Private names use cases

Mark S. Miller erights at
Tue Dec 21 12:29:22 PST 2010

On Tue, Dec 21, 2010 at 10:20 AM, Allen Wirfs-Brock
<allen at>wrote:

> See below:
> On Dec 21, 2010, at 9:03 AM, Mark S. Miller wrote:
> On Mon, Dec 20, 2010 at 9:21 AM, Allen Wirfs-Brock <allen at>wrote:
>> I've seen mentions in the recent thread that the goal of the "Private
>> Names" proposal was to support "private fields" for objects.  While that may
>> be a goal of some participants in the discussion, it is not what I would
>> state as the goal.
>> I have two specific use cases in mind for "private names":
>> 1) Allow JavaScript programmers, who choose to do so, to manage the direct
>> accessibly of object *properties*.  This may mean limiting access to
>> methods of a particular instance, or to methods of the same "class", or to
>> various friends or cohorts, etc.
>> 2) Allow third-party *property* extensions to built-in objects or
>> third-party frameworks that are guaranteed to not have naming conflicts
>>  with unrelated extensions to the same objects.
>> Of these two use cases, the second may be the more important.
> I'm glad you agree. By incremental fiddling, such as repairing the
> encapsulation leaks of private names, perhaps we could brings these two
> proposals closer together to try to find common ground. However, this second
> use case would still be a real difference. <
> uses your example of this use case. In light of your message, I just added a
> note on the crucial difference:
> For defensive programming, best practice in many environments will be to
> freeze the primordials early, as the dual of the existing best practice that
> one should not mutate the primordials. Evaluating the dynamic behaviour of
> Python applications <> (See
> also provides evidence that
> this will be compatible with much existing content. We should expect these
> best practices to grow during the time when people feel they can target ES5
> but not yet ES6.
> Consider if Object.prototype or Array.prototype were already frozen, as
> they should be, before the code above executes. Using soft fields, this
> extension works. Using private names, it is rejected.
> Not everybody in the JavaScript community agrees that this style of
> defensive programming is desirable or should be a "best practice".

How did "best practice in many environments" become a question of
whether "everybody
in the JavaScript community agrees" on this, or indeed anything?

>  On my blog, there was resistance expressed to JavaScript providing any
> sort of information hiding mechanism. I would not anticipate frozen
> primordials becoming the norm anytime soon.

How about "a norm"? I can't help feeling that your uses of "everyone" and
"the" is an attempt to (in the pejorative sense) strawman the discussion. I
am claiming only that it will become *a* norm, an important one, and one
that we should strive to support.

> Even if this style did become the norm, I don't see why you would argue in
> support of mechanisms that allow extension of frozen objects.  Isn't the
> whole point of freezing to prevent any extensions.

No. Let's distinguish two use cases. The point of shallow freezing is to
create a "tamper proof object" (the phrase I've been using in talks). If x
is a tamper proof object given to otherwise isolated subsystems A and B,
then the only interactions between them enabled by giving them x are those
that the author of x chooses to provide. This helps both security and
modularity. As David-Sarah points out, only if the author of x is in control
of what public interface they export can they know what changes they can
make without breaking clients.

Building on this, the point of transitive immutability is to create objects
that can be safely shared between isolated subsystems without thereby giving
them *any* means to interact. With out historic inability to freeze the
primordials (Object.prototype, etc), our only isolation mechanism was the
creation of separate frames. In the browser, separate same-origin frames by
themselves provides no security benefit, but are increasingly used for their
modularity benefit -- to ensure that various complex subsystems do not
interfere with each other. Notice that the clone code, whether expressed
using soft fields or private names, will not succeed at associating default
behavior with objects from other frames. If same-origin frames grows as the
best practice, we both lose.

When we are able to freeze the primordials, and combined with our desire to
remove the global object from the bottom of the scope chain, then we can
arrange to bring separate subsystems into one frame without interference.
Then the soft field clone works but the private names clone does not. The
private names clone *only* works if we bring these all into one frame and
(by not freezing the primordials) risk that these complex subsystems may
destructively interfere with each other silently -- with no diagnostic.

