How pick the extract characters from the string using javascript?

Html page:

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

<p id="P6"></p>

Javascript page:

<script>

    function myFunction() {

        var str = "Hello world!";

        var res = str.substring(1, 4);

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

    }

</script>

Output:

ell