How to process arrays?

Question

JavaScript

The problem of multiloop polyalphabetic substitution is given. Given plain text and 2 keys, I can't figure out how to process arrays with this text and keys to get the index of the new character. Screenshot of an example below.

Alphabet:

Answer 1

const al   = "АБВГДЕЁЖЗИКЛМНОПРСТУФХЦЧШЩЫЬЭЮЯ "; // Алфавит
let key1 = "ПЕРВЫЙ"; // первый ключ
let key2 = "БУКВА"; // второй ключ
let open_str = "ЭТО СТРОКА ОТКРЫТОГО ИСХОДНОГО ТЕКСТА"; // кодируемый текст

// т.к. в алфавите нет букв Й и Ъ, то производим соответствующую замену:
key1 = key1.replace("Й","И").replace("Ъ","Ь");
key2 = key2.replace("Й","И").replace("Ъ","Ь");
open_str = open_str.replace("Й","И").replace("Ъ","Ь");

// кодируем текст
let encoded_str = ([...open_str]).map( (e,i) => al[(al.indexOf(e) + al.indexOf(key1[i%key1.length]) + al.indexOf(key2[i%key2.length])) % 32] ).join("");

console.log(encoded_str);

in fact, a string can be accessed as an array - str[index]
about .map look here - https://developer.mozilla.org/en-US/docs/Web/JavaS...
if you need it right here on ARRAYS, then just make the corresponding variables "arrays" the
string into an array can be split like this str.split("") or [...str]
according to the script
al - string / array of letters of the alphabet
al.indexOf(char) - get the index of the letter in alphabet
by choosing a character from the key - because. the key is iterated, then we take the remainder of dividing the index of the encoded character by the length of the key, this will be the character from the key - key1[i% key1.length], then we get its code alphabetically - al.indexOf(key1[i% key1. length])
and you have a lot of errors in the table, for example, in the third line - the final character is not AND, but Z
, etc.

Answer 2

As I understand it:
There is an alphabet with matching codes: 0 - A, 1 - B, 2 - C, 3 - D, etc.
mod is the remainder of the sum divided by 32 (also "mod", sometimes denoted by a percent sign: "% ").
We add the numbers, divide, get the remainder, look at the letter based on the alphabet.