Get selected item ID to display in customized DispForm javascript

I'm using SharePoint Online and have a SharePoint-hosted app.

I have a page with a question form that send data in to SharePoint. I show my question form in my DispForm for my list (I added my app as an app part) on DispForm. When I select a item and press on view item I see my question form just as I like it but I don't know how to get the right ID of that item I want to show. Someone have any suggestion? I use

listItem.set_item('Title', $('#Title').val());

to send data in to my SharePoint list and

and get_fieldValues to read it back

$('#Title').val(item.get_fieldValues().Title);