Missing letters

function fearNotLetter(str) {
  //return str;
  var arr = str.split('');
  var temp = [];
  var start = str.charCodeAt(0);
  var end = str.charAt(str.length - 1).charCodeAt(0);
  for(var i = start; i < end + 1; i++){
    var item = String.fromCharCode(i);
    if(arr[0] !== item){
      temp.push(item);
    }else{
      arr.shift();
    }
  }
  if(temp.length === 0){
    return undefined;
  }else{
    return temp.join('');
  }
}

fearNotLetter("abce");