function chr (codePt) {
  // http://kevin.vanzonneveld.net
  // +   original by: Kevin van Zonneveld (http://kevin.vanzonneveld.net)
  // +   improved by: Brett Zamir (http://brett-zamir.me)
  // *     example 1: chr(75);
  // *     returns 1: 'K'
  // *     example 1: chr(65536) === '\uD800\uDC00';
  // *     returns 1: true
  if (codePt > 0xFFFF) { // Create a four-byte string (length 2) since this code point is high
    //   enough for the UTF-16 encoding (JavaScript internal use), to
    //   require representation with two surrogates (reserved non-characters
    //   used for building other characters; the first is "high" and the next "low")
    codePt -= 0x10000;
    return String.fromCharCode(0xD800 + (codePt >> 10), 0xDC00 + (codePt & 0x3FF));
  }
  return String.fromCharCode(codePt);
}

chr(75);
chr(65536) === '\uD800\uDC00';

'K'
true