How to know a div's nth child by pure js?

<div>One</div>
<div>Two</div>
<div id="myDiv">Three</div>
<div>Four</div>

<script>
var elm=document.getElementById("myDiv");
alert(elm.getChildNumber());
</script>

This code doesn't works.

Now how will I know the nth-child number of #myDiv ?

Thanks in advance!!