using Private name objects for declarative property definition.

Allen Wirfs-Brock allen at
Sat Jul 9 11:19:46 PDT 2011

On Jul 9, 2011, at 9:45 AM, Brendan Eich wrote:

> On Jul 9, 2011, at 8:48 AM, Brendan Eich wrote:
>> See above, there is nothing novel or evil in isName or isArray (another example) isGenerator.
> Also the Proxy.isTrapping, which in recent threads has been proposed to be renamed to Proxy.isProxy or Object.isProxy.
> This is a fine point, but we may as well hash it out, and es-discuss is the best place: do we want Object.isFoo predicates, in spite of precedent such as ES5's Array.isArray (which I've whined about as redundantly and a bit misleadingly named, since its argument can be any value -- Object seems a better home, lacking a Value "class" built-in constructor in which to bind Value.isArray).

A consideration here is that whatever conventions we follow sets a precedent for user written code. I don't think we want to encourage the addition of such classification functions to Object or Object.prototype.  So from that perspective Array.isArray, Function.isGenerator, and Proxy.isProxy are better exemplars than Object.isArray, etc.

Another consider is whether such classification schemes should use methods or data properties. An instance side, isFoo() method to be useful generally will also require the addition of an isFoo() method to Object.prototype while a instance side-data property can be tested without augmenting Object.prototype (({}.isFoo) is a falsey value).  Also most user code classification schemes will require some sort of instance-side tagging or testing to actually determine whether the instance is in the category. 

For those reasons, I think the best classification scheme is usually via an instance-side public data property with a truthy value. 

But there are some exceptions.  For the legacy built-ins such as Array is less risky from compatibility perspective to add properties to the constructor than to the prototype. However, for new built-in this isn't an issue. Another consider is meta layering: meta-layer functions shouldn't be exposed on application layer objects. Proxy.isProxy is a good example.  Proxy instances operate as application objects in the application layer.  The public face of application objects shouldn't be polluted with a meta-layer method such as isProxy.

> Back at the dawn of time, isNaN and isFinite as value predicates. Not relevant, since not type predicates, but worth a mention. adds saner (no argument coercion){NaN,Finite} predicates, and Number.isInteger for testing whether a number is an integral IEEE 754 double.
> For, Function.prototype.isGenerator has been proposed (and implemented in Firefox 4 and up).
> Why not Function.isGenerator? Because the prototype method is more usable when you have a function in hand and you want to know whether it's a generator. If you have any value x, you'll need (typeof x === 'object' && 'isGenerator' in x) guarding the x.isGenerator() call, but we judged that as the less common use-case.

Why not just make it a boolean-valued data property?

> Could be we were wrong, or that being consistent even in the hobgoblin/foolish sense is better. But Object.isGenerator crowds poor Object more.
> Notice how there's no need for Function.isFunction or Object.isFunction, due to typeof (function(){}) === 'function'. That's another dawn-of-time distinction that does not fit the primitiive types vs. object classification use-case I mentioned in the last message -- testing whether x is callable via typeof x === 'function', at least for native if not host objects, is the use-case here.

But this is a special case that does not extrapolate to other situations.

> Some of these questions are at this point more about aesthetics and Principles than about usability. And we have historic messiness in how things have grown.
In a utilitarian language like JS, usability and aesthetic considerations should be closely linked and consistency is a significant usability factor. .  Principles are a way to get consistent application of such  aesthetics . 

> Given Array.isArray, perhaps we are better off with Proxy.isProxy. The case for Function.prototype.isGenerator hangs on usability, so I'd like to let the prototype in Firefox ride for a bit.
> If someone wants to propose a better, extensible and uniform system of type predicate naming, please do.

a start of one is above.  The exceptions and special cases are where good judgement is required.


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