phpjs/functions/strings/chr.js

php.js offers community built php functions in javascript

function chr(codePt) {
  //  discuss at: http://phpjs.org/functions/chr/
  // original by: Kevin van Zonneveld (http://kevin.vanzonneveld.net)
  // improved by: Brett Zamir (http://brett-zamir.me)
  //   example 1: chr(75) === 'K';
  //   example 1: chr(65536) === '\uD800\uDC00';
  //   returns 1: true
  //   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);
}