Le 19/06/2011 16:53, Tom Van Cutsem a écrit :
> 2011/6/17 David Bruant <[email protected]
> <mailto:[email protected]>>
>
> Le 17/06/2011 22:20, Mark S. Miller a écrit :
>> Hmmm. This suggests a loophole that might help out on some of the
>> proxy cases we're concerned about. I know of no universal
>> constraint that a proxy's handler could violate if it could 1)
>> trap attempts to change the value of a writable non-configurable
>> property, and 2) could respond by either setting the value as it
>> likes or reporting a failed assignment. The handler must of
>> course be prevented from changing any other attribute, except to
>> change from writable to non-writable. AFAICT, this does not
>> weaken any assumptions that are safe to assume regarding ES5.1
>> non-native objects.
> Actually, since in the recent version of the strawman, the
> defineProperty trap is actualy trapped even for fixed properties,
> I think that what you're describing is happening already. Is it Tom?
>
>
> Yes, with one minor difference: as currently specced in the strawman,
> the proxy handler can trap attempts to change the value of any fixed
> non-configurable property, both writable and non-writable. Still, the
> handler will be prevented from changing writable:false to
> writable:true, which seems to be the important invariant to uphold.
>
> So, updates to non-writable, non-configurable fixed properties are
> currently trapped, but the handler can't actually change the
> property. Should the strawman be changed such that defineProperty is
> only trapped on writable, non-configurable fixed properties?
With the current fixed properties proposal, here is what happens in the
engine at an Object.defineProperty(o, name, pd) call:
----
if("name" has already been observed as non-configurable){
[[DefineOwnProperty]] (name, pd) on the fixed properties record //
ES5.1 - 8.12.9
// This call includes invariant checking code
}
else{
resPd = ToPropertydescriptor(call o's defineProperty trap with [o,
name, pd]);
if(!resPd.configurable){
add "name" to fixed properties record with resPd as property descriptor
}
}
----
What about doing the following:
----
// (Calling the trap in all cases)
resPd = ToPropertyDescriptor(call o's defineProperty trap with [o, name,
pd]);
if("name" has already been observed as non-configurable){
[[DefineOwnProperty]] (name, resPd) on the non-configurable properties
record // ES5.1 - 8.12.9
// This call includes the exact same invariant checking code
}
else{
if(!resPd.configurable){
add "name" to non-configurable properties record with resPd as
property descriptor
}
}
----
The same amount of engine code is called and we have the genericity of
calling the defineProperty trap every single time.
David
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