ES3.1 Object static methods rationale document
brendan at mozilla.org
Wed Jul 16 23:20:20 PDT 2008
On Jul 16, 2008, at 10:14 PM, Kris Zyp wrote:
>>> Arguably, some of the need for direct prototype access is
>>> alleviated by providing the clone method. However, there are still
>>> plenty of other situations where it is useful.
>> I observe that __proto__ in SpiderMonkey- and Rhino-based JS is
>> mostly used for cases covered by Object.create, with a minority use-
>> case that we've discussed before initializing it to null in object
>> initialisers to make maps (dictionaries).
> I am curious how Object.create covers this __proto__ use case of
> objects with a defined proto. Doesn't Object.create create a new
> object and
> copy properties over?
ES3.1 draft dated 15-July-08:
18.104.22.168 Object.create ( O [, Properties] )
The create method creates a new object with a specified prototype.
When the static create method is called, the following steps are taken:
1. If Type(O) is not Object throw a TypeError exception.
2. Create a new object as if by the expression new
Object() where Object is the standard built-in constructor with that
3. Call the standard built-in function
Object.defineProperties with arguments Result(2) and Properties.
4. Set the internal [[Prototype]] property of Result
(2) to Result(1).
5. Return Result(4).
> __proto__ allows objects with existing properties to
> have their proto defined in constant time, but isn't Object.create
> O(n), with n being the number of properties?
Object.create allows creation of a new Object instance with a
designated prototype object initializing [[Prototype]].
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