Why is this code not working?

Tell us what’s happening:

Your code so far

function fearNotLetter(str) {
  var x = str.charCodeAt(0);
  var y = str.charCodeAt(str.length-1);
  var aw = 0;
  var sub = [];
  var bw = 0;
  var str2 = str.split("");
  for(var i=x;i<=y;i++)
    {
      aw=0;
      for(var j=0;j<str2.length;j++)
        {
          if(str2[j]==String.fromCharCode[i])
            {
              aw=1;
              break;
            }
        }
      if(aw==0)
        {
          sub.push(String.fromCharCode(i));
          bw++;
        }
    }
  if(bw==0)
    {
      return undefined;
    }
  else
    {
      return sub;
    }
}

fearNotLetter("bcd");

Your browser information:

Your Browser User Agent is: Mozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/63.0.3239.84 Safari/537.36.

Link to the challenge:
https://www.freecodecamp.org/challenges/missing-letters

You did not include an answer to “Tell us what’s happening”. What have you tried? What isn’t working as expected? What tests are failing? What don’t you understand? What do you understand?