July TC39 meeting notes, day 1

Kevin Reid kpreid at google.com
Mon Aug 8 09:46:20 PDT 2011


On Mon, Aug 8, 2011 at 08:50, Andreas Rossberg <rossberg at google.com> wrote:
> I would welcome removing the extra receiver (or proxy) arguments from
> get and set traps. However, it seems to me that the main reason,
> currently, for having them is that they are needed by the default
> traps, in case the respective descriptor returned by
> getOwnPropertyDescriptor has a getter/setter (which need a receiver).

This is almost the rationale I gave earlier. To be precise, the
default traps themselves need not have behavior which is implementable
as an explicit trap (since they are not exposed as being functions
which take the same parameters as user-supplied traps do). I feel the
receiver should be provided so that user-supplied traps *can mimic the
default traps*, with variations or optimizations.

> Arguably, making a proxy trap return getters/setters seems a somewhat
> pointless use case anyway. But nevertheless we need to have some
> reasonable semantics for it.

It allows a proxy to pretend to be an object which supports
Object.defineOwnProperty normally.

It allows a proxy to emulate, or wrap, an ordinary object which
happens to have some accessor properties, while still being
transparent to reflection (which I understand is one of the goals of
the proxy facility).

(As it happens, this doesn't affect the use case which made me notice
this problem; there, I am defining an emulation of DOM nodes (i.e. the
accessors are such things as .innerHTML), and DOM nodes, being host
objects, are allowed to do anything, I am given to understand.
However, it is convenient to define my emulations as ordinary objects,
which are incidentally wrapped by a proxy to implement the
particularly magical parts.)


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