Vigenère Cipher using typescript in a funcional way

vigenere_cipher.ts

type CipherEquation = (Xi:number, Ki:number, L:number) => number;

const encoder = (Xi:number, Ki:number, L:number) => (Xi + Ki) % L;
const decoder = (Ci:number, Ki:number, L:number) => (Ci - Ki) >= 0 ? (Ci - Ki) % L : (Ci - Ki + L) % L;

function repeat_key(to_length:number, key):string {
  if(key.length < to_length) {
    return repeat_key(to_length, key.repeat(2));
  } else {
    return key.slice(0, to_length + 1);
  }
}

type VigenereCipher = (str:string, equation:CipherEquation) => string;

function vigenere_cipher(alphabet:string, key:string):VigenereCipher {
  const L = alphabet.length;

  return function(str, equation) {
    const repeated_key = repeat_key(str.length, key);
    return str
      .split("")
      .map((C, i) => {
        const Ci = alphabet.indexOf(C);
        const K = repeated_key[i];
        const Ki = alphabet.indexOf(K);

        if(Ci === -1) return C;

        const Ri = equation(Ci, Ki, L);

        return alphabet[Ri];
      })
      .join("");
  };
}

//Redhat client
const redhat_cipher = vigenere_cipher("abcdefghijklmnopqrstuvwxyz", "novyuspxqa");

const redhat_encode = (message:string) => redhat_cipher(message, encoder);
const redhat_decode = (message:string) => redhat_cipher(message, decoder);

redhat_encode("hello hacker!"); // usgji wxskrf!
redhat_decode("usgji wxskrf!"); // hello hacker!

//IBM client
const ibm_cipher = vigenere_cipher("abcdefghijklmnñopqrstuvwxyz", "gtjmdglsonpvbtm");

const ibm_encode = (message:string) => ibm_cipher(message, encoder);
const ibm_decode = (message:string) => ibm_cipher(message, decoder);

ibm_encode("hello thinker!"); // nxtwr ezwzzzs!
ibm_decode("nxtwr ezwzzzs!"); // hello thinker!