Caesars Cipher, how to pass on special characters?

JavaScript
1

Tell us what’s happening:

can someone help me figure out how to pass on the special symbols.
everything works fine except the special characters.

Your code so far

function rot13(str) { // LBH QVQ VG!
  var arr=[];
 
  for(var i=0;i<str.length;i++){
    
   arr[i]= str.charCodeAt(i)+13; 
    
  if(arr[i]>90){
    arr[i]=arr[i]%91 +65;
  }
    
  arr[i]=String.fromCharCode(arr[i]);
    
  
  }
  str=arr.join('');
  
  
return str;
}

// Change the inputs below to test
rot13("SERR CVMMN!");

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Link to the challenge: