfixed by: Kevin van Zonneveld (http://kevin.vanzonneveld.net)
  // +   bugfixed by: kirilloid
  // *     example 1: utf8_decode('Kevin van Zonneveld');
  // *     returns 1: 'Kevin van Zonneveld'

  var tmp_arr = [],
    i = 0,
    ac = 0,
    c1 = 0,
    c2 = 0,
    c3 = 0,
    c4 = 0;

  str_data += '';

  while (i < str_data.length) {
    c1 = str_data.charCodeAt(i);
    if (c1 <= 191) {
      tmp_arr[ac++] = String.fromCharCode(c1);
      i++;
    } else if (c1 <= 223) {
      c2 = str_data.charCodeAt(i + 1);
      tmp_arr[ac++] = String.fromCharCode(((c1 & 31) << 6) | (c2 & 63));
      i += 2;
    } else if (c1 <= 239) {
      // http://en.wikipedia.org/wiki/UTF-8#Codepage_layout
      c2 = str_data.charCodeAt(i + 1);
      c3 = str_data.charCodeAt(i + 2);
      tmp_arr[ac++] = String.fromCharCode(((c1 & 15) << 12) | ((c2 & 63) << 6) | (c3 & 63));
      i += 3;
    } else {
      c2 = str_data.charCodeAt(i + 1);
      c3 = str_data.charCodeAt(i + 2);
      c4 = str_data.charCodeAt(i + 3);
      c1 = ((c1 & 7) << 18) | ((c2 & 63) << 12) | ((c3 & 63) << 6) | (c4 & 63);
      c1 -= 0x10000;
      tmp_arr[ac++] = String.fromCharCode(0xD800 | ((c1>>10) & 0x3FF));
      tmp_arr[ac++] = String.fromCharCode(0xDC00 | (c1 & 0x3FF));
      i += 4;
    }
  }

  return tmp_arr.join('');
}

utf8_decode('Kevin van Zonneveld');

'Kevin van Zonneveld'