function fearNotLetter(str) {

  var arr = str.split("");

  var newArr = [];

  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 ) {

      newArr.push(item);

    }else {

      arr.shift();

    }

  }

  if (newArr.length === 0) {

    return undefined;

  }else {

    return newArr.join("");

  }

}

fearNotLetter("abce");