- From: Maciej Stachowiak <mjs@apple.com>
- Date: Thu, 15 Oct 2009 11:29:31 -0700
- To: Mike Shaver <mike.shaver@gmail.com>
- Cc: Allen Wirfs-Brock <Allen.Wirfs-Brock@microsoft.com>, "public-script-coord@w3.org" <public-script-coord@w3.org>, es-discuss Steen <es-discuss@mozilla.org>
On Oct 15, 2009, at 10:54 AM, Mike Shaver wrote:
> On Thu, Oct 15, 2009 at 1:47 PM, Allen Wirfs-Brock
> <Allen.Wirfs-Brock@microsoft.com> wrote:
>> Is the Mozilla document.all optimization contingent upon the
>> occurrence of the text "document.all"?
>
> No, but it's contingent on the property lookup being the
> truthyness-testing-expression. (I wouldn't call it an optimization;
> if anything it adds some cost to certain paths.)
>
>> What happens for:
>> var docAll = document.all;
>> if (docAll) alert("IE"); else alert("not IE");
>
> "IE"
Just as a minor point of technical correction - this will actually
alert "not IE" in Firefox because the right-hand sign of an assignment
is considered a detecting access. (Just tested to confirm.)
>
> var doc = document;
> if (doc.all) alert("IE"); else alert("not IE");
>
> "not IE"
>
> It was something that was intentionally targetted at the common case
> of the simple feature test, which is almost always "document.all".
> This is done by the host object implementation, which is given access
> to limited context ("is this access one that's being used to detect
> the presence of a property, versus fetching the value?").
>
> Mike
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Received on Thursday, 15 October 2009 18:37:12 UTC