vigenere with functions p1

vigenerefunction.js

function repeat_key(length){
  let new_key = this.key;
  
  while(new_key.length < length){
    new_key = new_key.repeat(2)
  }
  
  return new_key.slice(0, length + 1);
}

function encode(str) {
  const key = this.repeat_key(str.length)
  
  return str
    .split("")
    .map((x, i) => {
      const xi = this.alphabet.indexOf(x);
      const k = key[i];
      const ki = this.alphabet.indexOf(k);
    
      if(xi === -1) return x;
      
      const ei = (xi + ki) % this.l
    
      return this.alphabet[ei];
    })
    .join("");
}

function decode(str) {
  const key = this.repeat_key(str.length)
  
  return str
    .split("")
    .map((c, i) => {
      const ci = this.alphabet.indexOf(c);
      const k = key[i];
      const ki = this.alphabet.indexOf(k);
    
      if(ci === -1) return c;
    
      let di;
      if((ci - ki) < 0) di = di = (ci - ki + this.l) % this.l;
      else di = (ci - ki) % this.l;
    
      return this.alphabet[di];
    })
    .join("");
}