rot13

script.js

function rot13(str) { // LBH QVQ VG!
  var res = [];
  for(var i=0; i<str.length; i++){
    if(str[i] !== " " && str[i] !== "!" && str[i] !== "?" && str[i] !== "." && str[i] !== ","){
      var code = (str.charCodeAt(i) + 13);
      if(code > 90){
        code = code - 26;
      }
      //(c <= "Z" ? 90 : 122) >= (c = c.charCodeAt(0) + 13) ? c : c - 26);
      res.push(String.fromCharCode(code));
    }
    else{
      if(str[i] === " "){
        res.push(" ");
      }
      else if(str[i] === "!"){
        res.push("!");
      }
      else if(str[i] === "?"){
        res.push("?");
      }
      else if(str[i] === "."){
        res.push(".");
      }
      else if(str[i] === ","){
        res.push(",");
      }
    }
  }
  return res.join("");
}

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