How to clean up __proto__ (was: Why we need to clean up __proto__)

Mark S. Miller erights at
Wed Dec 28 17:39:37 PST 2011

[Oops. Actually changing the Subject: this time]
[change of Subject: as this reply starts the promised thread]

Hi Lasse, good. That is the direction Dave Herman suggested and which I
also find attractive. I have elaborated it at <>,
which I've also placed on the Agenda for the January EcmaScript meeting.

The most surprising punchline is in requirement #4:

Object.create(null), creating an object that does not inherit from
Object.prototype, also creates an object that does not inherit it’s
__proto__, even if that property has not been deleted. With this change,
objects-as-stringmaps for objects created by Object.create(null) would
avoid the __proto__ hazard, even in contexts where
Object.prototype.__proto__ has not been deleted. (FF already acts this way,
so my previous message was wrong in claiming that Object.create(null) fails
to avoid this platform on all non-IE browsers.)

Comments on this proposal appreciated.

On Wed, Dec 28, 2011 at 5:26 PM, Lasse Reichstein <
reichsteinatwork at> wrote:

> The biggest problem with __proto__, as I see it, isn't that you can
> hit it accidentally, but that even if you know what you are doing, you
> can't avoid it (and have to do expensive workarounds).
> It obviously depends on the implementation. In Mozilla
> Firefox/*Monkey, __proto__ is an inherited accessor on
> Object.prototype (just as __define[SG]etter__). It's not an
> own-property and you can shadow it.
> In Chrome/V8, it's an artificial own property on every object, and it
> can't be removed, shadowed or ignored. It tries to emulate Safari's
> original implementation, but probably fails some details (as your
> tests show, they aren't the same).
> I like the Firefox way much better, both as a user and as an
> implementor. Since we probably can't get rid of __proto__ totally, I
> would cheer if implementations would change to the Mozilla way.
> /L 'entirely my personal opinion'.
> --
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