Caesars Cipher

function rot13(str) { // LBH QVQ VG!
    var codeArr = str.split("");

    var decodedArr=[];

    var alphabet=["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","A","B","C","D","E","F","G","H","I","J","K","L","M"];

    for(var i=0;i<codeArr.length;i++){
        if(alphabet.indexOf(codeArr[i])===-1){
            decodedArr.push(codeArr[i]);
        }else{
            for(var j=0;j<alphabet.length;j++){
                if(codeArr[i]===alphabet[j]){
                    decodedArr.push(alphabet[j+13]);
                }
            }
        }
    }

  return decodedArr.join("");
}

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