function formatCodePoint(codePoint) {
  codePoint = codePoint || 0; // NaN --> 0
  var formatted = '';
  if (codePoint <= 0xFFFF) {
    formatted = zeroPaddedHex(codePoint, 4);
  } else {
    formatted = codePoint.toString(16).toUpperCase();
  }
  return 'U+' + formatted;
}
@param {number} codePoint  Valid Unicode code-point
@return {string}           A formatted Unicode code-point string
                           of the format U+XXXX, U+XXXXX, or U+XXXXXX