Click the button

<!DOCTYPE html>
<html>
<body>

<p id="demo">Click the button to return the number of characters in the string "Hello World!".</p>

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

<script>
function myFunction()
{
var str= prompt("My name:");
var n=str.length;
document.getElementById("demo").innerHTML=n;
}
</script>

</body>
</html>