On Apr 20, 2015, at 12:42 PM, Caitlin Potter wrote:
> Oh — he’s right, ValidateAndApplyPropertyDescriptor won’t throw in the
> example case, because the old descriptor is configurable. That’s kind of
> weird.
It is kind of weird, but that was what TC39 decided on back when ES5 was being
developed. The logic was that if a property is configurable then it is
possible to change all of its attributes by performing a [[DefineOwnProperty]]
with a complete property description. Because of that possibility, all
changes made via a partial property descriptor are also accepted. In other
words:
var o = Object.create(null, {x:{value: 0, writable: false, enumerable: true,
configurable:true}});
Object.defineProperty(o,' x', {value:2});
console.log(o.x); //2
The define property above is allowed because it could have been replaced with
the sequence :
Object.defineProperty(o,'x', {writable: true});
Object.defineProperty(o,'x', {value: 2, writable: false});
or even by:
delete o.x;
Object.defineProperty(o,'x', {value: 2, writable: false, enumerable: true,
configurable: true};)
hence, we might as well accept the single line version.
In retrospect, perhaps not such a good idea.
Allen
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