Add constructor for BeforeUnloadEvent

[ayg]

We normally allow authors to instantiate their own events (although I don't know why). These days we do this with constructors, so BeforeUnloadEvent should have a constructor. AFAICT, every other event type defined in the spec already has one.

[ayg]

(See whatwg/dom#445)

[zcorpan]

So this event was added in 246c3b4 which includes a note (still in the spec today)

+  <p class="note">There are no <code>BeforeUnloadEvent</code>-specific
+  initialization methods.</p>

email: https://lists.w3.org/Archives/Public/public-whatwg-archive/2008Dec/0146.html

irc ramblings: https://krijnhoetmer.nl/irc-logs/whatwg/20081212#l-99

So... at that time event constructors didn't exist. But at least it seems intentional to not include initBeoreUnloadEvent method.

If we add a constructor, you would be able to fire a beforeunload event (with the right interface) in onbeforeunload handler. Does the spec for handling this event need to change to avoid having the user get stuck with repeated dialogs, or something?

[ayg]

The constructor needn't allow you to initialize anything. It can take a plain EventInit dict, so it just lets you create it with no flags (beyond bubbles and cancelable, which you can set now with initEvent).

You can already fire a BeforeUnloadEvent in a beforeunload handler using createEvent(), in Chrome and Firefox. This is consistent with the way the rest of the platform works. Browsers already don't have to display the dialogs if they don't want to, so I think this can be left to a QoI issue, unless you have a specific concern that you think should be in the spec.

[zcorpan]

Oh I mistakenly thought "beforeunloadevent" was not in https://dom.spec.whatwg.org/#dom-document-createevent . Then I agree there are no new abilities.

Test for my scenario with createEvent: http://software.hixie.ch/utilities/js/live-dom-viewer/saved/5041

It seems the browsers I have don't show more than one dialog.

Looking at the spec, it's clear that the prompt can only appear because of the UA-generated event.