Caesars Cipher encryption

caesarsCipher.js

function rot13(str) {
  let key = 13;
  let lowerAlphabet = 'abcdefghijklmnopqrstuvwxyz'
  let upperAlphabet = 'abcdefghijklmnopqrstuvwxyz'.toUpperCase();
  let lowerAlphabetArr = lowerAlphabet.split('');
  lowerAlphabetArr.push(...lowerAlphabetArr.splice(0,key))
  let alphabetPushed = lowerAlphabetArr.join('')
  let alphabetPushedUp = alphabetPushed.toUpperCase();
  let strEncoded = [];
  
  for(let i = 0; i<str.length; i++){
    if(alphabetPushed[lowerAlphabet.indexOf(str[i])] !== undefined){
      strEncoded.push(alphabetPushed[lowerAlphabet.indexOf(str[i])])
    } else if(alphabetPushedUp[upperAlphabet.indexOf(str[i])] !== undefined){
      strEncoded.push(alphabetPushedUp[upperAlphabet.indexOf(str[i])])
    } else{
      strEncoded.push(str[i])
    }
  }
  return strEncoded.join('')
}

// rot13("SERR PBQR PNZC"); will return FREE CODE CAMP

caesarsCipherFccSolution.js

function rot13(encodedStr) {
  var lookup = {
  'A': 'N','B': 'O','C': 'P','D': 'Q',
  'E': 'R','F': 'S','G': 'T','H': 'U',
  'I': 'V','J': 'W','K': 'X','L': 'Y',
  'M': 'Z','N': 'A','O': 'B','P': 'C',
  'Q': 'D','R': 'E','S': 'F','T': 'G',
  'U': 'H','V': 'I','W': 'J','X': 'K',
  'Y': 'L','Z': 'M'
};
  var codeArr = encodedStr.split("");  // String to Array
  var decodedArr = []; // Your Result goes here
  // Only change code below this line

  decodedArr = codeArr.map(function(letter) {
    if(lookup.hasOwnProperty(letter)) {
      letter = lookup[letter];
    }
    return letter;
  });

  // Only change code above this line
  return decodedArr.join(""); // Array to String
}