You can use a variety of methods, one uses Javascript window.onload function in a simple function call from a script or from the body as in the solutions above, you can also use jQuery to do this but its just a modification of Javascript...Just add Jquery to your header by pasting
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.1.1/jquery.min.js"></script>
to your head section and open another script tag where you display the alert when the DOM is ready i.e. `
<script>
$("document").ready( function () {
alert("Hello, world");
});
</script>
`
This uses Jquery to run the function but since jQuery is a Javascript framework it contains Javascript code hence the Javascript alert function..hope this helps...