How to get the next letter of the alphabet in Javascript?

I am build an autocomplete that searches off of a CouchDB View.

I need to be able to take the final character of the input string, and replace the last character with the next letter of the english alphabet. (No need for i18n here)

For Example:

  • Input String = "b"
  • startkey = "b"
  • endkey = "c"


  • Input String = "foo"
  • startkey = "foo"
  • endkey = "fop"

(in case you're wondering, I'm making sure to include the option inclusive_end=false so that this extra character doesn't taint my resultset)

The Question
  • Is there a function natively in Javascript that can just get the next letter of the alphabet?
  • Or will I just need to suck it up and do my own fancy function with a base string like "" and indexOf()?


my_string.substring(0, my_string.length - 1)
      + String.fromCharCode(my_string.charCodeAt(my_string.length - 1) + 1)

// This will return A for Z and a for z.

function nextLetter(s){
    return s.replace(/([a-zA-Z])[^a-zA-Z]*$/, function(a){
        var c= a.charCodeAt(0);
            case 90: return 'A';
            case 122: return 'a';
            default: return String.fromCharCode(++c);

A more comprehensive solution, which gets the next letter according to how MS Excel numbers it's columns... A B C ... Y Z AA AB ... AZ BA ... ZZ AAA

This works with small letters, but you can easily extend it for caps too.

getNextKey = function(key) {
  if (key === 'Z' || key === 'z') {
    return String.fromCharCode(key.charCodeAt() - 25) + String.fromCharCode(key.charCodeAt() - 25); // AA or aa
  } else {
    var lastChar = key.slice(-1);
    var sub = key.slice(0, -1);
    if (lastChar === 'Z' || lastChar === 'z') {
      // If a string of length > 1 ends in Z/z,
      // increment the string (excluding the last Z/z) recursively,
      // and append A/a (depending on casing) to it
      return getNextKey(sub) + String.fromCharCode(lastChar.charCodeAt() - 25);
    } else {
      // (take till last char) append with (increment last char)
      return sub + String.fromCharCode(lastChar.charCodeAt() + 1);
  return key;

Here is a function that does the same thing (except for upper case only, but that's easy to change) but uses slice only once and is iterative rather than recursive. In a quick benchmark, it's about 4 times faster (which is only relevant if you make really heavy use of it!).

function nextString(str) {
    if (! str)
        return 'A'  // return 'A' if str is empty or null

    let tail = ''
    let i = str.length -1
    let char = str[i]
    // find the index of the first character from the right that is not a 'Z'
    while (char === 'Z' && i > 0) {
        char = str[i]
        tail = 'A' + tail   // tail contains a string of 'A'
    if (char === 'Z')   // the string was made only of 'Z'
        return 'AA' + tail
    // increment the character that was not a 'Z'
    return str.slice(0, i) + String.fromCharCode(char.charCodeAt(0) + 1) + tail


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