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Re: typeof document.all

From: Brendan Eich <brendan@mozilla.org>
Date: Mon, 12 Oct 2009 11:45:29 -0700
Message-Id: <95BA75C5-4F66-4DB9-895B-C43626C2B557@mozilla.org>
To: HTML WG <public-html@w3.org>
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). 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.


 > > I have no idea what the spec is saying about this, exactly (link
 > > appreciated), but document.hasOwnProperty("all") is false in  
Gecko, and
 > > will continue to be so, I suspect; that's just not an own  
property of
 > > that object.  The behavior is quite self-consistent, contrary to  
Thomas'
 > > claim above.

(Thomas seems to have been misled by the 'prototype' property of  
functions and
built-in constructors: document.prototype is an undefined property, of  
course;
some JS implementations support document.__proto__ as bz showed.)

 > http://www.whatwg.org/specs/web-apps/current-work/multipage/obsolete-features.html#dom-document-all
 >
 > See also the IDL block above it (which defines the prototype) and the
 > definition of HTMLAllCollection linked to from that section that  
defines
 > the variant of HTMLCollection used for this API (and for col.tags()).

If the prototype chain is not observable (by the book) why do the IDL
definitions matter?

The issue is falsy object value. Must this be normative?

/be
Received on Monday, 12 October 2009 18:46:33 GMT

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