title: Caesars Cipher Free Code Camp challenge - Error return with 12 red dots
body: <p>I am doing the Caesars Cipher challenge ON FREE CODE CAMP.
  I believe my code should be working but I am getting an error I don't understand. I get 12 red dots. i Believe it has something to do with a null value. I use <a href="http://pythontutor.com/visualize.html#mode=edit" rel="nofollow noreferrer">http://pythontutor.com/visualize.html#mode=edit</a> and my array returns empty strings. I don't know why.</p>
  
  <pre><code>function rot13(str) { 
      //str = str.split('');
      var arr = [];
      // var regex = /[^a-zA-Z]/g;
      for (var i = 0; i &lt; str.length; i++) {
          if (str.charCodeAt(i) &gt;= 65 &amp;&amp; str.charCodeAt(i) &lt;= 77) {
              arr.push(String.fromCharCode(str.charCodeAt[i] += 13));
          } else if(str.charCodeAt(i) &gt;= 78 &amp;&amp; str.charCodeAt(i) &lt;= 90){
              arr.push(String.fromCharCode(str.charCodeAt[i] -= 13));
          } else if (str.charCodeAt(i) &lt; 65){
              arr.push(str[i]);
          }
      }
      return arr.join('');
  }
  rot13("SERR PBQR PNZC");
  </code></pre>