Caesar's Cipher Free Code Camp problem(no spaces in solution)

I have been working on this code for FREECODECAMP but I've been stuck trying to get the answer to come out properly I can't get the code to come out with spaces in between the words. It is supposed to be Caesars cipher with a 13 digit shift in the letters. Can someone look at my code and tell me where I went wrong.

``

    function rot13(str) {
    let key = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
    let ans = '';
    let keyAlph = key.substring(13) + key.substring(0,13);

    for(let i =0; i < str.length; i++){ 
        let temp = str.charAt(i);
        let index = key.indexOf(temp);
       if(temp !== NaN){
        ans += keyAlph.[index];
      }
 
    }
       return ans;
    }
``

keyAlph.[index] should not contain that ".", and to add spaces between the words in the result, you can check if the current character is a space and add it to the answer string without applying the rot13 transformation, like this:

function rot13(str) {
  let key = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
  let ans = '';
  let keyAlph = key.substring(13) + key.substring(0,13);

  for(let i = 0; i < str.length; i++) { 
    let temp = str.charAt(i);
    let index = key.indexOf(temp);
    if(temp !== ' ') { // check if character is not a space
        ans += keyAlph[index];
    } else {
      ans += ' ';
    }
  }
  return ans;
}