How can you count the number of items in the array - what do you have to add to this code?

<!DOCTYPE html>
<html>
<body>

<p>Click the button to sort the array.</p>

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

<p id="demo"></p>

<script>
var fruits = ["Banana", "Orange", "Apple", "Mango"];
document.getElementById("demo").innerHTML = fruits;

function myFunction() {
fruits.sort();
document.getElementById("demo").innerHTML = fruits;
}
</script>

</body>
</html>

Many thanks