How find word located in last of given paragraph using javascrip?

Html Page:

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

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

Javascript Page:

<script>

function myFunction()

{

    var str = "Hello planet earth, you are a great planet.";

    var n = str.lastIndexOf("planet");

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

}

</script>

Output:

36