string replace function not working for emoticon with skin tone

Question

I have string with emoticons like str = . I have applied below string replacement function on above string

self.rx_unified = new RegExp('(' + a.join('|') + ')(\uD83C[\uDFFB-\uDFFF])?', "g");
return str.replace(self.rx_unified, function(m, p1, p2) {
  var val = self.map.unified[p1];
  if (!val) return m;
  var idx = null;
  if (p2 == '\uD83C\uDFFB') idx = '1f3fb';
  if (p2 == '\uD83C\uDFFC') idx = '1f3fc';
  if (p2 == '\uD83C\uDFFD') idx = '1f3fd';
  if (p2 == '\uD83C\uDFFE') idx = '1f3fe';
  if (p2 == '\uD83C\uDFFF') idx = '1f3ff';
  if (idx) {
    return self.replacement(val, null, null, {
      idx: idx,
      actual: p2,
      wrapper: ''
    });
  }
  return self.replacement(val);
});

But I am getting . Can anyone please help?