How to write a decoder if the algorithm is known?

Question

JavaScript

Hello, I'm curious how to make decoders based on an existing algorithm?
I am doing this for informational purposes.
There is an encoder code, I want to get, as it were, an inverse function (Just like in math analysis)

function shifr(text) {
            var alphabet = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";//АБВГДЕЁЖЗИЙКЛМНОПРСТУФХЦЧШЩЪЫЬЭЮЯабвгдеёжзийклмнопрстуфхцчшщъыьэюяӘІҢҒҮҰҚӨҺәіңғүұқөһ";
            var key = getkey();// 'nx9MwEJN2KZ6e1NPiCAaHTxn9QX0iXcjLUoY9fdpc6J';
            var len = alphabet.length;
            var table = new Array();
            for (var i = 0; i < len; i++) {
                table[i] = alphabet.substring(i) + alphabet.substring(0, i);
            }
            var word = text;
            var shfr = "";
            while (key.length < word.length)
                key += key;
            for (var i = 0; i < word.length; i++) {
                if (alphabet.indexOf(word[i]) > -1) {
                    shfr = shfr + table[alphabet.indexOf(key[i])][alphabet.indexOf(word[i])];
                    }
                else
                    shfr = shfr + word[i];
            }
            return shfr;
        }

Here's how it works:
"0" -> "W"
"00" -> "Wg"
"000" -> "WgE"
"0000" -> "WgEM"
I noticed that everything works statically, the getkey function is not bound to anything (everything is static)
Please help me figure it out.)))

Answer 1

On vskidku, it is necessary to generate simply the return shift table.

for (var i = 0; i < len; i++) {
    table[i] = alphabet.substring(0, i) + alphabet.substring(i);
}