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Re: Exceptions in event listeners triggered by dispatchEvent().

From: Simon Pieters <simonp@opera.com>
Date: Fri, 07 Jun 2013 15:09:30 +0200
To: "Glenn Maynard" <glenn@zewt.org>
Cc: "Domenic Denicola" <domenic@domenicdenicola.com>, "John Barton" <johnjbarton@google.com>, johnjbarton <johnjbarton@chromium.org>, "www-dom@w3.org" <www-dom@w3.org>, "Anne van Kesteren" <annevk@annevk.nl>
Message-ID: <op.wya754u2idj3kv@simons-macbook-pro.local>
On Wed, 05 Jun 2013 16:19:48 +0200, Glenn Maynard <glenn@zewt.org> wrote:

>> No. window.onerror is invoked directly without an event. ErrorEvent is  
>> for
>> the case when an uncaught error is fired on a Worker object (if the  
>> error
>> wasn't handled in the worker itself).
> Nope.  In Chrome and IE10, ErrorEvent is used for uncaught exceptions in
> the UI thread too, for the "error" event fired on window.  Firefox also
> fires "error" event listeners (not just the event handler), but doesn't  
> yet
> use ErrorEvent there.

Well then those browsers violate the spec and the original  
implementation... At least Presto implements the spec.

> (The spec looks incorrect here.  It bypasses event dispatch entirely and
> calls "onerror" directly, which forgets to fire event listeners.  It also
> doesn't make much sense to support error handling with event listeners in
> workers but not in the UI thread.  This should probably just fire an
> ErrorEvent, and the window.onerror special case should probably be in the
> event dispatch algorithm.)

You misunderstood the worker case. Errors in workers are handled the same  
as in window, i.e. onerror is invoked directly. However, if a worker error  
is not "handled", an ErrorEvent gets fired on the Worker object.

> If you want to catch an exception and have it behave as though the error
> was uncaught and gets sent to onerror, you need to make sure "error" is
> dispatched to event listeners.  If you simply call window.onerror  
> directly,
> event listeners won't be called.
> This works in Chrome, including calling the onerror event handler called
> with its special-cased arguments:
> http://zewt.org/~glenn/test-error-handler.html.  Firefox doesn't handle
> this yet, so you end up with the event handler called with incorrect
> arguments.  (IE10 doesn't seem to have any way to create an ErrorEvent
> object yet; the interface exists but isn't a constructor.)  I'll file a  
> bug
> on HTML.

Simon Pieters
Opera Software
Received on Friday, 7 June 2013 13:09:07 UTC

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