How to convert the string to lowercase letters in other method javascript?

Html page:

<button onclick="myFunction()">Try it</button>

<p id="P8"></p>

Javascript page:

<script>

    function myFunction() {

        var str = "Hello World!";

        var res = str.toLocaleUpperCase();

        document.getElementById("demo").innerHTML = res;

    }

</script>

Output:

HELLO WORLD!