function zeroPaddedHex(codePoint, len) {
  let codePointHex = codePoint.toString(16).toUpperCase();
  let numZeros = Math.max(0, len - codePointHex.length);
  var result = '';
  for (var i = 0; i < numZeros; i++) {
    result += '0';
  }
  result += codePointHex;
  return result;
}
@param {number} codePoint  Valid Unicode code-point
@param {number} len        Zero-padded minimum width of result
@return {string}           A zero-padded hexadecimal string (00XXXX)