Alternative of Python's Array.index() function in JavaScript that treats negative indices the same as Python's Array.index()?

TLDR: Is there any alternative of Python’s Array.index() method in JavaScript that treats negative indices the same as Python’s Array.index()?

For background, I’m writing a code for the Caesar cipher (Wikipedia link).

I have written the following code in Python:

alphabet = list('abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz') 
# list('abc') == ['a', 'b', 'c']

def caesar(text, shift, direction):
    endText = ''
    if direction == 'encode':
        shift = shift
    else:
        shift *= -1
    for i in text:
        oldIndex = alphabet.index(i)
        newIndex = oldIndex + shift
        newLetter = alphabet[newIndex]
        endText += newLetter
    print(endText)

caesar('zeus', 5, 'encode')  # 'ejzx'
caesar('ejzx', 5, 'decode')  # 'zeus'

When I enter a word that has letters that are towards the end of the English alphabet (i.e. containing letters such as v, w, x, y, z etc), it encodes the word and when I decode the result, it gives the word I encoded previously without any errors.

When I write the exact same code in JavaScript, and when I encode the same message I used in the Python code (‘zeus’), it gives me ejzx; however, when I decode ejzx, it gives me undefinedeus instead of zeus i.e. it does decode the eus, but not the initial z.

JavaScript:

 let alphabet = 'abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz'.split('');

 function caesar(text, shift, direction) {
   let endText = '';
   if (direction == 'encode') {
     shift = shift;
   } else if (direction == 'decode') {
     shift *= -1;
   }
   for (let i of text) {
     let oldIndex = alphabet.indexOf(i);
     let newIndex = oldIndex + shift;
     let newLetter = alphabet[newIndex];
     endText += newLetter;
   }
   console.log(endText)
 }


 caesar('zeus', 5, 'encode') // 'ejzx'
 caesar('ejzx', 5, 'decode') // 'undefinedeus'

I tried replacing the indexOf() with Array.at() (MDN link), but it returns undefinedundefinedundefinedundefined both in encoding and decoding.

How could I fix the code so that it works the same as Python?

>Solution :

You don’t have to replace indexOf() with Array.prototype.at(), instead of that
you have to replace alphabet[newIndex] to alphabet.at(newIndex);

let alphabet = 'abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz'.split('');

 function caesar(text, shift, direction) {
   let endText = '';
   if (direction == 'encode') {
     shift = shift;
   } else if (direction == 'decode') {
     shift *= -1;
   }
   for (let i of text) {
     let oldIndex = alphabet.indexOf(i);
     let newIndex = oldIndex + shift;
     let newLetter = alphabet.at(newIndex);
     endText += newLetter;
   }
   console.log(endText)
 }

Hope this helps.