typeof extensibility, building on my Value Objects slides from Thursday's TC39 meeting

Brendan Eich brendan at mozilla.com
Sat Aug 3 20:25:51 PDT 2013

Claude Pache wrote:
> Le 4 août 2013 à 01:39, Brendan Eich<brendan at mozilla.com>  a écrit :
>> Brendan Eich wrote:
>>> Claude Pache wrote:
>>>> Fixing `typeof` of old (null) and new value types would be a solution, but I'm rather definitely considering something like the defunct `Object.isObject()`
>> I forgot to add that my patch for https://bugzilla.mozilla.org/show_bug.cgi?id=749786 includes
>> Object.isObject(x) // false unless x is an object
>> Object.isValue(x)   // true for primitives and value objects
>> The idea: isObject returns true for all the new value object types and the reference-semantics objects found in JS today, while isValue returns true for all compare-by-value types, whether the legacy primitives (undefined, null, boolean, number, string) or the new-in-ES7 value objects.
>> /be
> Ok. What I am most interested, when I test for an "object", is roughly that I am able to get or set properties on it (ignoring restrictions or special cases like freezing, etc., and, in order to shorten the test, forgetting functions that are not supposed to occur).

Value objects are non-extensible, no-own-property (so effectively 
frozen), compare-by-value objects.

>   Or that it can be used like an object created by `new Object` (an Object-like). This probably coincides with `!Object.isValue`. Likely, I won't want to treat `1L` like `{ }` and differently from `1`.

Right, you want !Object.isValue(x).

> That numbers are non-objects but int64 are objects is much more an implementation detail than a relevant distinction for the programmer.

Yes, that's intentional. I'm not sure we need the 
Object.is{Object,Value} APIs but they might be handy. Could go in the 
to-be-named Reflect module instead.

> So, I consider that `typeof int64(0) === "object"` and your `Object.isObject` are about as useless than `typeof null === "object"`.

You could be right -- if it's useless we should cut it. But it does 
provide a bit of information that's not the same as !Object.isValue, and 
which is a bit clumsy to piece together otherwise.

> —Claude

More information about the es-discuss mailing list