Private Slots

Tom Van Cutsem at
Tue Jan 15 12:10:19 PST 2013

In defense of Kevin, I too would argue that private symbols add complexity
to the object model.
That doesn't mean I'm arguing in favor of removing private symbols, just
noting that we do pay a complexity price.

We should not deny that with the addition of private symbols, we had to
reconsider every existing ES5 operation to see whether it needed to be
adjusted so that it does not leak private symbols. We happen to be lucky
that Object.keys and Object.freeze can be thought of as using
Object.getOwnPropertyNames, so adjusting gOPN (and the Proxy API) closed
all leaks identified thus far. Going forward from here, we need to continue
to be vigilant about private symbol leaks.

I also think Kevin is making the fair point that for the 99% use case,
unique symbols are sufficient. For software engineering purposes, they
provide good enough encapsulation. "private" in Java is not really private
due to java.lang.reflect, but that doesn't stop people using it to express

Since Kevin is merely asking for a good explanation of why private symbols
are "in", I would say that it's because TC39 explicitly wanted symbols to
be usable for high-integrity encapsulation scenarios.

Kevin's reply to that is "if the use case is high-integrity encapsulation,
why aren't WeakMaps sufficient?"

The response to that is: WeakMaps add more boilerplate and more overhead. I
don't think the WeakMap boilerplate overhead is an actual issue for people
writing defensive code (such code already has to go so out-of-its-way, e.g.
by using early-bound primordials, I think it can deal with WeakMaps). The
performance argument still stands though.


2013/1/15 David Bruant <bruant.d at>

>  Le 15/01/2013 17:16, Kevin Smith a écrit :
>>  The variable was called "s" standing for secret. The example I gave was
>> dummy, but examples in the wild are legion. Take your favorite node
>> library. Anytime it defines a _property, this is where there should be a
>> private name.
>  Again, that's arguable.  Certainly it would be better expressed with a
> symbol.  But private?  Are most node authors attempting to write "secure"
> code?  Of course not!
> People writing Java code do use the private keyword. I think if
> encapsulation wasn't that hard to achieve, people would do it. I'm looking
> forward to see people use classes and modules without putting more effort
> that they do currently and yet write more secure code by default.
>    Node (by itself) does not provide a secure environment for untrusted
> code.
> What I know of Node makes me think it's not that much worse than any other
> platform and from experience, at least much better than anything I've
> played wit in PHP.
>   One of the goal was that no one had access to the "s" part of the
>> state. You need private symbols for that.
>  But why?  Where is this strict runtime privacy requirement coming from?
>  What I'm not understanding is the larger context in which mutually
> untrusting code supposedly shares raw (non-proxied) objects.
> The reason you can't write code in any blog comment these days is that
> people have given up on securing untrusted code. Likewise for emails. HTML
> is used for emails, but scripts aren't executed, because email client
> (web-based or not) have given up on the idea of securing email code.
>   Where is the real world code doing this?
> I think iGoogle does this. Otherwise, close to nowhere because people have
> given up.
> My guess is that they have given up because the language does not make
> easy to sandbox untrusted code. As soon as you have naturally written your
> code in JavaScript, it's unsafe and it takes a lot of work making it safe.
> People give up. Private symbols are one tool to lower the barrier to
> writing code secure by default.
>   For what applications?  Really, I want to know! : )
> They do not exist unfortunately. That's a chicken and egg problem. As soon
> as it'll be easier, people will probably restart thinking of what they want
> to do with the new tool they have in hand.
> That's what happened when JS engines got faster. Developers didn't stay
> still happy that their websites just got faster. Websites just started to
> get more and more JS.
>   In "Crockfordian" design, "name encapsulation" and "security
> encapsulation" are indistinguishable.  But unique symbols address "name
> encapsulation".  Is there really a need to address "security encapsulation"
> at the *object property level*?
> It seems that it lowers the barrier to writing secure code by default. If
> that's the only thing to gain, I'll take it.
>   Thinking more about the loading third-party code, I guess it's
>> technically doable to do without private names. It comes to the cost of
>> creating proxies doing the encapsulation for you. You provide a blacklist
>> of properties that must not be reflected and the third party never sees
>> them... In essence, you're listing the private properties and the proxy
>> does the book keeping (details about non-configurability aside).
>  Sure - and approaches like this (or simpler - people are clever!) can be
> factored away into a neat library, without having to mess with the
> underlying object model.
>  ES6 provides WeakMaps and Proxies.  Why not see what people do with
> those before introducing private slots?
> I wouldn't be opposed to that, but that's just my opinion. Still, private
> symbols allow property-like syntax. I haven't followed the latest
> developments of how classes and private symbols interact, but I'm not too
> worried it goes well. Assuming it does, it makes it easy for people to use
> actually private properties in their code, lowering the barrier to writing
> well-encapsulated code.
> David
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