Hi, I have recently read http://wiki.ecmascript.org/doku.php?id=strawman:es5_internal_nominal_typing and in particular "Array.isArray should test for presence of special [[DefineOwnProperty]] rather than testing [[Class]" (There is a typo, by the way) which has woken up one fundamental question which is "what is an array?". The consequence of this is "for what Array.isArray should return true or false?"
Starting with the defintion of "new Array" in the ES5 spec, we can see that an Array is created with a certain prototype value and the internal [[Class]] equal to "Array". Of course, the prototype doesn't define an Array by itself. Object.create(Array.prototype) will not return what we could consider as an Array. In a way, same thing for [[Class]], it's "just a string" and doesn't really identify what arrays are. One thing that caracterizes arrays is the internal connexion between "length" and numerical properties. This link can be found in the specific [[DefineOwnProperty]]. But I think it may be too quick to conclude that Arrays are the objects with this particular internal function. One could imagine that one day, other objects have this particular [[DefineOwnProperty]] internal methods, but other [[Get]] or [[Set]] or [[HasProperty]] internal methods. In my opinion, one thing that makes an Array what it is is all internal methods and the internal consistency they create altogether. It turns out they are all the same than the Object ones and only [[DefineOwnProperty]] is different, but still, the internal consistency we rely on when using Arrays exists because of the conjunction of all internal properties. But recently, I have created the ProxyArray library (https://github.com/DavidBruant/ProxyArray) where, on a proxy, I have recreated the internal array behavior and special relationship between 'length' and numerical indices. Leaving aside the fact that no ES engine could determine whether a proxy handler has the exact same semantics than actual array internal methods, my library doesn't create Arrays anyway. One of the reason is that, for instance, even if the engine could tell that the semantics is the same, it would certainly not apply Array-specific optimizations. I have mentionned "Array-specific optimizations". The same way, in the spec, for most Array.prototype method is written something like "The filter function is intentionally generic; it does not require that its this value be an Array object.". In these two contexts, what does "Array" mean? My answer to this last question and my (partial) conclusion to the discussion is: "An array is something created after a 'new Array()' call (where 'Array' is the built-in constructor define in the spec). " As a consequence, it englobes things created after a call to "Array()" (ES5 15.4.1.1 - Create and return a new Array object exactly as if the standard built-in constructor Array was used in a new expression with the same arguments (15.4.2).) or array literals (ES5 11.1.4 Array Initialiser - semantics Step 1 of "ArrayLiteral : [ Elisionopt ]" and "ElementList : Elisionopt AssignmentExpression"). In my opinion, at creation (direct or indirect "new Array()"), arrays should internally contain a marker saying they've being created with the built-in Array constructor. I think that the intention behind [[Class]] was to have such a marker. But in a way, it's "just a string" and we could imagine a world where the [[Class]] was settable on some objects. Nevertheless even if setting [[Class]] to arrays it wouldn't make them Arrays just because some string has the right value. Earlier, I said that it was a partial conclusion. I would like to addArray subclassesto the definition. The "Standardizing __proto__" thread led to the idea of standardizing a subclassing mechanism. In my opinion, objects constructed as subclassed arrays should benefit from the Array identity (return true to "Array.isArray") and as a consequence benefit from Array extras natively (in ES5, applying Array extras to non-Array objects is implementation-specific). Of course, all of this is just my opinion and is open to debate. David Influencial readings : - http://wiki.ecmascript.org/doku.php?id=strawman:es5_internal_nominal_typing - ES5 15.4.2.1 new Array (http://es5.github.com/#x15.4.2.1) - http://perfectionkills.com/how-ecmascript-5-still-does-not-allow-to-subclass-an-array/
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