"var" declarations shadowing properties from Window.prototype

Boris Zbarsky bzbarsky at MIT.EDU
Wed Aug 15 08:16:21 PDT 2012


On 8/15/12 3:48 AM, Axel Rauschmayer wrote:
> In FF 14, I’m getting the following results (if there is an element
> whose ID is "foo"):
>
>      $ "foo" in window
>      false
>      $ foo
>      ReferenceError: foo is not defined

This is an artifact of how the repl you're using (presumably the Web 
Console in Firefox) is implemented.  In particular, if you're using the 
Web Console its global is NOT the window, which allows you to declare 
variables visible in the repl but not leaking to the web page.  But it 
leads to some weird artifacts with the global scope polluter, since 
there isn't one here, really.

In a web page, the behavior is different, as you can test by using an 
actual <script> tag.


> That looks like what you described: The Global Scope Polluter
> auto-creates foo after the read access (*). However, (**) puzzles me: A
> getter for foo seems to be called (as a warning is displayed), but it
> returns `undefined`. How come?

Because you're testing in a somewhat bizarre environment.

> Also interesting: window instanceof EventTarget holds,
> but EventTarget.prototype is not in protos(window)

Yes, indeed.  Host objects can do some interesting things with 
instanceof.  ;)  Again, this will be fixed once we move Window to WebIDL.

-Boris


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