Caesars Cipher - JavaScript Project

Code Feedback
1

Hi Coders,

I completed this project using if and else if statements. How is this? Any feedback, please?

Thanks

function rot13(str) {
  let newStr = '';

  for(let i=0; i<str.length; i++){
    if(str.charCodeAt(i) < 65){
      newStr = newStr + String.fromCharCode(str.charCodeAt(i));
    } else {
      if(str.charCodeAt(i) + 13 > 90){
        newStr = newStr + String.fromCharCode(str.charCodeAt(i) - 13);
      } else if(str.charCodeAt(i) + 13 < 90){
        newStr = newStr + String.fromCharCode(str.charCodeAt(i) + 13);
      } else if(str.charCodeAt(i) + 13 === 90){
        newStr = newStr + String.fromCharCode(str.charCodeAt(i) + 13);
      }
  }

  }
  return newStr;
}

rot13("GUR DHVPX OEBJA SBK WHZCF BIRE GUR YNML QBT.");