934311 - Javascript modules don't share definition of Array (and probably all other standard classes like String etc.)

Status: RESOLVED INVALID

(Core :: General, defect)

Description

[Peter Lyons Kehl]

User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:27.0) Gecko/20100101 Firefox/27.0 (Beta/Release)
Build ID: 20131029134505

Steps to reproduce:

Have two simple Firefox extensions XPI, each defining one Javascript module (loadable via chrome://). One module exports a function, which creatse new array (via 'new Array' or via [..]). The other module exports a function, that takes a parameter and it checks whether (param instanceof Array).


Actual results:

An array created and returned from one JS module is not recognized as an array in another module.

If param was created as an array in a different JS module, (param instanceof Array) returns false.


Expected results:

Array instances should have same class (constructor) no matter what module.

If the current behaviour is a correct one (because of scopes or memory leaks), please
- explain why
- update https://developer.mozilla.org/en-US/docs/Mozilla/JavaScript_code_modules/Using so it clearly states that

Comment 1

[Boris Zbarsky [:bzbarsky]]

> param instanceof Array

You should be using Array.isArray.

Each module has its own global, of course; and per ES spec each global has a different Array instance.

Maybe all we need to do is document that each module has its own global?  I would have thought that's obvious from the way they work...

Comment 2

[Bobby Holley (:bholley)]

(In reply to Boris Zbarsky [:bz] from comment #1)
> Maybe all we need to do is document that each module has its own global?  I
> would have thought that's obvious from the way they work...

Yes.

Comment 3

[Peter Lyons Kehl]

In addition to mentioning/documenting that each module has its own global, please also make
https://developer.mozilla.org/en-US/docs/Mozilla/JavaScript_code_modules/Using refer to https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects so that the users know what is in 'global'.

Comment 4

[Bobby Holley (:bholley)]

(In reply to Peter Kehl from comment #3)
> In addition to mentioning/documenting that each module has its own global,
> please also make
> https://developer.mozilla.org/en-US/docs/Mozilla/JavaScript_code_modules/
> Using refer to
> https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/
> Global_Objects so that the users know what is in 'global'.

MDN is a wiki, which you can freely edit. If you have any thoughts about how to make documentation better, you can just do it! :-)