It makes perfect sense for Object.defineProperty, but maybe not so much sense for PutValue(). One idea was to just add an `return false if existingDescriptor.[[Writable]] is false.` Before receiver.[[DefineOwnProperty]]()`.
> On Apr 20, 2015, at 8:17 PM, Allen Wirfs-Brock <[email protected]> wrote: > > >> 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 > > > _______________________________________________ es-discuss mailing list [email protected] https://mail.mozilla.org/listinfo/es-discuss
