911135 - ES6 Array.prototype.{map, filter, slice, splice} result is based on "constructor" property

Status: RESOLVED DUPLICATE of bug 1165052

(Core :: JavaScript Engine, defect)

Description

[Brandon Benvie [:benvie]]

The Array methods that return new Array instances have been changed so that return an instance of whatever their "constructor" property is, if the receiver is an actual array ("an Exotic Array Object"). If the result of Get(O, "constructor") is constructable, then it will be constructed (with the size of the resulting array as the only argument, or 0 in the case of `filter`). If the value is not constructable, then a regular array is created.

> function FakeArray(len){ this.length = len; }
> var array = [0, 1, 2];
> array.constructor = FakeArray;
> array.slice() instanceof FakeArray; // true

See ES6 spec draft (August 2013 revision) sections
* Array.prototype.slice - 15.4.3.10
* Array.prototype.splice - 15.4.3.12
* Array.prototype.map - 15.4.3.19
* Array.prototype.filter - 15.4.3.20

Comment 1

[Till Schneidereit [:till]]

I doubt that that'll change anything, but we'll probably want to wait until the current discussion on es-discuss[1] has come to a conclusion.


[1]: http://esdiscuss.org/topic/array-prototype-slice-web-compat-issue

Comment 3

[Jason Orendorff [:jorendorff]]

Bug 1165052 subsumes this, right? We'll take care of it once subclassing lands.