I have written a function with the intention of populating an array with uni code characters for each of the letters in a string.

However the result I am being returned is an array of null values.

```js
function rot13(str) {
  var splitString = str.split("");
  var myArray = [];

  for (var i = 0; i < splitString.length; i++) {
    myArray.push(splitString[i].charCodeAt[i]);
  }
  return myArray;
}
console.log(rot13('cats'));
```