ROT13_Encoding.js

function rot13(str) { // LBH QVQ VG!
  const A_Z = ['A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z'];
  
  function repLetter(L) {
    if(L > 'M') {
      return A_Z[(A_Z.indexOf(L)-13)];
    }
    else return A_Z[(A_Z.indexOf(L)+13)];
  }
  
  return str.split('').map(e => {
    return /[A-Z]/.test(e)? repLetter(e):
     e;
  }).join('');
}

// Change the inputs below to test
rot13("SERR PBQR PNZC");