Polybius-Square-cipher | also called polybius checkerboard , which was a device | Encryption library

let row = letter.charCodeAt(i - letter.charCodeAt('a')) / 5 + 1; is not correct since the char code of the letter itself is letter.charCodeAt(0) and the char code of 'a' is 'a'.charCodeAt(0). Moreover, you need to use Math.floor to round down.

Thus, it should be let row = Math.floor((letter.charCodeAt(0) - 'a'.charCodeAt(0)) / 5) + 1;.

There is a similar problem with the col initialization and it should be let col = (letter.charCodeAt(0) - 'a'.charCodeAt(0)) % 5 + 1;.

Next, to check if the letter is k, there is no need to compare char codes, you can directly use letter === 'k'. For the comparison to see if the letter is greater than or equal to 'j', a similar issue exists and you can directly use >= 'j'.

Finally, you return in the first iteration of the loop, so it will only run once. Instead, you can build a string to output or return at the end.