the hashchange event of jQuery doesn't work if I open a page with hash directly

I placed a script tag at the end of a html page:

$(window).on ('hashchange', function (e) { alert (location.hash); });

It works if I click on buttons with hrefs like #a, but If I open links like localhost/aaa#a the alert function is not triggered.

So it looks like I have to detect the presence of a hash when the document is ready. But that looked wrong.

Is there a way to make it work in both situations?