This was the free code camps Caesar's Cipher, hard but I did it, not sure why it doesn't work though

freecodecamp Caesar's Cipher

function rot13(str) { // LBH QVQ VG!
  
  str = str.toUpperCase().split('');
  str.reduce(function(acc, item)  {
    var patt = /[a-zA-Z]/g;
    
    if(patt.test(item)){
      item = item.charCodeAt(0);
      
      if(item > 77) {
        return acc + String.fromCharCode(item - 13);
      } else if (item < 78){
        return acc + String.fromCharCode(item + 13);
      }
    } else {
      return acc + item; 
    }

  },[]);
}

// Change the inputs below to test
rot13("GUR DHVPX OEBJA QBT WHZCRQ BIRE GUR YNML SBK.");