Get Page name and hide button if file doesn't exist

I am creating a help system for my app.

I have created a custom button named Help on my list page for example.

What I need is the following, but I don't know how to do it.

I want to use the same code in each help button so I need to determine the page name that I'm running.

For example my list page url is www.ding.com/employee_list.php.

Assuming that's true, I then want to look for a file in the help directory named employee_list.html

If it doesn't exist, I want to hide the custom button.

If it does exist when the user clicks on the button I want to open the html in another window, by issueing this command
window.open('help/employee_list.html', '_blank');

But of course don't want to hard code it.

Hopefully I didn't totally confuse everyone.

Thanks in advance.

Alan