Find missing letter in list of alphabets

Question

I am trying to solve the following issue:

Find the missing letter in the passed letter range and return it. If all letters are present in the range, return undefine

Answer 1

function fearNotLetter(str) {

  var a = str.split('');
  var array = [];
  var j = 0;

  for (var i = 1; i < a.length; i++) {
    var d = a[i].charCodeAt(0);
    var c = a[i - 1].charCodeAt(0);

    var delta = d - c;

    if (delta != 1) {
      array[i] = String.fromCharCode(a[i - 1].charCodeAt(0) + 1);
    }
  }

  str = array.join('');

  if (str.length === 0) {
    return undefined;
  } else {
    return str;
  }
}

fearNotLetter('abcefr');