so I've wrote this function, i want to uppercase the vowels and lowercase every other letter,
problem the end result ends with the same string, I'm new to spread and for-each,
after i spread a string does it become an array?
when i manipulate letters does it suppose to become a string again with the manipulations or do i need to join it? why aren't the upper and lowercase functions don't work? 

the function: 

```
function upperCase(str) {
  var vowels = "aeiou";
  [...str].forEach(letter => {
    if (vowels.includes(letter)) letter.toUpperCase();
    letter.toLowerCase();
  });
  console.log(str);
}

```