[webkit-dev] Runtime setting for incomplete features

Maciej Stachowiak mjs at apple.com
Mon Sep 21 12:17:22 PDT 2009

On Sep 18, 2009, at 1:30 PM, Jeremy Orlow wrote:

> On Fri, Sep 18, 2009 at 12:59 PM, Alexey Proskuryakov  
> <ap at webkit.org> wrote:
> 18.09.2009, в 12:24, Jeremy Orlow написал(а):
> I'm not sure if we've hit any compatibility issues with this yet,  
> but it seems quite plausible that someone would compare  
> window.localStorage (or sessionStorage or database) to undefined  
> and, since it's null (vs. undefined), their script would assume it's  
> available.
> Note that they can also do '"localStorage" in window', which we  
> can't easily prevent from returning true.
> That's true.  And unfortunate.
> What would be involved in making it not return true?

It would be pretty complicated to do that based on a runtime setting.  
You would need a custom getter for any object that has properties  
which may appear or disappear based on settings. This is probably too  
complicated to be worth it. Or at least, if we added that level of  
code complexity I would begin to doubt the merits of supporting  
runtime enabling of Web platform features.

Which is a shame, because if ("localStorage" in window) is a generally  
a better way to feature test than if (window.localStorage). The latter  
idiom is problematic for attributes where a possible valid value is  
something that evaluates to false, or where computing the value can be  
expensive. For example, if (document.body.outerHTML) would be an awful  
way to test whether outerHTML is available.


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