>  why is the fact that the extension is accomplished using a side-channel
> any more acceptable to you.

Side channel has a particular technical meaning. This channel is overt, and
so is not a side channel.

>> Note that I emphasized "properties" rather than a new concept such as
>> "private fields".  I believe we should be trying to build upon the
>> conceptual foundation of the existing JavaScript object model whenever
>> possible. We should strive to avoid introducing new concepts such as
>> non-property fields into the object model.  (see
>>  for further thoughts on this
>> topic.)
> I find this latter point and your elaboration on that web page bizarre. It
> is the private names proposal that would change the object model, even if
> you consider these changes minor. The soft fields proposal does not change
> the object model *at all*. It has the semantics of a side table.
> I am speaking of the object model, as perceived to by a JavaScript
> programmer of moderate skill

As the comparison page shows, using your examples, the object model for
normal vanilla usage is identical.

> and also by JavaScript implementors.

The fastpath portion of the two are very similar. Both would place the state
in question of the objects being extended and reuse the existing
inherited-lookup machinery.

Regarding the fastpath, from the implementor perspective, the two proposals
have very similar object models. From the casual programmer perspective,
the object models are identical. And from the security and formal semantics
perspective, soft fields does not change the existing object model at all.

>  To me, an incremental extension of a concept that is already present
> (extending the set of values that can be used as a property name) is a much
> smaller extension to the object model than the introduction of a new form of
> per object state (whether called a private field, a soft field, or something
> else).
> The fact that you are proposing implementing you object extension as a
> look-aside table doesn't mean it isn't a conceptual extension to the object
> model perceived by JavaScript programmers.  That might arguably be the case
> if you were simply defining a set of conventions based upon Ephemeron tables
> for decorating objects with additional non-encapsulated state.  However, as
> soon as you tie into either the language's property access syntax ( . and
> []) as you have (perhaps reluctantly) proposed you have extended the
> conceptual object model.

For the casual programmer, the change to the object models are identical.

> It seems to me, the real point of difference here is whether or not we
> should add a syntactic mechanism that supports information hiding in the
> context of JavaScript objects.  Some constituents want this, others do not.
> If you don't care about syntactic support for information hiding than we
> already have solutions.

I will answer in a separate email.

> Ephemeron tables provide a side-band mechanisms for dynamically associating
> additional state with an objects.  You have demonstrated one way to do that
> in your soft fields proposal.   Even without Ephemeron tables, it is already
> possible to hide state within an object using closure capture.
> However, neither of these mechanisms are integrated with the fundamental
> concept of an "object" that is defined by the language specification and
> taught to JavaScript programmers.  They are based upon coding patterns that
> assume understanding of difficult concepts (Ephemeron or closure capture).
>  In addition, in the context of current implementations their performance
> characteristics will be inferior relative to standard property access.  If
> you care about these issues than you probably fall into the camp that wants
> some sort of syntactic and semantic language extension that explicitly
> supports object information hiding.

Syntactic yes. I will no longer argue about which syntax.
Semantic, to whom?
* Casual programmer: yup, we have the same one in mind.
* Implementor: yup, again, we have essentially the same small change in mind
regarding the fast path.
* Security and formal semantics: All for it, if it actually enhances
information hiding over what's possible otherwise.

So to answer your question, yes, I think we're all in essentially the same

> It seems both of your key points in this message support soft fields over
> private names.
> I don't think you even addressed my first point

I am not sure what point you are referring to. Do you mean

1) Allow JavaScript programmers, who choose to do so, to manage the direct
accessibly of object *properties*.  This may mean limiting access to methods
of a particular instance, or to methods of the same "class", or to various
friends or cohorts, etc.

? Other than terminology, I don't see how the proposals differ in this
regard. If the issue is that we call the new state "<adjective> properties"
so that it seems more familiar, fine.

> and your argument concerning the second point is based upon an assumption
> that I don't share regarding a particular "defensive programming" style.

What assumption is that? If the assumption in question is whether "everyone"
agrees that defensive programming should be "the norm", then I don't share
that assumption either.

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