using Private name objects for declarative property definition.

Brendan Eich brendan at
Sat Jul 9 09:45:04 PDT 2011

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).

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.

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.

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.

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.


More information about the es-discuss mailing list