Used charCodeAt to identify unicode point for char. In a while loop I compared the string version of the unicode alphabet with the char at the string.

Missing in sequence

function fearNotLetter(str) {
  var c = 0;
  var i = 0;

  if (str) {
    i = str.charCodeAt(0);
    while (c < str.length && i < 122) {
      if (String.fromCharCode(i) !== str.charAt(c)) {
        return String.fromCharCode(i);
      }
      i++;
      c++;
    }
    return undefined;
  }
}

fearNotLetter("abce");