[webkit-dev] Setting event handlers on the global context

Maciej Stachowiak mjs at apple.com
Sun Jul 19 16:28:52 PDT 2009

On Jul 19, 2009, at 11:10 AM, Adam Barth wrote:

> I think we should do what Firefox does in the window.onload case.  :)
> I'm not familiar with the history through.  Is there some particular
> reason we have our current behavior?

The current behavior is an accident of implementation, but I think the  
relevant applicable spec is somewhat ambiguous. A function declaration  
creates a new var-like binding which shadows the underlying onmessage  

The relevant spec here is ECMAScript, not HTML5. In ECMA-262, section  
10.1.3 is what would apply, I haven't checked the proper reference for  
the fifth edition draft. Regarding the declaration of a function with  
the same name as an existing property, it says: "If the variable  
object already has a property with this name, replace its value and  
attributes." So a vanilla property should get replaced with a  
DontDelete property, but in the case of a setter it's not clear  
whether it should be replaced or shadowed.

It would probably be good to match other browsers. I am curious what  
IE and Opera do here. I would also be curious if ECMAScript 5 is more  
clear about what to do when a function declaration has the same name  
as a setter property, since it supports getters and setters in the spec.

  his would be slightly, but not insanely tricky to fix. The relevant  
code is all in BytecodeGenerator::BytecodeGenerator(ProgramNode*  
programNode....), since this only affects global scope. The change  
could have far-reaching consequences so we'd have to be on the lookout  
for Web compatibility regressions if we change this.


> Adam
> On Sun, Jul 19, 2009 at 10:56 AM, Drew Wilson<atwilson at google.com>  
> wrote:
>> Yes, it happens with window.onload() too (I didn't mean to imply  
>> that it was
>> a worker-only issue).
>> This seems like precisely the type of inoperability that the HTML5  
>> spec
>> should address, but I figured I should get some input here before  
>> bringing
>> it up there.
>> -atw
>> On Sun, Jul 19, 2009 at 10:31 AM, Adam Barth <abarth at webkit.org>  
>> wrote:
>>> You should test the same thing with window.onload.  If I recall
>>> correctly, you'll see similar inoperability.
>>> Adam
>>> On Sun, Jul 19, 2009 at 9:29 AM, Drew Wilson<atwilson at google.com>  
>>> wrote:
>>>> I was writing a new worker unit test and I noticed that all of  
>>>> our unit
>>>> tests set event handlers in worker global context like so:
>>>> onmessage = function(event) { ... do something ... };
>>>> I note that Firefox also allows setting event handlers like this:
>>>> function onmessage(event) { ... do something ... };
>>>> In WebKit, the latter form creates a function at global scope named
>>>> "onmessage" but does not set it as an event handler.
>>>> I'm trying to figure out what the correct behavior should be - I've
>>>> asked
>>>> IanH, and his response was that he dimly recalls that both forms  
>>>> should
>>>> be
>>>> valid "(through a convoluted argument that I forget right now,  
>>>> but which
>>>> should be examined carefully by whoever implements this, and which
>>>> requires
>>>> careful examination of at least the ECMAScript spec and maybe  
>>>> also the
>>>> WebIDL and HTML5 specs)" - basically, he wasn't entirely certain  
>>>> and
>>>> thought
>>>> I should check here and with the Mozilla devs to get your opinions.
>>>> Anyone familiar enough with the various specs to make a definitive
>>>> argument
>>>> one way or the other?
>>>> -atw
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