My ECMAScript 7 wishlist

David Bruant bruant.d at
Fri Jun 6 09:23:23 PDT 2014

Le 06/06/2014 18:16, Nicholas C. Zakas a écrit :
> On 6/6/2014 8:38 AM, Mark S. Miller wrote:
>>     But the concern Nicholas raises doesn't seem to have this
>>     property. Reading a property that doesn't exist doesn't carry a
>>     security risk, does it? Object.preventUndeclaredGet doesn't
>>     really protect against anything like ES5 methods did.
>> That's true, but misses the point I was trying to make. For normal ES 
>> objects, it is already part of their API contract with their clients 
>> that the clients can do feature testing to detect the presence or 
>> absence of a method. The most common way to do such feature testing 
>> is to get the property and see if it is falsy. (Variations include, 
>> testing for undefined, testing for undefined or null, and testing if 
>> its typeof is "function".) It's fine if the provider of an 
>> abstraction does not wish to support this pattern. But it is not ok 
>> for one client of an object which does support it to prevent that 
>> object's other clients from successfully using feature detection.
> Sorry I was sleeping while most of this conversation was happening. :)
> I understand the point about feature detection, it would suck if some 
> random code did Object.preventUndeclaredGet() on an object you own and 
> were using feature detection on. I still wish for some way to do this 
> other than through proxies, but I agree that it would be nice for just 
> the object provider to be able to set this behavior.
It is possible via an API following the same pattern than revocable proxies:

     {object, toggle} = makeUndeclGetThrowObject()
     toggle(); // now the object throws when there is a [[Get]] on an 
undef property

Keep the toggle function locally so only trusted parties access it, 
share the object as you wish.
Admittedly more cumbersome than your solution or proxies :-p

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