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function bs(str){
  //storage for new string
  let newStr = '';
  //storage for character codes
  let arr = [];
  //loop to cycle through each letter in the string then return it into the array
  for(let i = 0; i < str.length; i++){
    let charCode = str.charCodeAt(i) + 1;
    //if char is z or Z loop back around to the approppriate a or A
    (str[i] == 'z') ? arr.push(97):
    (str[i] == 'Z') ? arr.push(65):
    //if the input is not a letter then return the text below
    return (charCode > 123) ? "Letters only, please" :
    (charCode <= 64) ? "Letters only, please" :
    (charCode <= 96 && charCode > 91) ? "Lettes only, please" : undefined;
  //decipher each char code in the array then return it to a string
    newStr = n => String.fromCharCode(n)).join('') 
    return newStr


What I have tried:

In this code I am trying to replace every character in a given string with the character following it in the alphabet. However I only want letters to be returned. My code was working fine before I tried to filter out non letters. Now when I enter letters I get undefined as my return. How do I fix this?
Updated 15-May-23 3:52am

1 solution

Bear in mind that characters sets are reasonable sensible: 'a' is less than 'b', which is less that 'c', and so on up to 'z'. And the same applies for the uppercase letters as well: 'A' < 'B' < 'C' ... < 'Z'
So, you can write a conditional that filters to just lowercase alphabetic characters like this:
if (charCode >= 'a' && charCode <='z')

Then ... it gets nasty. Javascript is a weakly (or poorly if you prefer) typed language - which means in this case that the type of variable changes depending on what you do with it.
So adding on to a character doesn't return a character - it returns a pair of characters: "x" + 1 == "x1" as the addition is assumed to be string concatenation. That why your code uses charCodeAt rather than charAt - but you need to convert it back when you try to build a string using string.fromCharCode:
let result = "";
str = "12Hello34";
for(let i = 0; i < str.length; i++) {
    let charCode = str.charCodeAt(i);
    if (charCode >= 65 && charCode <= 90 ) {
       if (charCode == 90) charCode = 65;
       else charCode = charCode + 1;
    else if (charCode >= 97 && charCode <= 122 ) {
       if (charCode == 122) charCode = 97;
       else charCode = charCode + 1;
    result += String.fromCharCode(charCode);
console.log (result);
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