"Subclassing" basic types in DOM - best method?

Peter van der Zee ecma at qfox.nl
Tue Nov 20 13:54:17 PST 2012

On Tue, Nov 20, 2012 at 10:31 PM, Rick Waldron <waldron.rick at gmail.com> wrote:
> On Tue, Nov 20, 2012 at 2:45 PM, Tab Atkins Jr. <jackalmage at gmail.com>
> wrote:
>> On Mon, Nov 19, 2012 at 9:46 PM, Brendan Eich <brendan at mozilla.com> wrote:
>> > Tab Atkins Jr. wrote:
>> >> If we did this, the only reason to continue subclassing Map is to get
>> >> instanceof checks to work.  Is this acceptable?
>> >
>> > I think it's either irrelevant (no one tests 'aUrlQuery instanceof Map')
>> > or else a potential problem (cross-frame instanceof).
>> People *do* perform those checks, though.  For example, in a method
>> that accepts either an array or other things, a quick "foo instanceof
>> Array" check is a clear, easy way to check what you've got.
> Be careful there, it's incredibly rare to see code that does that—which is

I don't agree. I see often see instanceof, both with Array and with
other objects. This danger you and everybody speaks of only applies to
cross-frame scripts. And while this danger is real (and I don't mean
to make it sound like it isn't), I think you should first consider the
amount of people actually doing cross frame scripting because it's not
something most people touch frequently, if at all. Any studies to get
such numbers?

- peter

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