How to prevent close of browser window/tab with javascript

In our application, we have a sequence of save requests being sent to middleware. We are using EmberJS which means that each save is actually a promise. I know we can use beforeunload to prompt a dialog for user to confirm but is there a way I can cancel/prevent close of the window/tab till all those promises resolve? We also need to show up a dialog (which has no close buttons) but just a message while these promises are resolving. Thanks in advance.