should be

“In addition to general attributes, an accessor property has the following *attributes* which related with a getter and a setter respectively”

thanks.

@kangax

thanks for additions.

Of course data descriptor is created _not only_ when corresponding object is empty, but also when “writable” or “value” properties are present (you only mention empty and get/set).

Yes, of course, but this case seems obvious. The main goal is to specify what happens when we have an empty descriptor object. Although, I added this case too.

You can also mention that `TypeError` is supposed to be thrown when creating a property descriptor out of an object that has both “value”/”writable” and “get”/”set” properties (as per 8.10.5 — ToPropertyDescriptor).

Yes, added.

It might not be obvious, but property can only be overwritten when its [[Configurable]] is not `false`. Since in this example we do not set [[Configurable]] explicitly and its default value is `false`, the property can not be overwritten and an error is thrown. Defining it with [[Configurable]] == true obviously “solves” the problem:

Yup, also a good addition. Although, it is mentioned in the [[Configurable]] attribute definition, it is good to specify this case explicitly.

By the way, the error is not thrown if reconfiguring attribute’s value (a getter in this case) is the same (because of SameValue check in 8.12.9). But practically such “re”-configuration seems useless.

I mentioned all that cases, though.

Btw, does generic descriptor (which should be created if object is empty) serve any useful purpose?

No, don’t think so. At least — not for ECMAScript program. Of course, it serves a useful purpose on implementation level. See again “8.12.9 [[DefineOwnProperty]]”, where checks such as IsGenericDescriptor and other are everywhere. There also said, that default value for absent attributes is false, so for the ECMAScript program it doesn’t matter.

Dmitry.

Few things you might want to add:

“all implementations creates a data property if corresponding descriptor is empty, and an accessor property if descriptor object has either a get or a set property”

Of course data descriptor is created _not only_ when corresponding object is empty, but also when “writable” or “value” properties are present (you only mention empty and get/set).

I find it easy to think about it in this way:

1) If object is empty or value/writable properties are present, data descriptor is created

2) If get/set properties are present, accessor descriptor is created

Btw, does generic descriptor (which should be created if object is empty) serve any useful purpose?

You can also mention that `TypeError` is supposed to be thrown when creating a property descriptor out of an object that has both “value”/”writable” and “get”/”set” properties (as per 8.10.5 — ToPropertyDescriptor).

Another “gotcha” I didn’t see in the article is what happens when you try to overwrite accessor property. For example:

var o = Object.defineProperty({ }, 'x', {
  get: function(){ return 'bar' }
});

Object.defineProperty(o, 'x', {
  get: function(){ return 'baz' }
});

o.x; // "bar"

It might not be obvious, but property can only be overwritten when its [[Configurable]] is not `false`. Since in this example we do not set [[Configurable]] explicitly and its default value is `false`, the property can not be overwritten and an error is thrown. Defining it with [[Configurable]] == true obviously “solves” the problem:

var o = Object.defineProperty({ }, 'x', {
  get: function(){ return 'bar' },
  configurable: true
});

Object.defineProperty(o, 'x', {
  get: function(){ return 'baz' }
});

o.x; // "baz"

Посмотрим. Возможно позже, когда я допишу все части этой новой серии на английском; параллельно вряд ли сейчас буду писать.

Но, этим может заняться кто-нибудь другой — естественно заинтересованный в JS и в этом цикле статей. Отсюда тогда будут ссылки. Постепенно, я смогу проверить качество и точность перевода в техническом плане.