W3C home > Mailing lists > Public > public-html@w3.org > October 2009

Re: typeof document.all

From: Brendan Eich <brendan@mozilla.org>
Date: Mon, 12 Oct 2009 12:50:30 -0700
Cc: HTML WG <public-html@w3.org>
Message-Id: <FFCFE2DC-6965-43B1-AA84-9B6003FA2DA8@mozilla.org>
To: Maciej Stachowiak <mjs@apple.com>
On Oct 12, 2009, at 12:22 PM, Maciej Stachowiak wrote:

> On Oct 12, 2009, at 11:45 AM, Brendan Eich wrote:
>
>> On Tue, 14 Jul 2009 07:22:24 +0000 (UTC), Ian Hickson wrote:
>> > On Mon, 13 Jul 2009, Boris Zbarsky wrote:
>> > >
>> > > In Gecko, document.all is in fact on the prototype of the  
>> document.
>> > > More precisely, trying to use document.all (as opposed to just  
>> test for
>> > > it) changes the prototype chain of the document such that its  
>> prototype
>> > > has the "all" property.  Try this in your favorite Gecko-based  
>> browser:
>> > >
>> > > <script>
>> > >   var oldProto = document.__proto__;
>> > >   alert(oldProto.hasOwnProperty("all"));
>> > >   document.all.length;
>> > >   alert(oldProto.hasOwnProperty("all"));
>> > >   alert(document.__proto__.hasOwnProperty("all"));
>> > >   alert(document.__proto__ == oldProto);
>> > >   alert(document.__proto__.__proto__ == oldProto);
>> > > </script>
>> > >
>> > > You should be seeing alerts with the following values: false,  
>> false,
>> > > true, false, true
>> >
>> > The "adding 'all' after it's used" behaviour doesn't match other  
>> browsers
>> > as far as I can tell.
>>
>> The prototype chain is not directly observable according to the ES  
>> specs, even
>> with ES5's Object.getPrototypeOf (host objects can lie).
>>
>> Reifying a property on first use is a common technique, an  
>> optimization (not
>> observable, so conforming).
>
> In this case it's observable - the code cite above observes it. I'm  
> not sure it matters in practice, but it is observable.

No, I'm talking about lazy creation here. Not observable.

What bz shows is using __proto__, of course he's observing stuff, but  
that's not what I was talking about. Sorry if I got pedantic there, I  
was trying to separate lazy creation from the non-existence vs. exists- 
with-undefined-value issue.


>> For document.all, the interoperation problem (if it
>> is a problem) is that Mozilla reifies depending on context. WebKit  
>> reifies
>> (eagerly or lazily, doensn't matter) independent of context, and  
>> with a falsy
>> object value.
>>
>> Confusion arises because undefined is a falsy value, which compares  
>> == itself
>> and null as well as converts to false in boolean contexts. But a  
>> property that
>> has value undefined is observably different from no property, even  
>> if one goes
>> only by the ES specs.
>
> Specifically, the in operator and the hasOwnProperty() method will  
> give different results.

You must be talking about the other issue I failed to mention: where  
on the prototype chain, if not directly on the referenced object, the  
existent property (lazily created or not) lives.

Good point, this is another potential interoperation issue, but only  
if we care about prototype chain structure as reflected by __proto__  
or ES5's Object.getPrototypeOf, and provided the host object  
implementation doesn't lie to conceal intermediate hidden prototypes  
and possibly thereby conform.


> I think using the object in a with() statement will also give  
> different results.

There should be no 'with' connection. Do you have a testcase?


>> If the prototype chain is not observable (by the book) why do the IDL
>> definitions matter?
>
> Web IDL defines which aspects of an IDL interface are prototype  
> properties and which are own properties. The prototype chain is  
> observable with nonstandard extensions, but also in part via  
> prototype properties present on DOM (pseudo-)constructors.

Right, of course -- but is 'all' supposed to be direct (or appear to  
be that way), and not on any visible prototype? Or must it (by  
hasOwnProperty) be a direct property of HTMLDocument.prototype?

This seems unimportant for interoperation based on what browsers  
actually do so far. Must it be specified? If it must, what about  
making it appear to be a direct ("own" in ES terms) property of  
document? IIRC IE does that.


>> The issue is falsy object value. Must this be normative?
>
> Some way of making document.all undetectable should be normative. We  
> can discuss the pros and cons of the Gecko and WebKit solutions to  
> this. It might also be possible to specify things in a way that both  
> the Gecko and WebKit behaviors end up conforming, for example by  
> listing a series of if tests that must pass or fail, thus leaving  
> some edge cases unspecified.

I'm loath to volunteer Mozilla changing to match WebKit, unsinking  
sunk costs and taking risks, but we might. Two issues stand in our way:

1. The ES-purity point of ToBoolean(Object) -> true manded by all  
ECMA-262 editions without host object exception, which is reflected in  
our implementation, so it's not just a purity issue for us (even if  
I'm not a purist about conformance).

2. The quirks mode conditioning of our undetected document.all  
emulation. We do not want document.all in standards mode. You agreed  
in principle on public-script-coord recently. Who else needs to agree  
for HTML5 to spec this as quirks-mode only (I may be showing my  
ignorance by asking, if HTML5 has no quirks-mode at all)?

/be
Received on Monday, 12 October 2009 19:51:30 GMT

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